Luca LARCHER - personale UniMoRe

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Luca LARCHER

Professore Ordinario
Dipartimento di Scienze e Metodi dell'Ingegneria

Pubblicazioni

2023 - A HydroDynamic Model for Trap-Assisted Tunneling Conduction in Ovonic Devices [Articolo su rivista]
Buscemi, F; Piccinini, E; Vandelli, L; Nardi, F; Padovani, A; Kaczer, B; Garbin, D; Clima, S; Degraeve, R; Kar, Gs; Tavanti, F; Slassi, A; Calzolari, A; Larcher, L
abstract

Electrical conduction in ovonic threshold switching (OTS) devices is described by introducing a new physical model where the multiphonon trap-assisted tun-neling (TAT) is coupled to a hydrodynamic theory. Static and transient electrical responses from Ge(x)Se(1-x )experimental devices are reproduced, outlining the role played by the material properties like mobility gap and defects in tuning the OTS performances. A clear physical interpretation of the mechanisms ruling the different OTS conduction regimes (off, threshold, on) is presented. A nanoscopic picture of the processes featuring the carrier transport is also given. The impact of geometry, temperature, and material mod-ifications on device performance is discussed providing physical insight into the optimization of OTS devices.

2023 - Device‐to‐Materials Pathway for Electron Traps Detection in Amorphous GeSe‐Based Selectors [Articolo su rivista]
Slassi, Amine; Medondjio, Linda‐sheila; Padovani, Andrea; Tavanti, Francesco; He, Xu; Clima, Sergiu; Garbin, Daniele; Kaczer, Ben; Larcher, Luca; Ordejón, Pablo; Calzolari, Arrigo
abstract

The choice of the ideal material employed in selector devices is a tough task both from the theoretical and experimental side, especially due to the lack of a synergistic approach between techniques able to correlate specific material properties with device characteristics. Using a material-to-device multiscale technique, we propose a reliable protocol for an efficient characterization of the active traps in amorphous GeSe chalcogenide. The resulting trap maps trace back the specific features of materials responsible for the measured findings, and connect them to an atomistic description of the sample. Our metrological approach can be straightforwardly extended to other materials and devices, which is very beneficial for an efficient material-device co-design and the optimization of novel technologies.

2023 - Insights into device and material origins and physical mechanisms behind cross temperature in 3D NAND [Relazione in Atti di Convegno]
Pesic, Milan; Beltrando, Bastien; Rollo, Tommaso; Zambelli, Cristian; Padovani, Andrea; Micheloni, Rino; Maji, Rita; Enman, Lisa; Saly, Mark; Bae, Yang Ho; Kim, Jung Bae; Yim, Dong Kil; Larcher, Luca
abstract

2023 - Towards a Universal Model of Dielectric Breakdown [Relazione in Atti di Convegno]
Padovani, Andrea; Torraca, Paolo La; Strand, Jack; Shluger, Alexander; Milo, Valerio; Larcher, Luca
abstract

2022 - Atomic Defects Profiling and Reliability of Amorphous Al2O3 Metal–Insulator–Metal Stacks [Articolo su rivista]
Torraca, P. La; Caruso, F.; Padovani, A.; Tallarida, G.; Spiga, S.; Larcher, L.
abstract

We present a comprehensive characterization of amorphous alumina (a-Al2O3) high- k dielectric in metal-insulator-metal (MIM) stacks, self-consistently extracting the space-energy distribution of a-Al2O3 atomic defects and the related bond-breaking process parameters. Active defects are profiled via simultaneous simulation of current-voltage ( I- V), capacitance-voltage ( CV), conductance-voltage (GV) (i.e., defect spectroscopy), and low-field I- V hysteresis analysis. The defect energies extracted ( ETH = 1.55 and 3.55 eV) are consistent with oxygen vacancies and aluminum interstitials. The voltage-dependent dielectric breakdown (VDDB) statistics of a-Al2O(3) is investigated using ramped voltage stress (RVS). The VDDB statistics show a complex and polaritydependent breakdown statistics, correlating with defect distributions, which allows estimating the a-Al2O3 bondbreaking parameters with the support of multiscale atomistic simulations of the breakdown process.

2022 - BELLO: A post-processing tool for the local-order analysis of disordered systems [Articolo su rivista]
Dianat, B.; Tavanti, F.; Padovani, A.; Larcher, L.; Calzolari, A.
abstract

The characterization of the atomic structure of disordered systems, such as amorphous, glasses and (bio)molecule in solution, is a fundamental step for most theoretical investigations. The properties of short- and medium-range local order structures are responsible for the electronic, optical and transport properties of these systems. Here, we present the BELLO open source code, a post-processing script-tool created for the automatic analysis and extraction of structural characteristics of disordered and amorphous systems. BELLO is agnostic to the code that generated single configurations or trajectories, it provides an intuitive access through a graphical user interface (GUI), and it requires minimal computational resources. Its capabilities include the calculation of the order parameter q, the folded structure identification, and statistical analysis tools such as atomic coordination number and pair/angle-distribution functions. The working principles of the code are described and tested on ab initio molecular dynamics trajectories of amorphous chalcogenides.

2022 - Dielectric breakdown in HfO2 dielectrics: Using multiscale modeling to identify the critical physical processes involved in oxide degradation [Articolo su rivista]
Strand, Jack; La Torraca, Paolo; Padovani, Andrea; Larcher, Luca; Shluger, Alexander L.
abstract

2022 - Electron-assisted switching in FeFETs: Memory window dynamics - retention - trapping mechanisms and correlation [Relazione in Atti di Convegno]
Pesic, M.; Beltrando, B.; Padovani, A.; Miyashita, T.; Kim, N. -S.; Larcher, L.
abstract

We investigate the impact of charge-trapping on ferroelectric (FE) switching and its influence on memory window (MW) and retention of FeFET. Fabricated FinFETs with ferroelectric gate stack were used to study defects properties (within HZO), trapping/FE-switching interplay, and dynamics. Starting from the electronic-assisted nucleation of FE domains, we investigated the interface charging and degradation, as well as their impact on the polarization compensation, MW and stabilization of the retention. Finally, a balance between those competing processes was analyzed and a retention model of FeFET (capturing behavior over device's lifetime) was developed.

2022 - Investigation of Coercive Field Shift During Cycling in HfZrOₓ Ferroelectric Capacitors [Articolo su rivista]
Cai, P.; Li, H.; Liu, Z.; Zhu, T.; Zeng, M.; Ji, Z.; Wu, Y.; Padovani, A.; Larcher, L.; Pesic, M.; Wang, R.; Huang, R.
abstract

To fully understand the mechanisms for shifting of coercive field ( $E_{c}$ ) during the bipolar stress cycling in doped HfO₂ ferroelectric (FE) material, we present a systematic study with both characterization and simulation on HfZrOₓ (HZO) capacitor. First, with the help of time-dependent dielectric breakdown (TDDB) results, defect redistribution is found to be localized during the bipolar stress cycling. Then, with the advanced simulation framework GinestraⓇ, influences of work function (WF) mismatch of the electrodes and defect distribution on the $E_{c}$ symmetry and breakdown property are discussed. It indicates that the shift of $E_{c}$ is mostly due to the redistribution of defects from FE phase to non-FE phase and grain boundaries. Furthermore, nucleation limited switching (NLS) model is adopted to investigate the switching dynamics during coercive field shift in more detail.

2022 - Pulse optimization and device engineering of 3D charge-trap flash for synaptic operation [Articolo su rivista]
Anik Kumar, Mondol; Padovani, Andrea; Larcher, Luca; Raiyan Chowdhury, S. M.; Zunaid Baten, Md
abstract

2022 - Reliability of Non-Volatile Memory Devices for Neuromorphic Applications: A Modeling Perspective (Invited) [Relazione in Atti di Convegno]
Padovani, A.; Pesic, M.; Nardi, F.; Milo, V.; Larcher, L.; Kumar, M. A.; Baten, M. Z.
abstract

The advent of Artificial Intelligence (AI) and big data era brought an unprecedented (and ever growing) need for fast and energy efficient computation that cannot be obtained with the classical von Neumann computing architecture. This paved the way to new technologies that try to mimic the human brain to leverage its energy efficient and distributed computing. Nonvolatile memory technologies seem to be the ideal solution for the hardware implementation of artificial neurons and synapses of Neural Network (NN) architectures and have been extensively investigated in the last years. However, they often suffer from limited linearity and symmetry, poor retention, and high variability, thus requiring significant advancements for their mass adoption in NNs. One of the cornerstones on which the required efforts must be based is certainly represented by device simulations, which can be effectively used to achieve a full understanding of the physics governing the device, which in turn is a pre-requisite for their design, optimization and successful exploitation in neuromorphic applications. In this scenario, we focus on Transition Metal Oxide (TMO)-based RRAM, Ferroelectric Tunnel Junctions (FTJ) and 3D-NAND Charge Trap devices and use simulations to address key issues related to neuromorphic operation (linearity and asymmetry) and reliability (variability and retention).

2022 - The electrons' journey in thick metal oxides [Articolo su rivista]
Caruso, F.; La Torraca, P.; Larcher, L.; Tallarida, G.; Spiga, S.
abstract

Originally introduced in electronic manufacturing to replace the SiO2 insulating layer, metal oxides are now extensively used in a multitude of electronic devices. Understanding charge transport mechanisms in metal oxides is of paramount importance for device optimization; however, a detailed and self-consistent discussion of electron conduction at all applied electric fields is lacking in the literature. In this work, we investigated the conduction mechanisms in three model systems, Al2O3, HfO2, and Al-doped HfO2 metal-insulator-metal capacitors, determining the path that the electrons travel within the metal oxide. Traps properties are extracted from experimental current-voltage characteristics using the Ginestra® simulation software. Furthermore, the analysis allowed to visualize the location of traps most involved in the conduction and the dominant transport mechanisms at each applied electric field. Despite the different oxide properties, a similar trend was recognized at low electric fields, the electron transport through the oxide is negligible, and the dominant contribution to the measured current is ascribed to the charge/discharge of traps located near the metal/oxide interfaces, leading to displacement currents. At high electric fields, the transport of electrons occurs through the defect rich oxides in the two following ways: if a large density of traps is energetically located near the electrodes Fermi level (as in HfO2), the electrons tunnel from trap to trap until they reach the anode; otherwise, when traps are closer to the conduction band (as in Al2O3 and AlHfO), the electrons tunnel from the cathode into one trap and then into the oxide conduction band, interacting only with traps near the cathode.

2022 - Variability and disturb sources in ferroelectric 3D NANDs and comparison to Charge-Trap equivalents [Relazione in Atti di Convegno]
Pesic, M.; Padovani, A.; Rollo, T.; Beltrando, B.; Strand, J.; Agrawal, P.; Shluger, A.; Larcher, L.
abstract

We investigate physical mechanisms driving the retention and disturb of charge-trap (CT) based and ferroelectric-(FE) based 3D NAND string. Combining a calibrated CT 3D NAND model and calibrated material properties of the FE material (extracted from FE-FinFET), we extrapolate and compare the existing workhorse with the low-power, high-speed contender. We show that: (1) a inherently discretized FE-stabilization combined with the polycrystalline nature of HZO, and interface charge compensation guarantees MLC capability; (2) FE 3D NAND offers higher ON currents that enable further Z-scaling. Furthermore, we develop a retention model and show that independently of the inherited discretization of the storage layer, lateral charge migration (of the parasitically trapped charge that stabilizes polarization) combined with pass voltage (disturb) can cause retention loss of FE 3D NAND. Finally, integration (layer-cut) and material engineering approaches are suggested for mitigation and guaranteeing stable operation of the string.

2021 - Analysis and Simulation of Interface Quality and Defect Induced Variability in MgO Spin-Transfer Torque Magnetic RAMs [Articolo su rivista]
Sikder, B.; Lim, J. H.; Kumar, M. A.; Padovani, A.; Haverty, M.; Kamal, U.; Raghavan, N.; Larcher, L.; Pey, K. -L.; Baten, M. Z.
abstract

Device-to-device variability of CoFeB/MgO based STT-MRAMs is studied based on experiments and simulations taking into account the influence of interface quality, temperature variation and device dimensionality. Metal-induced gap states resulting from electron transfer at the ferromagnet-tunnel barrier interface significantly influence the effective energy barrier height of these devices irrespective of their diameters. Switching voltage and parallel - antiparallel resistance values vary by as much as 43% and 30% respectively for about 13% variation of the energy barrier, whereas the tunneling magnetoresistance remains typically unaffected. WRITE cycles of highly scaled STT-MRAMs are therefore more susceptible to device-to-device variations resulting from microscopic variations in the interface quality, rather than the READ cycles. Such variations are observed to be independent of temperature, as well as spatial distribution of the defects.

2021 - Circuit model for thermoviscous propagation in annular waveguides [Articolo su rivista]
Ricci, Y.; La Torraca, P.; Larcher, L.
abstract

This paper presents a circuit model of the thermoviscous acoustic wave propagation in waveguides with annular cross section. The model, validated against finite element method simulations of the input acoustic impedance, captures the annular waveguide behavior with good accuracy within a frequency bandwidth consistent with the lumped-element approximation. The cascading of multiple circuit models easily allows extending the bandwidth while preserving the same accuracy. The circuit model was derived from the low reduced frequency (LRF) wave propagation model in rectangular layers, representing a valid approximation of the complex LRF solution in annular waveguides. The simplified analytical description allows for the formulation of a compact T-network model comprised of standard circuit elements. This circuit model can be implemented in circuit simulators to accelerate both the analysis and engineering of devices having elements with annular cross section, such as micro-electro-mechanical systems devices or microphones.

2021 - Design and Fabrication of a Pillar-based Piezoelectric Microphone exploiting 3D-Printing Technology [Articolo su rivista]
Ricci, Y.; Sorrentino, A.; La Torraca, P.; Cattani, L.; Cotogno, M.; Cantarella, G.; Orazi, L.; Castagnetti, D.; Lugli, P.; Larcher, L.
abstract

This letter presents a 3-D-printed piezoelectric microphone with enhanced voltage sensitivity. The sensitivity is improved by a combination of a single-pillar mechanical design and a specific polyvinylidene fluoride (PVDF)-film electrode patterning. The moving part of the mechanical structure and the chassis are 3D-printed as a single unit and trimmed by laser cutting, allowing for a simple fabrication of the device. The measured sensitivity of 1 mV/Pa (±6 dB) in the bandwidth 500–2500 Hz agrees with simulations, showing an improvement over similar pillar-based piezoelectric sensor solutions. The sensitivity performance is shown to be comparable to existing microphones with different technologies. The microphone is also characterized by excellent linearity within the measurable range. 3D-printing technique can thus be adopted for the manufacturing of low cost and highly customizable microphone sensors.

2021 - Electron trapping in ferroelectric HfO2 [Articolo su rivista]
Izmailov, R. A.; Strand, J. W.; Larcher, L.; O'Sullivan, B. J.; Shluger, A. L.; Afanas'Ev, V. V.
abstract

Charge trapping study at 300 and 77 K in ferroelectric (annealed Al- or Si-doped) and nonferroelectric (unannealed and/or undoped) HfO2 films grown by atomic layer deposition reveals the presence of "deep"and "shallow"electron traps with volume concentrations in the 1019-cm-3 range. The concentration of deep traps responsible for electron trapping at 300 K is virtually insensitive to the oxide doping by Al or Si but slightly decreases in films crystallized by high-temperature annealing in oxygen-free ambient. This behavior indicates that the trapping sites are intrinsic and probably related to disorder in HfO2 rather than to the oxygen deficiency of the film. Electron injection at 77 K allowed us to fill shallow electron traps energetically distributed at ∼0.2 eV. These electrons are mobile and populate states with thermal ionization energies in the range ∼0.6-0.7 eV below the HfO2 conduction band (CB). The trap energy depth and marginal sensitivity of their concentration to crystallization annealing or film doping with Si or Al suggests that these traps are associated with boundaries between crystalline grains and interfaces between crystalline and amorphous regions in HfO2 films. This hypothesis is supported by density functional theory calculations of electron trapping at surfaces of monoclinic, tetragonal, and orthorhombic phases of HfO2. The calculated trap states are consistent with the observed thermal ionization (0.7-1.0 eV below the HfO2 CB) and photoionization energies (in the range of 2.0-3.5 eV below the HfO2 CB) and support their intrinsic polaronic nature.

2021 - Extraction of Defects Properties in Dielectric Materials from I-V Curve Hysteresis [Articolo su rivista]
Torraca, P. L.; Caruso, F.; Padovani, A.; Spiga, S.; Tallarida, G.; Larcher, L.
abstract

Atomic defects in high-k materials affect the performance, reliability, variability, and scaling potential of electronic devices. Their characterization is thus of paramount importance, and methods exploiting electrical measurements are highly demanded. In this work we present a novel method for extracting the defect properties from I-V curve hysteresis measured at low electric field in thick metal-insulator-metal (MIM) stacks. The I-V curve hysteresis allows detecting the defects located near the electrode-insulator interfaces and aligned with the stack Fermi level, and extracting their properties. The defects are profiled cross-correlating the information provided by the low-field current hysteresis and the high-field steady-state current. This technique can be applied to MIM stacks fabricated in Back-End-of-Line for capacitors, embedded memories and thin film transistors.

2021 - Mechanism of Retention Degradation after Endurance Cycling of HfO2-based Ferroelectric Transistors [Abstract in Atti di Convegno]
Zhou, H.; Ocker, J.; Pesic, M.; Padovani, A.; Trentzsch, M.; Dunkel, S.; Muller, J.; Beyer, S.; Larcher, L.; Koushan, F.; Muller, S.; Mikolajick, T.
abstract

In this study, we provide insight into the mechanism of retention degradation after endurance cycling of HfO2-based ferroelectric field-effect transistors (FeFETs). Transfer characteristics of the FeFET are compared with the current-voltage response of the ferroelectric capacitors (FeCAP) for better understanding of the retention loss mechanism after cycling. Furthermore, a multiscale simulation by using the Ginestra™ modeling platform is conducted and the results show that charge trapping stabilizes the polarization switching and improves the retention behavior. Retention after cycling experiments are shown as well as pathways to reduce this degradation mechanism.

2021 - Standards for the Characterization of Endurance in Resistive Switching Devices [Articolo su rivista]
Lanza, M.; Waser, R.; Ielmini, D.; Yang, J. J.; Goux, L.; Sune, J.; Kenyon, A. J.; Mehonic, A.; Spiga, S.; Rana, V.; Wiefels, S.; Menzel, S.; Valov, I.; Villena, M. A.; Miranda, E.; Jing, X.; Campabadal, F.; Gonzalez, M. B.; Aguirre, F.; Palumbo, F.; Zhu, K.; Roldan, J. B.; Puglisi, F. M.; Larcher, L.; Hou, T. -H.; Prodromakis, T.; Yang, Y.; Huang, P.; Wan, T.; Chai, Y.; Pey, K. L.; Raghavan, N.; Duenas, S.; Wang, T.; Xia, Q.; Pazos, S.
abstract

Resistive switching (RS) devices are emerging electronic components that could have applications in multiple types of integrated circuits, including electronic memories, true random number generators, radiofrequency switches, neuromorphic vision sensors, and artificial neural networks. The main factor hindering the massive employment of RS devices in commercial circuits is related to variability and reliability issues, which are usually evaluated through switching endurance tests. However, we note that most studies that claimed high endurances >106 cycles were based on resistance versus cycle plots that contain very few data points (in many cases even <20), and which are collected in only one device. We recommend not to use such a characterization method because it is highly inaccurate and unreliable (i.e., it cannot reliably demonstrate that the device effectively switches in every cycle and it ignores cycle-to-cycle and device-to-device variability). This has created a blurry vision of the real performance of RS devices and in many cases has exaggerated their potential. This article proposes and describes a method for the correct characterization of switching endurance in RS devices; this method aims to construct endurance plots showing one data point per cycle and resistive state and combine data from multiple devices. Adopting this recommended method should result in more reliable literature in the field of RS technologies, which should accelerate their integration in commercial products.

2021 - Toward Hole-Spin Qubits in Si p -MOSFETs within a Planar CMOS Foundry Technology [Articolo su rivista]
Bellentani, L.; Bina, M.; Bonen, S.; Secchi, A.; Bertoni, A.; Voinigescu, S. P.; Padovani, A.; Larcher, L.; Troiani, F.
abstract

Hole spins in semiconductor quantum dots represent a viable route for the implementation of electrically controlled qubits. In particular, the qubit implementation based on Si p-MOSFETs offers great potentialities in terms of integration with the control electronics and long-term scalability. Moreover, the future down scaling of these devices will possibly improve the performance of both the classical (control) and quantum components of such monolithically integrated circuits. Here, we use a multiscale approach to simulate a hole-spin qubit in a down-scaled Si-channel p-MOSFET, the structure of which is based on a commercial 22-nm fully depleted silicon-on-insulator device. Our calculations show the formation of well-defined hole quantum dots within the Si channel and the possibility of a general electrical control, with Rabi frequencies of the order of 100MHz for realistic field values. A crucial role of the channel aspect ratio is also demonstrated, as well as the presence of a favorable parameter range for the qubit manipulation.

2021 - Variability sources and reliability of 3D-FeFETs [Relazione in Atti di Convegno]
Pesic, M.; Beltrando, B.; Padovani, A.; Gangopadhyay, S.; Kaliappan, M.; Haverty, M.; Villena, M. A.; Piccinini, E.; Bertocchi, M.; Chiang, T.; Larcher, L.; Strand, J.; Shluger, A. L.
abstract

Discovery of ferroelectricity (FE) in binary oxides enables the advent of FE memories and a plethora of novel CMOS compatible building blocks spanning from the logic domain to high-density storage and neuromorphic computing. In this paper we develop the first comprehensive model of vertical Ferroelectric Field Effect Transistor, V-FeFET, to identify sources of variability, understand retention problems, and point a path to improving reliability and enabling high-density storage FE memories with extended endurance.

2020 - Application and benefits of target programming algorithms for ferroelectric HfO2transistors [Relazione in Atti di Convegno]
Zhou, H.; Ocker, J.; Padovani, A.; Pesic, M.; Trentzsch, M.; Dunkel, S.; Mulaosmanovic, H.; Slesazeck, S.; Larcher, L.; Beyer, S.; Muller, S.; Mikolajick, T.
abstract

The ferroelectric HfO2 based field effect transistor (FeFET) has been under research for many years and shows unique properties for applications in the field of emerging memories and in-memory computing. This work for the first time demonstrates how a target programming algorithm can improve the FeFET device characteristics with respect to endurance performance and variability for small device geometries. With this technique the threshold voltage Vt of the memory cell can be targeted to any desired value, which is essential for multilevel cells and analog in-memory computing as used in AI accelerators. The switching, trapping and detrapping characteristics of the cell and their influence on the target programming algorithm are presented. The trapping and leakage characteristics are modelled using the GinestraTM simulation software to extract the trap distribution in ferroelectric HfO2. Finally, a model for the underlying mechanism of the endurance degradation is proposed.

2020 - Corrigendum to “Linearization of thermoacoustic loudspeakers by adaptive predistortion” [Sens. Actuators A: Phys. 297 (2019) 111551] (Sensors and Actuators: A. Physical (2019) 297, (S0924424719307903), (10.1016/j.sna.2019.111551)) [Articolo su rivista]
La Torraca, P.; Ricci, Y.; Bobinger, M.; Pavan, P.; Larcher, L.
abstract

The authors would like to add the following text as acknowledgement The authors would like to thank ASK industries S.P.A. for financial support and technical assistance. This work is supported by the Italian Ministry of Economic Development (MISE)'s FUND FOR THE SUSTAINABLE GROWTH (F.C.S) under grant agreement (CUP) B48I15000130008, project VASM (“Vehicle Active Sound Management”). The authors acknowledge the support of the Tiziano Nili (Project Leader) and Luca Cattani (Team Leader) The authors would like to apologise for any inconvenience caused.

2020 - Effect of electric field on defect generation and migration in HfO2 [Articolo su rivista]
Strand, J. W.; Cottom, J.; Larcher, L.; Shluger, A. L.
abstract

Understanding the effect of electric fields on defect creation and diffusion in metal oxides is of fundamental importance for developing accurate models of oxide degradation in electronic devices and dielectric breakdown. We use the Berry phase operator method within density functional theory to calculate how an applied electric field affects barriers for the creation of oxygen vacancy-interstitial defect pairs (DPs) and diffusion of interstitial O ions in monoclinic (m-)HfO2. The results demonstrate that even close to breakdown fields, barriers for DP generation exceed 6 eV in the perfect m-HfO2 lattice. Simulated injection of extra electrons from electrodes significantly lowers barriers for the creation of DPs, which are further reduced by the field to around 1 eV. Thus, bias application facilitates the injection of electrons into the oxide; these extra electrons reduce energy barriers for the creation of O vacancies, and these barriers as well as those for O ion diffusion are further lowered by the field. We find that, within a linear regime, the electric field modulates the barrier height by a dot product between the electric field and the electric dipole at the zero-field transition state to good accuracy.

2020 - Hot Electrons as the Dominant Source of Degradation for Sub-5nm HZO FeFETs [Relazione in Atti di Convegno]
Tan, A. J.; Pesic, M.; Larcher, L.; Liao, Y. -H.; Wang, L. -C.; Bae, J. -H.; Hu, C.; Salahuddin, S.
abstract

In this work, we demonstrate FDSOI ferroelectric FETs (FeFETs) incorporating 4.5 nm hafnium zirconium oxide, which show a 0.5V memory window at +/-3.3V and a program/erase speed of 1 u s. In typical FeFETs where geq 9 nm thick ferroelectric (FE) gate oxides have been used, bulk charge trapping has been identified as the main mechanism for endurance degradation and shrinkage of the memory window (MW). By contrast, we find that the role of bulk trapping in our devices with a much thinner FE layer is minimal. Through a combination of cryogenic temperature-dependent electrical measurements and simulations using the Ginestra ™ modeling platform, we identify and prove that hot electron-induced hole damage during the application of negative gate biases is the primary source of endurance degradation and MW closure in FeFETs with scaled oxide layers.

2020 - Multiscale modeling of CeO2/La2 O3 stacks for material/defect characterization [Relazione in Atti di Convegno]
Dianat, B.; Padovani, A.; Larcher, L.
abstract

Presence of defects in high-k dielectric materials will affect device's electrical properties, thus, defect/material characterization is of great importance. We present a simulation-based methodology relying on an accurate description of charge trapping and transport that is useful to extract relevant information on material and defect characteristics. This methodology was applied to cerium oxide and lanthanum oxide high-k dielectric materials and as a result, material properties alongside defect characteristics were extracted. Consequently, main charge conduction mechanism was identified to be trap-assisted tunneling (TAT).

2020 - Properties of intrinsic point defects and dimers in hexagonal boron nitride [Articolo su rivista]
Strand, J.; Larcher, L.; Shluger, A. L.
abstract

Hexagonal boron nitride (hBN) is a wide gap 2D layered material with good insulating properties. Intrinsic point defects in hBN play an important role in its applications as a dielectric in 2D electronic devices. However, the electronic properties of these defects are still poorly understood. We have calculated the structure and properties of a wide range of intrinsic point defects in the bulk of hBN using hybrid density functional theory (DFT). These include vacancies and interstitial states of B and N as well as di-and tri-vacancies. For each isolated defect, multiple charge states are calculated, and for each charge state multiple spin states are investigated. Positions of defect charge transition levels in the band gap of hBN are calculated. In particular, we predict that B vacancies are likely to be negatively charged in contact with graphene and other metals. Calculations of the interaction between vacancies predict that divacancies in both B and N sublattices are strongly binding. Moreover, the interaction of single B and N vacancies in adjacent layers induces the creation of-N-N-and-B-B-molecular bridges, which greatly distort the local structure, leading to local bond weakening. These results provide further insight into the properties of defects which can be responsible for degradation of hBN based devices.

2019 - A Sensitivity Map-Based Approach to Profile Defects in MIM Capacitors From I-V, C-V, and G-V Measurements [Articolo su rivista]
Padovani, A.; Kaczer, B.; Pesic, M.; Belmonte, A.; Popovici, M.; Nyns, L.; Linten, D.; Afanas'Ev, V. V.; Shlyakhov, I.; Lee, Y.; Park, H.; Larcher, L.
abstract

We present a defect spectroscopy technique to profile the energy and spatial distribution of defects within a material stack from leakage current (J-V), capacitance (C-V), and conductance (G-V) measurements. The technique relies on the concept of sensitivity maps (SMs) that identify the bandgap regions, where defects affect those electrical characteristics. The information provided by SMs are used to reproduce J-V, C-V, and G-V data measured at different temperatures and frequencies by means of physics-based simulations relying on an accurate description of carrier-defect interactions. The proposed defect spectroscopy technique is applied to ZrO2-based metal-insulator-metal structures of different compositions for dynamic random-access memory capacitor applications. The origin of the observed voltage, temperature, and frequency dependencies of the I-V, C-V, and G-V data is understood, and the atomic structure of the relevant stack defects is identified.

2019 - Acoustic characterization of laser-induced graphene film thermoacoustic loudspeakers [Relazione in Atti di Convegno]
Torraca, P. L.; Larcher, L.; Lugli, P.; Bobinger, M.; Romero, F. J.; Rivadeneyra, A.; Ricci, Y.; Cattani, L.; Morales, D. P.; Rodriguez, N.; Salinas-Castillo, A.
abstract

We report on the superior performance of laser-induced graphene (LIG) film thermoacoustic (TA) loudspeakers. LIG films are fabricated by laser-ablation of polyimide foil and contacted by screen-printing of silver paste, allowing for a facile, ambient condition fabrication process. The LIG film TA loudspeakers achieve up to 50 dB SPL at 1m distance, showing a 10 dB higher efficiency with respect to the nanostructured TA loudspeakers with polyimide substrate reported in literature.

2019 - Advanced modeling and characterization techniques for innovative memory devices: The RRAM case [Capitolo/Saggio]
Puglisi, Francesco Maria; Padovani, Andrea; Pavan, Paolo; Larcher, Luca
abstract

2019 - Boron Vacancies Causing Breakdown in 2D Layered Hexagonal Boron Nitride Dielectrics [Articolo su rivista]
Ranjan, A.; Raghavan, N.; Puglisi, F. M.; Mei, S.; Padovani, A.; Larcher, L.; Shubhakar, K.; Pavan, P.; Bosman, M.; Zhang, X. X.; O'Shea, S. J.; Pey, K. L.
abstract

Dielectric breakdown in 2D insulating films for future logic device technology is not well understood yet, in contrast to the extensive insight we have in the breakdown of bulk dielectric films, such as HfO2 and SiO2. In this letter, we investigate the stochastic nature of breakdown (BD) in hexagonal boron nitride (h-BN) films using ramp voltage stress and examine the BD trends as a function of stress polarity, area, and temperature. We present evidence that points to a non-Weibull distribution for h-BN BD and use the multi-scale physics-based simulations to extract the energetics of the defects that are precursors to BD, which happens to be boron vacancies.

2019 - Deconvoluting charge trapping and nucleation interplay in FeFETs: Kinetics and Reliability [Relazione in Atti di Convegno]
Pesic, M.; Padovani, A.; Slcsazeck, S.; Mikolajick, T.; Larcher, L.
abstract

Discovery of ferroelectric (FE) behavior in HfO 2 removed the compatibility roadblocks between the state-of-the-art CMOS and FE memories. Even though FE FETs (FeFETs) are scaled into 22 nm nodes and beyond, the limits of the technology as well as the physical mechanisms and reliability are still under research. In this paper we successfully developed a multiscale modeling platform to understand the interplay between the FE switching and charge trapping. Starting from the nucleation theory and rigorous charge transport modeling we present for the first time a self-consistent modeling framework we used for investigation of reliability and variability in FeFETs.

2019 - Design, prototyping and validation of a new PVDF acoustic sensor [Relazione in Atti di Convegno]
Sorrentino, A.; Ricci, Y.; Castagnetti, D.; Larcher, L.
abstract

This work focuses on the design, prototyping and testing of a new millimeter-size 3D printed acoustic sensor based on a Polyvinylidene Fluoride (PVDF) film. The piezoelectric PVDF has been widely used for sensors development in many applications due their high sensitivity and low cost. The aim of this work is to design an acoustic sensor that collects the acoustic wave and convey the most part of it as a force on the piezoelectric PVDF film in order to increase the sensitivity of the microphone. The work presents a detailed analytical model describing the mechanical behavior of the system. Through a metaheuristic optimization algorithm, we found the optimal geometric parameters of the system that maximize the acoustic force on the piezoelectric film. Analytical results show that the proposed solution exhibits a good value of sensitivity in the frequency range 10-10000 Hz. The proposed acoustic sensor was manufactured through 3D printing in ABS material and the tests focused on investigating the sensitivity of the system at different frequencies.

2019 - Investigating the Statistical-Physical Nature of MgO Dielectric Breakdown in STT-MRAM at Different Operating Conditions [Relazione in Atti di Convegno]
Lim, J. H.; Raghavan, N.; Padovani, A.; Kwon, J. H.; Yamane, K.; Yang, H.; Naik, V. B.; Larcher, L.; Lee, K. H.; Pey, K. L.
abstract

Ultra-thin dielectric breakdown (BD) has been studied in-depth for SiO 2 and HfO 2 in CMOS devices in the past. In general, the degradation physics and model governing BD in these materials are assumed to hold true for MgO. This study provides evidences that this assumption may not be true by investigating in detail the statistical nature of BD in MgO dielectrics for wide range of operating conditions, relevant to its application as spin transfer torque magnetic random access memory (STT-MRAM). Our analysis shows that - MgO BD is polarity dependent; lifetime is lower for bipolar (AC) stress; defect generation is clustered in space and time; self-heating dominates for low frequencies; temperature within the percolation path exhibits fast transients (thermal runaway); Weibull model does not apply to BD statistics and defect generation (F + ) is charge fluence driven (and field assisted) with power law model being most suited for lifetime extrapolation.

2019 - Investigation of I-V linearity in TaO x -Based RRAM devices for neuromorphic applications [Articolo su rivista]
Sung, C.; Padovani, A.; Beltrando, B.; Lee, D.; Kwak, M.; Lim, S.; Larcher, L.; Della Marca, V.; Hwang, H.
abstract

We perform experiments and device simulations to investigate the origin of current-voltage (I-V) linearity of TaO x -based resistive switching memory (RRAM) devices for their possible application as electronic synapses. By using electrical characterization and simulations, we link the electrical characteristics (linear or nonlinear I-V) to the microscopic properties of the conductive filament (CF). Our findings indicate that the shape and the thermal properties of the CF region are crucial to achieve linear I-V characteristics. These results allow optimizing the I-V curve linearity of TaO x -based RRAM devices, explaining the wide range of linear I-V characteristics experimentally observed on RRAM device obtained. When weight sum operation using SPICE simulations is performed, the read current is improved under the condition of linear I-V characteristics due to current loss minimization.

2019 - Linearization of thermoacoustic loudspeakers by adaptive predistortion [Articolo su rivista]
La Torraca, P.; Ricci, Y.; Bobinger, M.; Pavan, P.; Larcher, L.
abstract

In this work we present a novel driving technique for thermoacoustic (TA) loudspeakers. The proposed technique allows linearizing the pressure response of TA loudspeakers while reducing the average power dissipation on the device, and thus its working temperature. This is achieved exploiting an adaptive predistortion algorithm, implemented through digital signal processing. The controlled TA loudspeakers show exceptionally low values of total harmonic distortion and intermodulation distortion in their pressure response, exceeding the performance of previously proposed techniques, and a significantly reduced working temperature.

2019 - Mixed-Mode Stress in Silicon-Germanium Heterostructure Bipolar Transistors: Insights from Experiments and Simulations [Articolo su rivista]
Puglisi, F. M.; Larcher, L.; Pavan, P.
abstract

Recently, a wide class of market segments (e.g., health, material science, security, and communications) is tackled by circuits fabricated in BiCMOS technology, integrating silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs) and passives. Currently, the reliability of SiGe HBT devices is a major concern, and much attention is given to self-heating (SH), that limits device performance and regulates their degradation during stress. Moreover, its relevance is supposed to increase with device scaling. In this paper, we explore the reliability issues of SiGe HBTs by combining dedicated experiments and TCAD simulations. We develop and calibrate a TCAD model that is then used to investigate SH effects in both operating and stress conditions. Results show the important role played by the back-end-of-line (BEOL) and by the substrate thermal resistance in dissipating the heat generated by impact ionization. The location at which defects are generated during stress and the microscopic properties of the defects are determined experimentally by means of dedicated noise measurements. Including defects in the TCAD model allows reproducing the degradation observed in stress experiments. Simulations of the SH effects on a stressed device in measurement conditions revealed the presence of a hole hot spot that suggests a possible physical mechanism involved in the degradation slowdown at long stress times reported in the literature.

2019 - Modeling of field cycling behavior of ferroelectric hafnia-based capacitors [Capitolo/Saggio]
Pesic, M.; Larcher, L.
abstract

The discovery of ferroelectricity within doped hafnia revived interest in ferroelectric memories and opened the door for the integration of CMOS-compatible ferroelectrics into state-of-the-art semiconductor technology. In contrast to classic perovskite-based ferroelectric memories, hafnia-based equivalents suffer from limited endurance strength and changes of the memory window during the lifetime of the devices. Modeling and simulation are necessary not only to investigate the physical mechanisms occurring during the memory operation, but also to provide design guidelines that would lead to the engineering of the novel device with superior performance. Therefore, in this chapter, modeling approaches increasing the understanding of processes and mechanisms occurring within hafnia-based ferroelectric memories are discussed. Initially, dielectric processes and models describing pure dielectric properties are reviewed. Afterward, the models required for simulating the ferroelectric history dependent behavior are covered. Finally, the physical mechanisms responsible for the wake-up and fatigue of the ferroelectric memories are presented through the comprehensive model of the ferroelectric capacitor.

2019 - Multiscale modeling for application-oriented optimization of resistive random-access memory [Articolo su rivista]
La Torraca, P.; Puglisi, F. M.; Padovani, A.; Larcher, L.
abstract

Memristor-based neuromorphic systems have been proposed as a promising alternative to von Neumann computing architectures, which are currently challenged by the ever-increasing computational power required by modern artificial intelligence (AI) algorithms. The design and optimization of memristive devices for specific AI applications is thus of paramount importance, but still extremely complex, as many dierent physical mechanisms and their interactions have to be accounted for, which are, in many cases, not fully understood. The high complexity of the physical mechanisms involved and their partial comprehension are currently hampering the development of memristive devices and preventing their optimization. In this work, we tackle the application-oriented optimization of Resistive Random-Access Memory (RRAM) devices using a multiscale modeling platform. The considered platform includes all the involved physical mechanisms (i.e., charge transport and trapping, and ion generation, diusion, and recombination) and accounts for the 3D electric and temperature field in the device. Thanks to its multiscale nature, the modeling platform allows RRAM devices to be simulated and the microscopic physical mechanisms involved to be investigated, the device performance to be connected to the material's microscopic properties and geometries, the device electrical characteristics to be predicted, the effect of the forming conditions (i.e., temperature, compliance current, and voltage stress) on the device's performance and variability to be evaluated, the analog resistance switching to be optimized, and the device's reliability and failure causes to be investigated. The discussion of the presented simulation results provides useful insights for supporting the application-oriented optimization of RRAM technology according to specific AI applications, for the implementation of either non-volatile memories, deep neural networks, or spiking neural networks.

2019 - Printed Technology Solutions for Audio Transducers [Relazione in Atti di Convegno]
Torraca, P. L.; Ricci, Y.; Albrecht, A.; Bobinger, M.; Pavan, P.; Cattani, L.; Becherer, M.; Lugli, P.; Larcher, L.
abstract

We propose novel printed technology solutions for the fabrication of electro-acoustic audio transducers. Flexible, transparent thermoacoustic (TA) loudspeakers are fabricated using spray coating of silver nanowires (AgNWs) on a 75 μ m thin polyimide substrate. The fabricated devices show interesting optical and electric properties for integration in audio systems. The thermal and the acoustic characterizations shows very good agreement with the models proposed in literature. The proposed technology achieves up to 40 dB SPL at 1m distance, retaining its properties even under large substrate deformations.

2019 - Recommended Methods to Study Resistive Switching Devices [Articolo su rivista]
Lanza, Mario; Wong, H. -S. Philip; Pop, Eric; Ielmini, Daniele; Strukov, Dimitri; Regan, Brian C.; Larcher, Luca; Villena, Marco A.; Yang, J. Joshua; Goux, Ludovic; Belmonte, Attilio; Yang, Yuchao; Puglisi, Francesco M.; Kang, Jinfeng; Magyari-Köpe, Blanka; Yalon, Eilam; Kenyon, Anthony; Buckwell, Mark; Mehonic, Adnan; Shluger, Alexander; Li, Haitong; Hou, Tuo-Hung; Hudec, Boris; Akinwande, Deji; Ge, Ruijing; Ambrogio, Stefano; Roldan, Juan B.; Miranda Castellano, Enrique Alberto; Suñe, Jordi; Pey, Kin Leong; Wu, Xing; Raghavan, Nagarajan; Wu, Ernest; Lu, Wei D.; Navarro, Gabriele; Zhang, Weidong; Wu, Huaqiang; Li, Runwei; Holleitner, Alexander; Wurstbauer, Ursula; Lemme, Max C.; Liu, Ming; Long, Shibing; Liu, Qi; Lv, Hangbing; Padovani, Andrea; Pavan, Paolo; Valov, Ilia; Jing, Xu; Han, Tingting; Zhu, Kaichen; Chen, Shaochuan; Hui, Fei; Shi, Yuanyuan
abstract

Resistive switching (RS) is an interesting property shown by some materials systems that, especially during the last decade, has gained a lot of interest for the fabrication of electronic devices, with electronic nonvolatile memories being those that have received the most attention. The presence and quality of the RS phenomenon in a materials system can be studied using different prototype cells, performing different experiments, displaying different figures of merit, and developing different computational analyses. Therefore, the real usefulness and impact of the findings presented in each study for the RS technology will be also different. This manuscript describes the most recommendable methodologies for the fabrication, characterization, and simulation of RS devices, as well as the proper methods to display the data obtained. The idea is to help the scientific community to evaluate the real usefulness and impact of an RS study for the development of RS technology.

2019 - Statistical Simulation to Predict Variability of TANOS Program/Erase Characteristics for Non-Volatile Memory Applications [Relazione in Atti di Convegno]
Baten, M. Z.; Kumar, M. A.; Padovani, A.; Larcher, L.; Pramanik, D.
abstract

The impact of device-to-device statistical variation on Program/Erase (P/E) transients of planar TANOS devices is investigated using a multi-scale simulation approach. Atomic-level material and defect properties are first extracted from experimental results and then employed to study variability of the flat-band voltage shift. Erase characteristics are observed to be more affected by statistical variation. Interestingly, by adjusting temperature, gate voltage and/or blocking layer thickness, an optimized operating condition can be reached such that variability in 3D TANOS arrays is minimized.

2019 - Understanding and Variability of Lateral Charge Migration in 3D CT-NAND Flash with and Without Band-Gap Engineered Barriers [Relazione in Atti di Convegno]
Padovani, A.; Pesic, M.; Kumar, M. A.; Blomme, P.; Subirats, A.; Vadakupudhupalayam, S.; Baten, Z.; Larcher, L.
abstract

3D NAND Flash represents the unmatchable non- volatile memory concerning the bit-cost scaling efficiency and a role model for all emerging memories. Yet some reliability features of these devices i.e. quantification of the threshold voltage shift due to the lateral and vertical migration/loss of charges (LCL and VCL, respectively) is not fully understood. In this study we use a multi-scale modeling approach start from identification of the defects responsible for the charge trapping and quantify the LCL and VLC. As a part of engineering of the barrier we investigate also band-gap engineered (BGE) devices. We show that LCL dominates the charge loss during the retention and that highest portion of injected charge ends up in Al2O3 layer of BGE device.

2019 - Understanding current instabilities in conductive atomic force microscopy [Articolo su rivista]
Jiang, Lanlan; Weber, Jonas; Puglisi, Francesco Maria; Pavan, Paolo; Larcher, Luca; Frammelsberger, Werner; Benstetter, Guenther; Lanza, Mario
abstract

Conductive atomic force microscopy (CAFM) is one of the most powerful techniques in studying the electrical properties of various materials at the nanoscale. However, understanding current fluctuations within one study (due to degradation of the probe tips) and from one study to another (due to the use of probe tips with different characteristics), are still two major problems that may drive CAFM researchers to extract wrong conclusions. In this manuscript, these two issues are statistically analyzed by collecting experimental CAFM data and processing them using two different computational models. Our study indicates that: (i) before their complete degradation, CAFM tips show a stable state with degraded conductance, which is difficult to detect and it requires CAFM tip conductivity characterization before and after the CAFM experiments; and (ii) CAFM tips with low spring constants may unavoidably lead to the presence of a ~1.2 nm thick water film at the tip/sample junction, even if the maximum contact force allowed by the setup is applied. These two phenomena can easily drive CAFM users to overestimate the properties of the samples under test (e.g., oxide thickness). Our study can help researchers to better understand the current shifts that were observed during their CAFM experiments, as well as which probe tip to use and how it degrades. Ultimately, this work may contribute to enhancing the reliability of CAFM investigations.

2019 - Understanding the Impact of Annealing on Interface and Border Traps in the Cr/HfO2/Al2O3/MoS2 System [Articolo su rivista]
Zhao, Peng; Padovani, Andrea; Bolshakov, Pavel; Khosravi, Ava; Larcher, Luca; Hurley, Paul K.; Hinkle, Christopher L.; Wallace, Robert M.; Young, Chadwin D.
abstract

Top-gated, few-layer MoS2 transistors with HfO2 (6 nm)/Al2O3 (3 nm) gate dielectric stacks are fabricated and electrically characterized by capacitance-voltage (C-V) measurements to study electrically active traps (D-it) in the vicinity of the Al2O3/MoS2 interface. Devices with low D-it and high D-it are both observed in C-V characterization, and the impact of H-2/N-2 forming gas annealing at 300 and 400 degrees C on the D-it density and distribution is studied. A 300 degrees C anneal is able to reduce the D-it significantly, while the 400 degrees C anneal increases defects in the gate stack. Simulation with modeled defects suggests a sizable decrease in D-it, half the amount of positive fixed charge in the dielectric, and slightly increased unintentional doping in MoS2 after a 300 degrees C anneal. In the as-fabricated devices displaying high D-it levels, the energy distribution of the D-it located at the Al2O3/MoS2 interface is continuous from the conduction band edge of MoS2 down to 0.13-0.35 eV below the conduction band edge. A plausible D-it origin in our experiments could come from the unexpected oxygen atoms that fill the sulfur vacancies during the UV-O-3 functionalization treatment. The border trap concentration in Al2O3 is the same, both before and after the anneal, suggesting a different origin of the border traps, possibly due to the low-temperature atomic-layer-deposited process.

2018 - A 28 GHz Scalable Beamforming System for 5G Automotive Connectivity: An Integrated Patch Antenna and Power Amplifier Solution [Relazione in Atti di Convegno]
Aluigi, L.; Orecchini, G.; Larcher, L.
abstract

This paper presents a scalable 28 GHz beamforming system for 5G automotive connectivity, based on 2×2 TRX beamforming core chips and patch antenna arrays. In particular, the design of two key elements of the system is presented, namely the patch antenna and the power amplifier. The antenna has a gain of 7.5 dB. At 28 GHz the power amplifier delivers a maximum output power of 13.5 dBm, with an output referred P1dB of 13 dBm, a gain of 21.8 dB and peak PAE of 15.3%. Exploiting these results, the proposed system can deliver an EIRP of 34.5 dBm with a full 360-degree coverage of the azimuth angle.

2018 - AMPLIFICATORE PER IL PILOTAGGIO DI UN CARICO CAPACITIVO [Brevetto]
Migliazza, Giovanni; Lorenzani, Emilio; Larcher, Luca
abstract

La presente invenzione riguarda in generale il settore dell’elettronica. Più in particolare, la presente invenzione riguarda un amplificatore di tipo ibrido parallelo per il pilotaggio di un carico capacitivo, come ad esempio testine di stampa piezoelettriche.

2018 - Built-In Bias Generation in Anti-Ferroelectric Stacks: Methods and Device Applications [Articolo su rivista]
Pesic, M.; Li, T.; Di Lecce, V.; Hoffmann, M.; Materano, M.; Richter, C.; Max, B.; Slesazeck, S.; Schroeder, U.; Larcher, L.; Mikolajick, T.
abstract

The discovery of ferroelectric (FE) properties in binary oxides has enabled CMOS compatible and scalable FE memories. Recently, we reported a simple approach to introduce non-volatility into state-of-the-art dynamic random-access memory stacks that show anti-FE (AFE) behavior. By employing a pair of electrodes with different work functions, a built-in bias is generated. Consequently, this bias modulates the energy potential of the AFE and enables two stable non-volatile states. Using this approach, a significant endurance improvement compared to hafnia-based FE memories can be obtained. In this paper, we investigate the possibility to bypass the usage of asymmetric workfunction electrodes. Using the interface-engineering approach, based on fixed charge or dipole formation, we show two additional methods for built-in bias generation within AFE layer stacks. By characterizing the film properties and performance of AFE capacitors, we compare and investigate retention and endurance of both work function-difference-based and interface-based AFE non-volatile memory. Finally, for the first time we present the concept of a binary oxide-based AFE tunnel junction that leverages both an interface and work function engineered AFE stack.

2018 - Characterization and Modeling of Low-Cost Contact-Mode Triboelectric Devices for Energy Harvesting [Relazione in Atti di Convegno]
Bertacchini, A.; Lasagni, M.; Sereni, G.; Larcher, L.; Pavan, P.
abstract

In this paper we investigate the effect of the contact force in Contact-Mode TriboElectric Devices (CM-TED). In this kind of devices, the generated output voltage and the electrical energy harvested from mechanical impacts depend on the contact force. The number of impacts and the contact force influences also the surface charge density of the triboelectric layers of CM-TED. This is confirmed by the measurements carried out on the low-cost CM-TED prototypes we realized using commercial silicone as triboelectric material. The effect of the impact force has been included into a device model suitable for both dielectric-to-dielectric and contact-to-dielectric triboelectric devices. The model predicts the output voltage and power at given conditions and it can be used to design ultra-low cost triboelectric energy harvesters. The realized prototypes provide up to 5.5µW when subjected to repetitive impacts with a contact force of 65N.

2018 - Defect spectroscopy from electrical measurements: A simulation based technique [Relazione in Atti di Convegno]
Larcher, L.; Padovani, A.; Pramanik, D.; Kaczer, B.; Palumbo, F.
abstract

We present in this paper a novel defect spectroscopy technique for extracting defect and material properties of gate oxides and dielectrics used for memory devices (e.g. DRAM, RRAM). The method is based on the correlate simulation of electrical characteristics (IV, CV, GV, BTI), to allow the determination of the energy distribution and depth profile of atomic defects within the material bandgap. This novel defect spectroscopy technique is applied to MOSFET gate stacks with Si and InGaAs, and to DRAM capacitors.

2018 - Design and Simulation of Out-of-Plane Nanomaterial-Based Thermocouples [Relazione in Atti di Convegno]
Falco, A.; Lugli, P.; Loghin, F.; Rivadeneyra, A.; Larcher, L.; Bertacchini, A.
abstract

Our contribution indicates a novel solution for the development of the field of low-cost, printed thermocouples for energy harvesting and autonomous thermal sensors. Particularly, our work stems from the observation that the totality of printed thermocouples presented in literature relies on the conversion of a thermal gradient parallel to the plane of material deposition. This strategy is inherently inefficient and rarely applicable in real life scenarios. To overcome these issues, we introduced a novel concept of printed thermocouples, which use 3D-printing to define a vertical structure, upon which an out-of-plane thermal gradient can form and be harvested. Here, we perform thermoelectrical multi-physics simulations, employing parameters of nanomaterials extracted from literature, to show how this approach can lead to generate hundreds of microwatt in typical work conditions. Given their elevated thermopower, these structures could be employed both as autonomous sensors and energy harvesters in Internet of Things applications.

2018 - Extracting Atomic Defect Properties From Leakage Current Temperature Dependence [Articolo su rivista]
Larcher, Luca; Padovani, Andrea; Puglisi, Francesco Maria; Pavan, Paolo
abstract

In modern electronic devices, a variety of novel materials have been introduced such as transition metal oxides, chalcogenides, ferroelectric, and magnetic materials. The electrical response of such materials, used also as active layers, is strongly affected by atomic defects, which affect device performances, variability, and reliability. Extracting the defect properties (i.e., density, energy, and atomic nature) is, thus, crucial to both engineer the performances of electron devices and correctly project their scaling potential and reliability. In this paper, we propose a simple method to extract the atomic properties of defects from the thermal activation energy of the leakage current using a charge trapping relaxation model.

2018 - Guest editorial TDMR IIRW special section [Articolo su rivista]
Larcher, L.
abstract

2018 - High Efficiency Thermoacoustic Loudspeaker Made with a Silica Aerogel Substrate [Articolo su rivista]
La Torraca, P.; Bobinger, M.; Pavan, P.; Becherer, M.; Zhao, S.; Koebel, M.; Cattani, L.; Lugli, P.; Larcher, L.
abstract

The extremely low thermal effusivity of the silica aerogel is exploited to develop a high efficiency thermoacoustic (TA) loudspeaker with solid substrate. The deposition of the electrically conductive, low heat capacity active film on the silica aerogel surface is achieved with both the spray coating of silver nanowires and the sputter coating of gold films. The uniform spray coating of the hydrophobic silica aerogel is enabled by a low pressure plasma treatment, which however impairs its robustness. The spray-coated samples prove to be fragile when subjected to elevated temperatures and thus not suitable for TA applications. Sputter coating, not requiring any treatment of the aerogel surface, allows the fabrication of working TA loudspeaker samples with a 100 nm gold active film. The electroacoustic response of the gold-sputtered silica aerogel TA loudspeaker is characterized at different input power levels. The experimental results are compared with those present in literature, showing an improved efficiency with respect to the other TA loudspeakers with solid substrate.

2018 - Multiscale Modeling of Ferroelectric Memories: Insights into Performances and Reliability [Relazione in Atti di Convegno]
Pesiu, M.; Di Lecce, V.; Pramanik, D.; Larcher, L.
abstract

Despite large efforts in research of HfO 2 -based ferroelectric (FE) random access memories (FRAM), mechanisms underlying the device behavior of and its reliability (premature degradation) are poorly understood. To tackle this issue, we used a multiscale modeling framework that allows investigating the interplay between the FE switching, defects and polycrystalline nature of the HfO 2 material. This multiscale model allows connecting the electrical performances of FE devices (e.g. switching) to the atomic material properties, including defects and morphology (e.g. material phase). We used this simulation platform to both study wake-up process and the device-to-device variability in different memory architectures, i.e. capacitor-based FRAM and ferroelectric tunnel junction (FTJ) and the ferroelectric FET (FeFET) subjected to high field program/erase stress.

2018 - Multiscale modeling of neuromorphic computing: From materials to device operations [Relazione in Atti di Convegno]
Larcher, L.; Padovani, A.; Di Lecce, V.
abstract

In this paper, a multiscale modeling platform for neuromorphic computing devices connecting the atomic material properties to the electrical device performances is presented. The main ingredients of the modeling platform are discussed in view of the different technologies (e.g. RRAM, PCM, FTJ) proposed for 3D integrated neuromorphic computing.

2018 - On the frequency response of nanostructured thermoacoustic loudspeakers [Articolo su rivista]
La Torraca, P.; Bobinger, M.; Servadio, M.; Pavan, P.; Becherer, M.; Lugli, P.; Larcher, L.
abstract

In this work, we investigate the thermal and acoustic frequency responses of nanostructured thermoacoustic loudspeakers. An opposite frequency dependence of thermal and acoustic responses was found independently of the device substrate (Kapton and glass) and the nanometric active film (silver nanowires and nm-thick metal films). The experimental results are interpreted with the support of a comprehensive electro-thermo-acoustic model, allowing for the separation of the purely thermal effects from the proper thermoacoustic (TA) transduction. The thermal interactions causing the reported opposite trends are understood, providing useful insights for the further development of the TA loudspeaker technology.

2018 - Physical and circuit modeling of HfO2 based ferroelectric memories and devices [Relazione in Atti di Convegno]
Pesic, M.; Di Lecce, V.; Hoffmann, M.; Mulaosmanovic, H.; Max, B.; Schroeder, U.; Slesazeck, S.; Larcher, L.; Mikolajick, T.
abstract

The discovery of ferroelectric properties in polycrystalline HfO2 has revived the interest in ferroelectric (FE) memories, which shows scaling feasibility allowing targeting high-density storage applications. In order to provide engineering guidelines for FE memory devices it is crucial to establish a correlation between the electrical device performances and the underlying physical mechanisms. In this work, we will discuss physical and circuit modeling approaches for FE memories connecting the FE HfO2 materials properties to the electrical performances of memory cells, artificial synapse for neuromorphic and in memory computing applications.

2018 - Random Telegraph Noise in Resistive Random Access Memories: Compact Modeling and Advanced Circuit Design [Articolo su rivista]
Puglisi, Francesco Maria; Zagni, Nicolo; Larcher, Luca; Pavan, Paolo
abstract

In this paper, we report about the derivation of a physics-based compact model of random telegraph noise (RTN) in HfO2-based resistive random access memory (RRAM) devices. Starting from the physics of charge transport, which is different in the high resistive states and low resistive states, we explore the mechanisms responsible for RTN exploiting a hybrid approach, based on self-consistent physics simulations and geometrical simplifications. Then, we develop a simple yet effective physics-based compact model of RTN valid in both states, which can be steadily integrated in state-of-the-art RRAM compact models. The RTN compact model predictions are validated by comparison with both a large experimental data set obtained by measuring RRAM devices in different conditions, and data reported in the literature. In addition, we show how the model enables advanced circuit simulations by exploring three different circuits for memory, security, and logic applications.

2018 - Random telegraph noise in 2D hexagonal boron nitride dielectric films [Articolo su rivista]
Ranjan, A.; Puglisi, F. M.; Raghavan, N.; O'Shea, S. J.; Shubhakar, K.; Pavan, P.; Padovani, A.; Larcher, L.; Pey, K. L.
abstract

This study reports the observation of low frequency random telegraph noise (RTN) in a 2D layered hexagonal boron nitride dielectric film in the pre- and post-soft breakdown phases using conductive atomic force microscopy as a nanoscale spectroscopy tool. The RTN traces of the virgin and electrically stressed dielectric (after percolation breakdown) were compared, and the signal features were statistically analyzed using the Factorial Hidden Markov Model technique. We observe a combination of both two-level and multi-level RTN signals in h-BN, akin to the trends commonly observed for bulk oxides such as SiO2 and HfO2. Experimental evidence suggests frequent occurrence of unstable and anomalous RTN traces in 2D dielectrics which makes extraction of defect energetics challenging.

2018 - Role of electron and hole trapping in the degradation and breakdown of SiO2 and HfO2 films [Relazione in Atti di Convegno]
Gao, D. Z.; Strand, J.; El-Sayed, A. -M.; Shluger, A. L.; Padovani, A.; Larcher, L.
abstract

We investigated possible mechanisms for correlated defect production in amorphous (a) SiO2 and HfO2 films under applied stress bias using ab initio simulations. During bias application, electron injection into these films may lead to the localization of up to two electrons at intrinsic trapping sites which are present due to the natural structural disorder in amorphous structures. Trapping two electrons weakens Si-O and Hf-O bonds to such an extent that the thermally activated creation of Frenkel defects, O vacancies and O2- interstitial ions, becomes efficient even at room temperature. Bias application affects defect creation barriers and O2- interstitial diffusion. The density of trapping sites is different in a-SiO2 and a-HfO2. This leads to qualitatively different degradation kinetics, which results from different correlation in defect creation in the two materials. These effects affect TDDB statistics and its dependence on the film thickness.

2018 - Self-Heating Effect in Silicon-Germanium Heterostructure Bipolar Transistors in Stress and Operating Conditions [Relazione in Atti di Convegno]
Puglisi, Fm; Ghillini, M; Larcher, L; Pavan, P
abstract

In recent times many systems in a wide range of application fields (e.g., health, material science, security, and communications) exploit the mm-and sub-mm-wave spectrum, which dramatically sped up the growth of the BiCMOS technology integrating silicon germanium (SiGe) heterojunction bipolar transistors (HBTs) and passives. Today, the reliability of such devices is of primary concern, and particular attention is given to the device self-heating (SH), the importance of which is supposed to increase with the device scaling. In this work we develop a TCAD model for SiGe HBT devices that is used to investigate the SH effects in SiGe HBTs both in operating and stress conditions. We underline the different role played by impact ionization and carriers' and lattice heating on the device degradation. Results show the important role played by the back end-of-line (BEOL) and by the substrate thermal resistance in dissipating the heat generated by impact ionization and hot carriers. Simulations of the SH effects in stress conditions excluded annealing as the possible reason for the degradation dynamics reported in the literature, while simulations of stressed devices in measurement conditions revealed the presence of a hole hot spot that suggests a possible physical mechanism involved in the degradation slowdown at long stress times reported in the literature.

2018 - Solution-Processing of Copper Nanowires for Transparent Heaters and Thermo-Acoustic Loudspeakers [Articolo su rivista]
Bobinger, Marco; La Torraca, Paolo; Mock, Josef; Becherer, Markus; Cattani, Luca; Angeli, Diego; Larcher, Luca; Lugli, Paolo
abstract

In this study, we present a copper nanowires (CuNWs) based spray deposition process for the fabrication of transparent heaters and thermo-acoustic loudspeakers. We developed a scalable and solution-based synthesis process for CuNWs, which allows to fabricate spray deposited transparent electrodes that show performances comparable to indium tin oxide based TEs, at much lower material and deposition costs. Without any post-processing, the CuNWs films exhibit a sheet resistance as low as 12.6 Ω/□ at a high transparency of 77%. CuNW-based transparent heaters and thermo-acoustic loudspeakers are accurately characterized and modeled in both the thermal and the acoustic domain, showing performances aligned with the state-of-the art.

2018 - Time-dependent dielectric breakdown statistics in SiO2 and HfO2 dielectrics: Insights from a multi-scale modeling approach [Relazione in Atti di Convegno]
Padovani, A.; Larcher, L.
abstract

We use a multi-scale modeling framework to investigate time dependent dielectric breakdown (TDDB) distributions in SiO2-and HfO2-based stacks. We show that the low and thickness independent Weibull slope (β) observed in HfO2 is due to the high intrinsic defect density and to the spatial correlation of the defect generation process. We investigate the origin of the double slope observed on TDDB distributions in IL-HfO2 stacks: we have found that it is related to the stochastic nature of the bond-breakage process. This is important for a correct evaluation of the lifetime of logic devices.

2018 - Understanding and Optimization of Pulsed SET Operation in HfOx-Based RRAM Devices for Neuromorphic Computing Applications [Articolo su rivista]
Padovani, A.; Woo, J.; Hwang, H.; Larcher, L.
abstract

We use experiments and device simulations to investigate pulsed SET operation of HfO2-based RRAM devices for their possible use as electronic synapses. The application of a train of identical pulses only allows for an abrupt change of the device current, which is not suitable for synaptic devices. By using simulations, we link the microscopic properties and changes of the conductive filament during the pulsed operation to the measured conductance and its dependence on pulse voltage, width, and number. The results allow us to derive guidelines that we use to design optimized SET pulses (or pulse trains) allowing extending the conventional binary operation of HfO2-based RRAMs to the multi-level cell operation required by electronic synapses.

2018 - "Velcro-type" crackles predict specific radiologic features of fibrotic interstitial lung disease [Articolo su rivista]
Sgalla, Giacomo; Walsh, Simon L. F.; Sverzellati, Nicola; Fletcher, Sophie; Cerri, Stefania; Dimitrov, Borislav; Nikolic, Dragana; Barney, Anna; Pancaldi, Fabrizio; Larcher, Luca; Luppi, Fabrizio; Jones, Mark G.; Davies, Donna; Richeldi, Luca
abstract

Background: "Velcro-type" crackles on chest auscultation are considered a typical acoustic finding of Fibrotic Interstitial Lung Disease (FILD), however whether they may have a role in the early detection of these disorders has been unknown. This study investigated how "Velcro-type" crackles correlate with the presence of distinct patterns of FILD and individual radiologic features of pulmonary fibrosis on High Resolution Computed Tomography (HRCT). Methods: Lung sounds were digitally recorded from subjects immediately prior to undergoing clinically indicated chest HRCT. Audio files were independently assessed by two chest physicians and both full volume and single HRCT sections corresponding to the recording sites were extracted. The relationships between audible "Velcro-type" crackles and radiologic HRCT patterns and individual features of pulmonary fibrosis were investigated using multivariate regression models. Results: 148 subjects were enrolled: bilateral "Velcro-type" crackles predicted the presence of FILD at HRCT (OR 13.46, 95% CI 5.85-30.96, p < 0.001) and most strongly the Usual Interstitial Pneumonia (UIP) pattern (OR 19.8, 95% CI 5.28-74.25, p < 0.001). Extent of isolated reticulation (OR 2.04, 95% CI 1.62-2.57, p < 0.001), honeycombing (OR 1.88, 95% CI 1.24-2.83, < 0.01), ground glass opacities (OR 1.74, 95% CI 1.29-2.32, p < 0.001) and traction bronchiectasis (OR 1.55, 95% CI 1.03-2.32, p < 0.05) were all independently associated with the presence of "Velcro-type" crackles. Conclusions: "Velcro-type" crackles predict the presence of FILD and directly correlate with the extent of distinct radiologic features of pulmonary fibrosis. Such evidence provides grounds for further investigation of lung sounds as an early identification tool in FILD.

2017 - A microscopic mechanism of dielectric breakdown in SiO2films: An insight from multi-scale modeling [Articolo su rivista]
Padovani, A.; Gao, D. Z.; Shluger, A. L.; Larcher, L.
abstract

Despite extensive experimental and theoretical studies, the atomistic mechanisms responsible for dielectric breakdown (BD) in amorphous (a)-SiO2are still poorly understood. A number of qualitative physical models and mathematical formulations have been proposed over the years to explain experimentally observable statistical trends. However, these models do not provide clear insight into the physical origins of the BD process. Here, we investigate the physical mechanisms responsible for dielectric breakdown in a-SiO2using a multi-scale approach where the energetic parameters derived from a microscopic mechanism are used to predict the macroscopic degradation parameters of BD, i.e., time-dependent dielectric breakdown (TDDB) statistics, and its voltage dependence. Using this modeling framework, we demonstrate that trapping of two electrons at intrinsic structural precursors in a-SiO2is responsible for a significant reduction of the activation energy for Si-O bond breaking. This results in a lower barrier for the formation of O vacancies and allows us to explain quantitatively the TDDB data reported in the literature for relatively thin (3-9 nm) a-SiO2oxide films.

2017 - A multiscale modeling approach for the simulation of OxRRAM devices [Relazione in Atti di Convegno]
Padovani, A.; Larcher, L.; Woo, J.; Hwang, H.
abstract

We present a multiscale modeling platform that exploits ab-initio calculation results and a material-related description of the most relevant defect-related phenomena in dieledtrics (charge trapping and transport, degradation and atomic species motion) to interpret and understand the electrical characteristics of OxRAM memory devices for non-volatile memories and neuromorphic applications. Simulation results provide a deep and quantitative understanding of the factors controlling device operation. The proposed multiscale modeling platform represents a powerful tool for investigating material properties and optimizing device performances and reliability.

2017 - A new verilog-A compact model of random telegraph noise in oxide-based RRAM for advanced circuit design [Relazione in Atti di Convegno]
Puglisi, Francesco Maria; Zagni, Nicolo'; Larcher, Luca; Pavan, Paolo
abstract

In this work, we propose for the first time a Verilog-A physics-based compact model of Random Telegraph Noise (RTN) in Resistive Random Access Memory (RRAM) devices. Starting from the physics of the RTN mechanism in both high (HRS) and low (LRS) resistive states, and combining experimental data with physics-based simulations, we develop and validate a complete compact model of RTN in RRAM devices. The model accounts for the intrinsic randomness in the number of defects contributing to the RTN and their properties. Moreover, it can be readily integrated in existing RRAM device compact models, extending their capabilities. The model is implemented in Verilog-A, and its effectiveness is demonstrated by using it to design the building block of a Truly-Random Number Generator circuit exploiting the RTN randomness as an entropy source.

2017 - Ab initio modelling of oxygen vacancy arrangement in highly defective HfO2 resistive layers [Articolo su rivista]
Sementa, Luca; Larcher, Luca; Barcaro, Giovanni; Montorsi, Monia
abstract

We report ab initio results for sub-stoichiometric HfOx with different oxygen vacancy densities, useful in exploring microscopic mechanisms that govern the operation of RRAM devices. We demonstrate that oxygen vacancy filaments are energetically more stable than randomly distributed defects. Furthermore, the stability of the filaments increases with the number of confined oxygen vacancies. Energetic and structural analyses show that bonds between neighboring coordinative unsaturated Hf atoms promote filament stability, and electron trapping, due to electron injection, increases the cohesive energy until the injection is moderate. The highly oxygen deficient configuration of the filaments leads to a substantial lowering of the HfOx band gap, which locally increases the conductivity of the system. Charge injection and electric fields modify the mobility of oxygen ions in the proximity of the filament. The simulations suggest that oxygen ion diffusion can lead to an asymmetric reduction of filament thickness and thus to its progressive disruption where the vacancy cohesion energy is lower.

2017 - Ab-initio Modelling of the Arrangement of Oxygen Vacancies in high-defective HfO2 resistive layers [Articolo su rivista]
Sementa, Luca; Larcher, Luca; Barcaro, Giovanni; Montorsi, Monia
abstract

2017 - Characterization and modelling of transparent heaters based on solution-processed copper nanowires [Relazione in Atti di Convegno]
Bobinger, Marco; Mock, Josef; Becherer, Markus; Torraca, Paolo La; Angeli, Diego; Larcher, Luca; Lugli, Paolo
abstract

In this study, we present an environmentally friendly and solution-based synthesis for copper nanowires (CuNWs) at a moderate process temperature. Transparent electrodes (TEs) are fabricated by spray-deposition and evaluated in terms of their electro-optical performance. Using ImageJ, the CuNW diameters are determined in an automated and reproducible way. Without any post-processing, the films show a sheet resistance as low as 12.6 Ohm/sq at a high transparency of 77 %. Further, CuNW-based transparent heaters are characterized and accurately modelled using the Crank-Nicolson finite method that accounts for the heat losses and the resistance-temperature dependence of the films.

2017 - Coexistence of Grain‐Boundaries‐Assisted Bipolar and Threshold Resistive Switching in Multilayer Hexagonal Boron Nitride [Articolo su rivista]
Pan, Chengbin; Ji, Yanfeng; Xiao, Na; Hui, Fei; Tang, Kechao; Guo, Yuzheng; Xie, Xiaoming; Puglisi, Francesco Maria; Larcher, Luca; Miranda, Enrique; Jiang, Lanlan; Shi, Yuanyuan; Valov, Ilia; Mcintyre, Paul; Waser, Rainer; Lanza, Mario
abstract

The use of 2D materials to improve the capabilities of electronic devices is a promising strategy that has recently gained much interest in both academia and industry. However, while the research in 2D metallic and semiconducting materials is well established, detailed knowledge and applications of 2D insulators are still scarce. In this paper, the presence of resistive switching (RS) in multilayer hexagonal boron nitride (h-BN) is studied using different electrode materials, and a family of h-BN-based resistive random access memories with tunable capabilities is engineered. The devices show the coexistence of forming free bipolar and threshold-type RS with low operation voltages down to 0.4 V, high current on/off ratio up to 106, and long retention times above 10 h, as well as low variability. The RS is driven by the grain boundaries (GBs) in the polycrystalline h-BN stack, which allow the penetration of metallic ions from adjacent electrodes. This reaction can be boosted by the generation of B vacancies, which are more abundant at the GBs. To the best of our knowledge, h-BN is the first 2D material showing the coexistence of bipolar and threshold RS, which may open the door to additional functionalities and applications.

2017 - Correlated Effects on Forming and Retention of Al Doping in HfO2-Based RRAM [Articolo su rivista]
Alayan, Mouhamad; Vianello, Elisa; De Salvo, Barbara; Perniola, Luca; Padovani, Andrea; Larcher, Luca
abstract

Editor's note: Retention time is one of the key parameters of emerging memories, which define the time duration the data can be retained when the power supply is removed. In this work, the authors investigate the forming voltage and the data retention of aluminum (Al)-doped HfO2-based RRAM devices and suggest a way to improve the device's data retention time. - - Yiran Chen, Duke University

2017 - Experimental and Simulation Studies of the Effects of Heavy-Ion Irradiation on HfO2-Based RRAM Cells [Articolo su rivista]
Alayan, M.; Bagatin, M.; Gerardin, S.; Paccagnella, A.; Larcher, L.; Vianello, E.; Nowak, E.; De Salvo, B.; Perniola, L.
abstract

HfO2-based resistive RAMs have been irradiated with high-linear energy transfer heavy ions and subjected to an extensive characterization, showing that the cells are immune from upsets. No relevant changes were observed in the irradiated cells on resistance distribution and programming voltages. The irradiation experiment has been performed without any applied bias (retention mode). Reasons for the observed hardness are discussed using physics-based simulations. Moreover, simulations put in evidence that the cell might be sensitive if it is struck during a read operation, since the applied read voltage prevents the instantaneous recombination of the generated defects due to the Coulomb interaction between oxygen ions and vacancies.

2017 - Heavy-ion upset immunity of RRAM cells based on thin HfO2 layers [Relazione in Atti di Convegno]
Alayan, M.; Bagatin, M.; Gerardin, S.; Paccagnella, A.; Vianello, E.; Nowak, E.; De Salvo, B.; Larcher, L.; Perniola, L.
abstract

HfO2-based resistive RAMs have been irradiated with high-LET heavy ions and subjected to an extensive characterization, showing that the cells are immune from upsets. Reasons for the observed hardness are discussed.

2017 - Infrared, transient thermal, and electrical properties of silver nanowire thin films for transparent heaters and energy-efficient coatings [Articolo su rivista]
Bobinger, Marco; Angeli, Diego; Colasanti, Simone; La Torraca, Paolo; Larcher, Luca; Lugli, Paolo
abstract

n this study, we investigate the infrared and electrical propertiesas well as the thermal response of transparent silvernanowire (AgNW) based thin-ﬁlm heaters, when subjected toJoule heating. Controlling the number of layers and hence thedeposition time, our spray-coating technique allows to modulatethe thermal and electrical properties of the thin ﬁlms in a precisemanner. In addition, this technique enables the fabrication ofhomogeneous and large-area heaters, which, in terms of theirelectro-optical properties, nicely compare to the performances ofstate-of-the-art AgNW transparent electrodes. The thermal response and the electrical properties are accurately reproducedby a purposely developed physical model, which shows that thetemperature dependence of the AgNW ﬁlm resistance is loweredby a factor of 2 compared to bulk silver, independently of thenumber of deposited layers. Compared to uncoated glass,the emissivity decreases by 58% at a coverage rate of 58%. At thesame time, the AgNW ﬁlm can sustain a transparency as high as81.3%. Therefore, AgNW-based thin ﬁlms can be used as a low-emissivity coating, for e.g., energy-efﬁcient window glazingapplications. Finally, we accurately determine the fragmentationtemperature of AgNWs, which sets the ultimate limitation of usefor heating applications.

2017 - Linking Conductive Filament Properties and Evolution to Synaptic Behavior of RRAM Devices for Neuromorphic Applications [Articolo su rivista]
Woo, Jiyong; Padovani, Andrea; Moon, Kibong; Kwak, Myounghun; Larcher, Luca; Hwang, Hyunsang
abstract

We perform a comparative study of HfO2 and Ta2O5 resistive switching memory (RRAM) devices for their possible application as electronic synapses. By means of electrical characterization and simulations, we link their electrical behavior (digital or analog switching) to the properties and evolution of the conductive filament (CF). More specifically, we identify that bias-polarity-dependent digital switching in HfO2 RRAM is primarily related to the creation and rupture of an oxide barrier. Conversely, the modulation of the CF size in Ta2O5 RRAM allows bias-polarity-independent analog switching with multiple states. Therefore, when the Ta2O5 RRAM is used to implement a synapse in multilayer perceptron neural networks operated by back-propagation algorithms, patterns in handwritten digits can be recognized with high accuracy.

2017 - Localized characterization of charge transport and random telegraph noise at the nanoscale in HfO2 films combining scanning tunneling microscopy and multi-scale simulations [Articolo su rivista]
Thamankar, R.; Puglisi, Francesco Maria; Ranjan, A.; Raghavan, N.; Shubhakar, K.; Molina, J.; Larcher, Luca; Padovani, Andrea; Pavan, Paolo; O'Shea, S. J.; Pey, K. L.
abstract

Charge transport and Random Telegraph Noise (RTN) are measured successfully at the nanoscale on a thin polycrystalline HfO2 film using room temperature Scanning Tunneling Microscopy (STM). STM is used to scan the surface of the sample with the aim of identifying grains and grain boundaries, which show different charge transport characteristics. The defects responsible for charge transport in grains and grain boundaries are identified as positively charged oxygen vacancies by matching the localized I-V curves measured at the nanoscale with the predictions of physics-based multi-scale simulations. The estimated defect densities at grains and grain boundaries agree with earlier reports in the literature. Furthermore, the current-time traces acquired by STM at fixed bias voltages on grains show characteristic RTN fluctuations. The high spatial resolution of the STM-based RTN measurement allows us to detect fluctuations related to individual defects that typically cannot be resolved by the conventional device-level probe station measurement. The same physical framework employed to reproduce the I-V conduction characteristics at the grains also successfully simulates the RTN detected at the nanoscale. We confirm that charge trapping at defects not directly involved in charge transport can induce significant current fluctuations through Coulombic interactions with other defects in the proximity that support charge transport.

2017 - Multiscale modeling of defect-related phenomena in high-k based logic and memory devices [Relazione in Atti di Convegno]
Padovani, Andrea; Larcher, Luca; Puglisi, Francesco Maria; Pavan, Paolo
abstract

We present a multiscale modeling platform that exploits ab-initio calculation results and a material-related description of the most relevant defect-related phenomena in dielectrics (charge trapping and transport, degradation and atomic species motion) to interpret the reliability and electrical characteristics of logic and memory devices. The model is used to identify and characterize the dielectric defects responsible for the charge transport and degradation in SiOx/high-k (HK) bi-layer logic devices and to investigate the kinetics of forming and switching operations of Hf-based RRAM memories. Simulation results provide a deep and quantitative understanding of the factors controlling device operation and reliability. The proposed multiscale modeling platform represents a powerful tool for investigating material properties and optimizing device performances and reliability.

2017 - Multiscale modeling of oxide RRAM devices for memory applications: from material properties to device performance [Articolo su rivista]
Larcher, Luca; Padovani, Andrea
abstract

RRAM devices have been subjected to intense research efforts and are proposed for nonvolatile memory and neuromorphic applications. In this paper we describe a multiscale modeling platform connecting the microscopic properties of the resistive switching material to the electrical characteristics and operation of RRAM devices. The platform allows self-consistently modeling the charge and ion transport and the material structural modifications occurring during RRAM operations and reliability, i.e., conductive filament creation and partial disruption. It allows describing the electrical behavior (current, forming, switching, cycling, reliability tests) of RRAM devices in static and transient conditions and their dependence on external conditions (e.g., temperature). Thanks to the kinetic Monte Carlo approach, the inherent variability of physical processes is properly accounted for. Simulation results can be used both to investigate material properties (including atomic defect distributions) and to optimize stack and bias pulses for optimum device performances and reliability.

2017 - Physical modeling and characterization of thermo-acoustic loudspeakers made of silver nano-wire films [Articolo su rivista]
La Torraca, P.; Bobinger, M.; Pavan, Paolo; Seeber, B.; Lugli, P.; Larcher, L.
abstract

Recent developments of ultra-low heat capacity nanostructured materials revived the interest in the thermo-acoustic (TA) loudspeaker technology, which shows important advantages compared to the classical dynamic loudspeakers as they feature a lower cost and weight, flexibility, conformability to the surface of various shapes, and transparency. The development of the TA loudspeaker technology requires accurate physical models connecting the material properties to the thermal and acoustic speaker's performance. We present here a combined theoretical and experimental analysis of TA loudspeakers, where the electro-thermal and the thermo-acoustic transductions are handled separately, thus allowing an in-depth description of both the pressure and temperature dynamics. The electro-thermal transduction is analyzed by accounting for all the heat flow processes taking place between the TA loudspeaker and the surrounding environment, with focus on their frequency dependence. The thermo-acoustic conversion is studied by solving the coupled thermo-acoustic equations, derived from the Navier-Stokes equations, and by exploiting the Huygens-Fresnel principle to decompose the TA loudspeaker surface into a dense set of TA point sources. A general formulation of the 3D pressure field is derived summing up the TA point source contributions via a Rayleigh integral. The model is validated against temperature and sound pressure level measured on the TA loudspeaker sample made of a Silver Nanowire random network deposited on a polyimide substrate. A good agreement is found between measurements and simulations, demonstrating that the model is capable of connecting material properties to the thermo-acoustic performance of the device, thus providing a valuable tool for the design and optimization of TA loudspeakers.

2017 - Random telegraph noise: Measurement, data analysis, and interpretation [Relazione in Atti di Convegno]
Puglisi, Francesco Maria; Padovani, Andrea; Larcher, Luca; Pavan, Paolo
abstract

Abstract: In this paper, we delve into one of the most relevant defects-related phenomena causing failures in the operation of modern nanoscale electron devices, namely Random Telegraph Noise (RTN). Due to its detrimental impact on devices and circuits performances, RTN mechanism must be thoroughly understood, which requires establishing a self-consistent framework encompassing automated measurement techniques, data analysis algorithms, and physics-based modeling. This platform is not only required to understand the physics of RTN-related failures, but also to enable RTN analysis as a tool to investigate device reliability. Starting from the analysis of RTN signal statistical properties, we propose a set of guidelines to perform correct RTN measurements and data analysis, in order to get reliable results that are needed for an unbiased physical interpretation. This is achieved by combining automated experiments with sophisticated data analysis, consistency check, and comprehensive physics-based simulations. RTN analysis is then applied to two different devices for logic and memory applications, respectively: FinFETs and RRAMs. Particularly, the analysis of the statistical properties of RTN simultaneously measured on the drain and on the gate current of FinFETs allows understanding the details of the defects generation during stress. The analysis of RTN measured during the read operation in RRAM devices allows understanding the physical origin of RTN in these devices and identifying the defects species involved in this phenomenon.

2017 - Scaling perspective and reliability of conductive filament formation in ultra-scaled HfO2 Resistive Random Access Memory [Relazione in Atti di Convegno]
Puglisi, Francesco Maria; Celano, Umberto; Padovani, Andrea; Vandervorst, Wilfried; Larcher, Luca; Pavan, Paolo
abstract

In this paper we report about the scaling perspective of ultra-scaled HfO2 Resistive Random Access Memory devices. Due to filamentary conduction, the scalability of these devices is considered to be ultimately limited by the size of the conductive filament. However, even though the precise size and shape of the filament is not fully elucidated, it is widely accepted that its size is mainly controlled by the current compliance. In turn, the latter sets the operating current level of the cell. The reduction of the current level is nevertheless accompanied by performance instabilities, which are the main reliability threat for low-current operations. The resulting tradeoff raises concerns about the scalability potential of RRAM devices. In this work, we combine device-level measurements, Conductive Atomic-Force Microscopy (C-AFM), and physics-based simulations of HfO2 RRAM devices to elucidate the reason for these instabilities. Results clarify the scaling perspectives of ultra-low cell size (< 10Ã10 nm2) RRAMs and their reliability.

2017 - Self-rectifying behavior and analog switching under identical pulses using Tri-layer RRAM crossbar array for neuromorphic systems [Relazione in Atti di Convegno]
Alayan, M.; Vianello, E.; Larcher, L.; Padovani, A.; Levisse, A.; Castellani, N.; Charpin, C.; Bernasconi, S.; Molas, G.; Portal, J. M.; De Salvo, B.; Perniola, L.
abstract

This work analyzes the self-rectifying behavior and the response under identical pulses of tri-layer RRAMs in crossbar arrays to implement the synapse function. Our finding shows that tri-layer RRAMs allow to achieve a stable Low Resistance State (LRS) without complete oxide breakdown. The first RRAM layer works as tunneling barrier allowing to achieve on-state half-bias nonlinearity. Thanks to LRS nonlinearity the power consumption during synaptic programming is reduces of one order of magnitude. Analog switching under identical pulses allows to emulate synaptic plasticity. The multilevel states of conductance have been explained by the enlargement of the conductive filament (CF) in the broken oxide by means of physical based simulations.

2017 - Tailoring the Aqueous Synthesis and Deposition of Copper Nanowires for Transparent Electrodes and Heaters [Articolo su rivista]
Bobinger, Marco; Mock, Josef; LA TORRACA, Paolo; Becherer, Markus; Lugli, Paolo; Larcher, Luca
abstract

Due to the high abundance of copper on the earth and its high intrinsic electrical conductivity, copper nanowires (CuNWs) represent a promising material for transparent electrodes. In this work, an environmentally friendly and scalable synthesis that requires a low process temperature is studied. The optimum temperature is found at 79 Â°C, which results in nanowires with the lowest diameters. The as-synthesized solution is sprayed to transparent conducting films, which are in turn subjected to various post-treatments such as thermal sintering or washing with propionic acid to enhance their electro-optical performance. Following both the optimum protocol for the synthesis and post-treatment, a sheet resistance of 10.3 Î© â »â 1at a transparency of 83.4% is achieved. Moreover, the CuNW-films are tested as transparent heaters and show a homogeneous heat distribution. For the electrical properties of the films, a temperature dependence of resistance that is lowered around 28% compared to the one for bulk copper is found.

2017 - 1.29-W/mm² 23-dBm 66-GHz Power Amplifier in 55-nm SiGe BiCMOS With In-Line Coplanar Transformer Power Splitters and Combiner [Articolo su rivista]
Pepe, Domenico; Zito, Domenico; Pallotta, Andrea; Larcher, Luca
abstract

This letter presents a four-way parallel-series power amplifier (PA) in 55-nm SiGe BiCMOS with in-line coplanar transformers for output power combining, and input/interstage power splitting. The in-line geometry allows an area efficient impedance matching design and an effective input signals routing to the individual PA stages without phase mismatch. The measurements show that the PA delivers a maximum output power of 23.4 dBm, an output-referred P1dB of 20 dBm, a gain of 23.8 dB, and a maximum power added efficiency of 12.5%, at 66 GHz, with a record power density (output power/active area) of 1.29 W/mm² among PAs on silicon technologies operating beyond 40 GHz.

2016 - A multi-scale methodology connecting device physics to compact models and circuit applications for OxRAM technology [Articolo su rivista]
Puglisi, Francesco Maria; Deleruyelle, Damien; Portal, Jean Michel; Pavan, Paolo; Larcher, Luca
abstract

RRAM technology relying on transitional metal oxides (namely OxRAM) is about to reach the industrial stage. Nevertheless the physical-based understanding of the material and process implications at device and circuit levels is still not completely clear, hindering the full industrial exploitation of the OxRAM technology. In this context, this article presents a multi-scale methodology that connects the microscopic material properties to the electrical behavior of OxRAM devices at the circuit level. Microscopic models describing OxRAM operation (i.e., forming, resistive switching) and variability (e.g., cycle-to-cycle, RTN) will be reviewed and used for the development of compact models that will allow investigating the potential of this technology at the circuit level. An overview of some innovative applications involving OxRAM will be finally presented.

2016 - Anomalous random telegraph noise and temporary phenomena in resistive random access memory [Articolo su rivista]
Puglisi, Francesco Maria; Larcher, Luca; Padovani, Andrea; Pavan, Paolo
abstract

In this paper we present a comprehensive examination of the characteristics of complex Random Telegraph Noise (RTN) signals in Resistive Random Access Memory (RRAM) devices with TiN/Ti/HfO2/TiN structure. Initially, the anomalous RTN (aRTN) is investigated through careful systematic experiment, dedicated characterization procedures, and physics-based simulations to gain insights into the physics of this phenomenon. The experimentally observed RTN parameters (amplitude of the current fluctuations, capture and emission times) are analyzed in different operating conditions. Anomalous behaviors are characterized and their statistical characteristics are evaluated. Physics-based simulations considering both the Coulomb interactions among different defects in the device and the possible existence of defects with metastable states are exploited to suggest a possible physical origin of aRTN. The same simulation framework is also shown to be able to predict other temporary phenomena related to RTN, such as the temporary change in RTN stochastic properties or the sudden and iterative random appearing and vanishing of RTN fluctuations always exhibiting the same statistical characteristics. Results highlight the central role of the electrostatic interactions among individual defects and the trapped charge in describing RTN and related phenomena.

2016 - Bipolar Resistive RAM Based on HfO2: Physics, Compact Modeling, and Variability Control [Articolo su rivista]
Puglisi, Francesco Maria; Larcher, Luca; Padovani, Andrea; Pavan, Paolo
abstract

In this paper, we thoroughly investigate the characteristics of the TiN/Ti/HfO/TiN resistive random access memory (RRAM) device. The physical mechanisms involved in the device operations are comprehensively explored from the atomistic standpoint. Self-consistent physics simulations based on a multi-scale approach are employed to achieve a complete understanding of the device physics. The latter includes different charge and ion transport phenomena, as well as structural modifications occurring during the device operations. The main sources of variability are also included by connecting the electrical response of the device to the atomistic material properties. The detailed understanding of the device physics allows developing a physics-based compact model describing the device switching in different operating conditions, including also the effects of cycling variability. Random telegraph noise (RTN), which constitutes an additional variability source, and its relations with cycling variability are analyzed. A statistical link between the programmed resistance and the worst-case RTN effect is found and exploited to include RTN effects in the compact model. Finally, we show how implementing an advanced programming scheme tailored on the device physics allows optimal control over variability and RTN, eventually achieving reliable and RTN-resilient two-bits/cell operations.

2016 - Boron nitride as two dimensional dielectric: Reliability and dielectric breakdown [Articolo su rivista]
Ji, Yanfeng; Pan, Chengbin; Zhang, Meiyun; Long, Shibing; Lian, Xiaojuan; Miao, Feng; Hui, Fei; Shi, Yuanyuan; Larcher, Luca; Wu, Ernest; Lanza, Mario
abstract

Boron Nitride (BN) is a two dimensional insulator with excellent chemical, thermal, mechanical, and optical properties, which make it especially attractive for logic device applications. Nevertheless, its insulating properties and reliability as a dielectric material have never been analyzed in-depth. Here, we present the first thorough characterization of BN as dielectric film using nanoscale and device level experiments complementing with theoretical study. Our results reveal that BN is extremely stable against voltage stress, and it does not show the reliability problems related to conventional dielectrics like HfO2, such as charge trapping and detrapping, stress induced leakage current, and untimely dielectric breakdown. Moreover, we observe a unique layer-by-layer dielectric breakdown, both at the nanoscale and device level. These findings may be of interest for many materials scientists and could open a new pathway towards two dimensional logic device applications.

2016 - Characterization and modeling of the thermal and electrical properties of transparent silver nanowire thin-film heaters [Relazione in Atti di Convegno]
Bobinger, Marco; Colasanti, Simone; Lugli, Paolo; Angeli, Diego; La Torraca, Paolo; Larcher, Luca
abstract

In this study, we present the characterization and the modeling of transparent silver nanowire thin-film heaters in terms of their transient thermal response when subjected to Joule heating and their electrical properties. The electrical properties, which showed a conductance-temperature dependence that is reduced down to a factor of 2 compared to the value for bulk silver, could be modeled accurately by simulation results. In addition, our transparent electrode deposition technique, i.e. spray-coating, allowed for an excellent reproducibility and provided homogeneous and large films that compare to state-of-the-art silver nanowire transparent electrode performance.

2016 - Defect spectroscopy and engineering for nanoscale electron device applications: A novel simulation-based methodology [Relazione in Atti di Convegno]
Larcher, Luca; Sereni, Gabriele; Vandelli, Luca
abstract

In this work we present a novel simulation-based methodology for the defect spectroscopy in dielectric materials. The cross-correlated simulation of electrical characteristics (IV, CV, GV, BTI, charge pumping and noise) is exploited to profile the properties and energy-space distribution of the defects within the oxide bandgap. This novel defect spectroscopy technique will be applied to three case studies, i.e. Si- MOSFET gate stack optimization with either Si and beyond Si channel (InGaAs), and STO MIM DRAM capacitor scaling. The integration of these methods into the process optimization will lead to a strong reduction of the time/cost required for the development of novel device architectures.

2016 - Electrical defect spectroscopy and reliability prediction through a novel simulation-based methodology [Relazione in Atti di Convegno]
Larcher, Luca; Sereni, Gabriele; Padovani, Andrea; Vandelli, Luca
abstract

The semiconductor technology development requires a full understanding of material implications at the device level. This requires connecting the microscopic/atomic properties of the material (e.g. defect) to the macroscopic electrical characteristics of the device. In this scenario, we developed a new methodology, supported by a multi-scale modeling and simulation (MS) software [1], [2], which allows extracting from the simulations of the electrical characterization measurements (I-V, C-V, G-V, BTI, Charge-Pumping, noise, stress) the material and device properties that can be used for the technology development, the design of novel devices and the analysis of the device reliability also at statistical level (TDDB, leakage currents), Fig. 1.

2016 - Extraction of the Defect Distributions in DRAM Capacitor Using I-V and C-V Sensitivity Maps [Articolo su rivista]
Sereni, Gabriele; Larcher, Luca; Kaczer, Ben; Popovici, Mihaela Ioana
abstract

We propose a novel spectroscopic technique that allows determining the defect density distributions within metal-electrode-sandwiched dielectric based on J-V and C-V characteristics. The technique relies on determining "sensitivity regions" corresponding to the energy-spatial coordinates of defects affecting J-V and C-V curves. This technique is then demonstrated on RuOx/SrTiOx/RuOx metal-insulator-metal structures for DRAM capacitor applications.

2016 - Force Impact Effect in Contact-Mode Triboelectric Energy Harvesters: Characterization and Modeling [Relazione in Atti di Convegno]
Lasagni, Marco; Pavan, Paolo; Bertacchini, Alessandro; Larcher, Luca
abstract

In this paper we investigate the effect of the contact force on the voltage generated by Contact-Mode Triboelectric Energy Harvesting Devices (CM-TEHD). The electrical energy harvested from mechanical shocks increases with the contact force. In order to investigate the role of the contact force in the triboelectric energy generation, we developed a physical model, which allows understanding the physical mechanisms of this process, while predicting the output voltage and power at given conditions. Prototypes of the CM-TEHD made of low-cost commercial silicone were fabricated using a very low cost process. The prototypes provide up to 5.5µW when subjected to repetitive impacts with a contact force of 65N.

2016 - Low leakage stoichiometric SrTiO3 dielectric for advanced metal-insulator-metal capacitors [Articolo su rivista]
Popovici, Mihaela; Kaczer, Ben; Afanas'Ev, Valeri V.; Sereni, Gabriele; Larcher, Luca; Redolfi, Augusto; Van Elshocht, Sven; Jurczak, Malgorzata
abstract

Metal-insulator-metal capacitors (MIMCAP) with stoichiometric SrTiO3 dielectric were deposited stacking two strontium titanate (STO) layers, followed by intermixing the grain determining Sr-rich STO seed layer, with the Ti-rich STO top layer. The resulted stoichiometric SrTiO3 would have a structure with less defects as demonstrated by internal photoemission experiments. Consequently, the leakage current density is lower compared to Sr-rich STO which allow further equivalent oxide thickness downscaling. Schematic of MIMCAP with stoichiometric STO dielectric formed from bottom Sr-rich STO and top Ti-rich STO after intermixing during crystallization anneal.

2016 - Monitoring Stress-Induced Defects in HK/MG FinFETs Using Random Telegraph Noise [Articolo su rivista]
Puglisi, Francesco Maria; Costantini, Felipe; Kaczer, Ben; Larcher, Luca; Pavan, Paolo
abstract

In this letter, we report on nFinFETs degradation during stress exploiting ID and IG noise analysis. We employed a stress/measure approach to monitor device characteristics at different levels of cumulative stress. IG-VG and ID-VG indicators suggest defects generation to occur away from the channel. This is confirmed by the quantitative analysis of ID and IG stationary RTN signals at operating conditions, which show no correlation as opposite to what reported for planar FETs. Moreover, we analyze for the first time the ID-t and IG-t non-stationary instabilities during stress. The results confirm that the generation of defects responsible for SILC occurs away from the channel. Only in highly stressed devices, ID-t and IG-t curves observed during stress exhibit anti-correlation, due to comparable values of the gate and drain current levels originated by the high defect density. Hence, in nFinFETs, ID and IG RTN/instabilities might originate from mechanisms involving different entities.

2016 - Moving graphene devices from lab to market: Advanced graphene-coated nanoprobes [Articolo su rivista]
Hui, F.; Vajha, P.; Shi, Y.; Ji, Y.; Duan, H.; Padovani, A.; Larcher, L.; Li, X. R.; Xu, J. J.; Lanza, M.
abstract

After more than a decade working with graphene there is still a preoccupying lack of commercial devices based on this wonder material. Here we report the use of high-quality solution-processed graphene sheets to fabricate ultra-sharp probes with superior performance. Nanoprobes are versatile tools used in many fields of science, but they can wear fast after some experiments, reducing the quality and increasing the cost of the research. As the market of nanoprobes is huge, providing a solution for this problem should be a priority for the nanotechnology industry. Our graphene-coated nanoprobes not only show enhanced lifetime, but also additional unique properties of graphene, such as hydrophobicity. Moreover, we have functionalized the surface of graphene to provide piezoelectric capability, and have fabricated a nano relay. The simplicity and low cost of this method, which can be used to coat any kind of sharp tip, make it suitable for the industry, allowing production on demand.

2016 - Multiscale modeling of electron-ion interactions for engineering novel electronic device and materials [Relazione in Atti di Convegno]
Larcher, Luca; Puglisi, Francesco Maria; Padovani, Andrea; Vandelli, Luca; Pavan, Paolo
abstract

In this work, we present a multiscale simulation platform as a viable tool to engineer novel electron devices. The tool connects the specific material properties (as atomic defects, interfaces, material morphology) to the electrical behavior of the device, representing a virtual space for the design of novel electrons device purposely exploiting atom-electron interactions. This simulation platform is based on the modeling the microscopic interactions and chemical reactions (e.g. bond breaking) between electrons and atomic species (ions, vacancies, dangling bonds). In this work, we show how this tool can be used to design resistive memory devices based on binary oxides. The fundamental importance of the complex interplay between charge carriers and atomic species is highlighted by showing how these interactions determine many electrical characteristics of the device, including charge transport, structural modifications associated with resistive switching, variability, and noise fluctuations.

2016 - Multiscale modeling of electron-ion interactions for engineering novel electronic devices and materials [Relazione in Atti di Convegno]
Larcher, Luca; Puglisi, Francesco Maria; Padovani, Andrea; Vandelli, Luca; Pavan, Paolo
abstract

2016 - Nanoscale homogeneity and degradation process of two dimensional atomically thin hexagonal boron nitride dielectric stacks [Relazione in Atti di Convegno]
Ji, Y.; Pan, C.; Hui, F.; Shi, Y.; Jiang, L.; Xiao, N.; Grustan-Gutierrez, E.; Larcher, L.; Lanza, M.
abstract

In this paper we analyze the reliability of atomically thin hexagonal boron nitride (A-BN) dielectric stacks subjected to electrical stresses. The 2D insulating stacks are grown by chemical vapor deposition, meaning that (unlike exfoliated nanosheets) they can cover large areas and are suitable for the fabrication of scalable devices using photolithography tools. By comparing HfO2 and A-BN stacks with similar equivalent oxide thickness we find that the 2D dielectric shows a striking stable conduction when subjected to sequences of ramped voltage stresses, indicating that it is much more stable versus electrical-field-induced defects. These results point A-BN as superb dielectric for electronic devices.

2016 - Operations, Charge Transport, and Random Telegraph Noise in HfOx Resistive Random Access Memory: a Multi-scale Modeling Study [Articolo su rivista]
Puglisi, Francesco Maria; Larcher, Luca; Padovani, Andrea; Pavan, Paolo
abstract

In this work we explore the mechanisms responsible for Random Telegraph Noise (RTN) fluctuations in HfOx Resistive Random Access Memory (RRAM) devices. The statistical properties of the RTN are analyzed in many operating conditions exploiting the Factorial Hidden Markov Model (FHMM) to decompose the multilevel RTN traces in a superposition of two-level fluctuations. This allows the simultaneous characterization of individual defects contributing to the RTN. Results, together with multi-scale physics-based simulations, allows thoroughly investigating the physical mechanisms which could be responsible for the RTN current fluctuations in the two resistive states of these devices, including also the charge transport features in a comprehensive framework. We consider two possible options, which are the Coulomb blockade effect and the possible existence of metastable states for the defects assisting charge transport. Results indicate that both options may be responsible for RTN current fluctuations in HRS, while RTN in LRS is attributed to the temporary screening effect of the charge trapped at defect sites around the conductive filament.

2016 - Physical Mechanisms behind the Field-Cycling Behavior of HfO2-Based Ferroelectric Capacitors [Articolo su rivista]
Pesic, Milan; Fengler, Franz Paul Gustav; Larcher, Luca; Padovani, Andrea; Schenk, Tony; Grimley, Everett D.; Sang, Xiahan; Lebeau, James M.; Slesazeck, Stefan; Schroeder, Uwe; Mikolajick, Thomas
abstract

Novel hafnium oxide (HfO2)-based ferroelectrics reveal full scalability and complementary metal oxide semiconductor integratability compared to perovskite-based ferroelectrics that are currently used in nonvolatile ferroelectric random access memories (FeRAMs). Within the lifetime of the device, two main regimes of wake-up and fatigue can be identified. Up to now, the mechanisms behind these two device stages have not been revealed. Thus, the main scope of this study is an identification of the root cause for the increase of the remnant polarization during the wake-up phase and subsequent polarization degradation with further cycling. Combining the comprehensive ferroelectric switching current experiments, Preisach density analysis, and transmission electron microscopy (TEM) study with compact and Technology Computer Aided Design (TCAD) modeling, it has been found out that during the wake-up of the device no new defects are generated but the existing defects redistribute within the device. Furthermore, vacancy diffusion has been identified as the main cause for the phase transformation and consequent increase of the remnant polarization. Utilizing trap density spectroscopy for examining defect evolution with cycling of the device together with modeling of the degradation results in an understanding of the main mechanisms behind the evolution of the ferroelectric response.

2016 - Probing defects generation during stress in high-κ/metal gate FinFETs by random telegraph noise characterization [Relazione in Atti di Convegno]
Puglisi, Francesco Maria; Costantini, Felipe; Kaczer, Ben; Larcher, Luca; Pavan, Paolo
abstract

In this work, we report about defects generation in the oxide layer of n-FinFETs during stress. Defects generation is probed using RTN traces collected at both the drain and the gate. A stress/measure approach is used to monitor the characteristics of the device, including RTN, at different levels of cumulative stress. Indicators derived from IG-VG and ID-VG measurements suggest defects generation to occur away from the channel. This is confirmed by the RTN analysis, which shows that drain and gate RTN events are completely uncorrelated. The detailed analysis of the RTN properties at different stress levels shows that an increase of the gate leakage is accompanied by changes in the gate RTN properties, while the drain RTN properties are rarely affected by the stress. This further proves that stress is associated with defects generation deep in the oxide layer, far away from the channel. This result is in contrast to what reported for planar FETs and suggests that, in n-FinFETs, the root cause of ID RTN might differ from the one causing SILC and IG RTN.

2016 - Random telegraph noise in HfOx Resistive Random Access Memory: From physics to compact modeling [Relazione in Atti di Convegno]
Puglisi, Francesco Maria; Pavan, Paolo; Larcher, Luca
abstract

In this paper we propose a compact model of Random Telegraph Noise in HfOx-based Resistive Random Access Memory devices. Starting from the physics of charge transport, we first focus on the RTN phenomenon in the two different resistive states (HRS and LRS). We separately explore the microscopic mechanisms responsible for Random Telegraph Noise (RTN) current fluctuations in HfOx RRAM devices in HRS and LRS, exploiting a self-consistent physics-based simulation framework accounting for many charge transport mechanisms and their alterations. Then, we develop a simple yet effective compact model of RTN valid in both states, which can be easily integrated in state-of-the-art compact RRAM device models. The compact model predictions are validated by comparison with both a large experimental dataset obtained by measuring RRAM devices in different conditions, and data found in the literature.

2016 - Root cause of degradation in novel HfO2-based ferroelectric memories [Relazione in Atti di Convegno]
Pesic, Milan; Fengler, Franz P. G.; Slesazeck, Stefan; Schroeder, Uwe; Mikolajick, Thomas; Larcher, Luca; Padovani, Andrea
abstract

HfO2-based ferroelectrics reveal full scalability and CMOS integratability compared to perovskite-based ferroelectrics that are currently used in non-volatile ferroelectric random access memories (FeRAMs). Up to now, the mechanisms responsible for the decrease of the memory window have not been revealed. Thus, the main scope of this study is an identification of the root causes for the endurance degradation. Utilizing trap density spectroscopy for examining defect evolution with cycling of the device studied together with modeling of the degradation resulted in an understanding of the main mechanisms responsible for degradation of the ferroelectric behavior.

2016 - Single vacancy defect spectroscopy on HfO2 using random telegraph noise signals from scanning tunneling microscopy [Articolo su rivista]
Thamankar, R.; Raghavan, N.; Molina, J.; Puglisi, Francesco Maria; O'Shea, S. J.; Shubhakar, K.; Larcher, Luca; Pavan, Paolo; Padovani, Andrea; Pey, K. L.
abstract

Random telegraph noise (RTN) measurements are typically carried out at the device level using standard probe station based electrical characterization setup, where the measured current represents a cumulative effect of the simultaneous response of electron capture/emission events at multiple oxygen vacancy defect (trap) sites. To better characterize the individual defects in the high-j dielectric thin film, we propose and demonstrate here the measurement and analysis of RTN at the nanoscale using a room temperature scanning tunneling microscope setup, with an effective area of interaction of the probe tip that is as small as 10 nm in diameter. Two-level and multi-level RTN signals due to single and multiple defect locations (possibly dispersed in space and energy) are observed on 4 nm HfO2 thin films deposited on n-Si (100) substrate. The RTN signals are statis- tically analyzed using the Factorial Hidden Markov Model technique to decode the noise contribu- tion of more than one defect (if any) and estimate the statistical parameters of each RTN signal (i.e., amplitude of fluctuation, capture and emission time constants). Observation of RTN at the nanoscale presents a new opportunity for studies on defect chemistry, single-defect kinetics and their stochastics in thin film dielectric materials. This method allows us to characterize the fast traps with time constants ranging in the millisecond to tens of seconds range.

2016 - System With RF Power Delivery Capabilities for Active Safety Enhancement in Industrial Vehicles Using Interchangeable Implements [Articolo su rivista]
Bertacchini, Alessandro; Napoletano, Giacomantonio; Scorcioni, Stefano; Larcher, Luca; Pavan, Paolo
abstract

In this paper, an active system for safety enhancement in industrial and off-highway vehicles using interchangeable implements is presented. The system, applied to the real case study of automatic identification of implements connected to a telehandler, is developed by adopting a hardware–software codesign approach. It is comprised of two devices: the Illuminator-Gateway Device (IGD) and the End Device (ED). Differently from other similar solutions, the system embeds a complete radio frequency (RF) power delivery system that is compliant with the regulations in force in Europe and in North America to extend the battery lifetime of the ED. In particular, the IGD, positioned on the free end of the telescopic arm of the telehandler, supplies the RF energy required for the operations of the ED and acts as a gateway sending the data received from the ED to the other Electronic Control Units (ECUs) of the vehicle. The ED, instead, is mounted on the connected implement, collects the RF energy delivered by the IGD, and wirelessly sends the unique identifier, the key parameters, and the calculated effective working time of the implement. This information can be used by the main ECU of the vehicle for safety-related purposes and programmed maintenance. Experimental results show that the implemented RF power delivery system is able to gather up to 63% of the power required by the ED when it is on duty, thus significantly extending its battery lifetime.

2016 - Wireless sensor systems with energy harvesting capabilities for safety enhancement in agricultural vehicles [Relazione in Atti di Convegno]
Bertacchini, Alessandro; Larcher, Luca
abstract

This paper presents an overview of a multi-sensor wireless system applied to agricultural vehicles. The data provided by the different kinds of ad-hoc developed wireless sensors can be used as starting point for the implementation of an automatic system for the active safety enhancement of the vehicles. In order to guarantee a lifetime comparable with the one of the implement or trailer where they are mounted, each sensor has his own energy harvesting system able to gather energy from the vibrations occurring in the environment where the sensor operates.

2016 - 2D h-BN based RRAM devices [Relazione in Atti di Convegno]
Puglisi, Francesco Maria; Larcher, Luca; Pan, C.; Xiao, N.; Shi, Y.; Hui, F.; Lanza, M.
abstract

This paper presents two dimensional (2D) RRAM devices exploiting multilayer hexagonal boron nitride (h-BN) as active switching layer. Different electrodes including Cu, Ni-doped Cu (CuNi) and graphene (G) are considered. The devices show low set/reset voltages, high on/off current ratio, good endurance and very low overall variability. Experimental results are interpreted using a novel simulation framework, which proves that the memory behavior is enabled by the manipulation of a boron (B)-deficient conductive filament (CF). The cyclical release and diffusion of B ions are the key physical mechanisms responsible for switching.

2015 - A Complete Statistical Investigation of RTN in HfO₂-Based RRAM in High Resistive State [Articolo su rivista]
Puglisi, Francesco Maria; Larcher, Luca; Padovani, Andrea; Pavan, Paolo
abstract

In this paper, we investigate the random telegraph noise (RTN) in hafnium-oxide resistive random access memories in high resistive state (HRS). The current fluctuations are analyzed by decomposing the multilevel RTN signal into two-level RTN traces using a factorial hidden Markov model approach, which allows extracting the properties of the traps originating the RTN. The current fluctuations, statistically analyzed on devices with a different stack reset at different voltages, are attributed to the activation and deactivation of defects in the oxidized tip of the conductive filament, assisting the trap-assisted tunneling transport in HRS. The physical mechanisms responsible for the defect activation are discussed. We find that RTN current fluctuations can be due to either the coulomb interaction between oxygen vacancies (normally assisting the charge transport) and the electron charge trapped at interstitial oxygen defects, or the metastable defect configuration of oxygen vacancies assisting the electron transport in HRS. A consistent microscopic description of the phenomenon is proposed, linking the material properties to the device performance.

2015 - A microscopic physical description of RTN current fluctuations in HfOx RRAM [Relazione in Atti di Convegno]
Puglisi, Francesco Maria; Pavan, Paolo; Vandelli, Luca; Padovani, Andrea; Bertocchi, Matteo; Larcher, Luca
abstract

In this work we explore the microscopic mechanisms responsible for Random Telegraph Noise (RTN) current fluctuations in HfOx Resistive Random Access Memory (RRAM) devices. The statistical properties of the RTN current fluctuations are analyzed in a variety of reading conditions by exploiting the Factorial Hidden Markov Model (FHMM) to decompose the complex RTN traces in a superimposition of two-level fluctuations. We investigate the physical mechanisms that could be responsible for the RTN current fluctuations by considering two options that are the Coulomb blockade effect and the metastable-to-stable transition of defect assisting the Trap- Assisted-Tunneling (TAT) charge transport. Physics-based simulations show that both options allow reproducing the RTN current fluctuations. The electron TAT via oxygen vacancy defects, responsible for the current in High Resistive State (HRS), is significantly altered by the electric field caused by electron trapping at defects (i.e. neutral interstitial oxygen), not directly involved in charge transport. Similarly, the transition of oxygen vacancies into a stable-slow defect configuration (still unidentified in HfOx) can temporarily switch off the current, thus explaining the RTN.

2015 - A New Physical Method Based on CV--GV Simulations for the Characterization of the Interfacial and Bulk Defect Density in High-k/III--V MOSFETs [Articolo su rivista]
Sereni, Gabriele; Vandelli, Luca; Veksler, Dmitry; Larcher, Luca
abstract

We propose a new defect characterization technique for high-k dielectric stacks in III-V MOSFETs. This technique allows extracting the defect density from the simulations of the C-V and G-V characteristics at different frequencies. The simulation is performed using a physical distributed compact model, where the trap-assisted capture and emission processes are described in the framework of the multiphonon trap-assisted tunneling theory, including lattice relaxation. The technique, tested on InGaAs MOS devices with different gate-stacks, allows profiling the interfacial and bulk defects in the (E, z) domain. The extracted map, consistent with previous report, allows reproducing C-V and G-V curves on the whole frequency and gate voltage ranges and monitoring the quality of dielectric stacks for the optimization of the manufacturing process.

2015 - A novel ball joint wear sensor for low-cost structural health monitoring of off-highway vehicles [Articolo su rivista]
Castagnetti, Davide; Bertacchini, Alessandro; Spaggiari, Andrea; Lesnjanin, A.; Larcher, Luca; Dragoni, Eugenio; Arduini, M.
abstract

Mechatronic agricultural machines and equipment are continuously increasing their complexity and cost. In order to ensure their efficiency and reliability and preserve their value, it is important to actively monitor damaging and wear occurring on critical components. This approach needs the introduction of sensors on the machine, which allow continuous monitoring of the residual life of components. This work presents the development and testing of a wear sensor for a ball joint which can be applied for monitoring and diagnostic in off-highway vehicles, automotive and the industrial fields. Many peculiar features make this sensor innovative and contribute to the advance of the technology in the sector: there are no other active sensors for this specific and safety-critical joint; it has an ultra-low power consumption and can be self-powered through energy harvesting; it implements wireless connectivity; it is simple, small size and low cost. This wear sensor for the ball joint is firstly aimed at monitoring the damage of the ball joint placed between the steering actuator and the wheel spindle, since failure of the joint leads to complete loss of steering action. However, the sensor can be applied to any application involving a safety-critical ball joint (e.g. the front suspension of a vehicle). The present work describes the conceptual design and development of the whole mechatronic sensor, which includes the mechanical joint sensor and the electronic board which manages the system. Validation of the ball joint wear sensor, which was performed on an agricultural tractor through tests on a track, proved the reliability of the proposed solution.

2015 - A novel technique exploiting C-V, G-V and I-V simulations to investigate defect distribution and native oxide in high-κ dielectrics for III-V MOSFETs [Articolo su rivista]
Sereni, Gabriele; Larcher, Luca; Vandelli, Luca; Veksler, D.; Kim, T.; Koh, D.; Bersuker, Gennadi
abstract

In this paper we present a novel defect spectroscopy technique to investigate the properties of high-κ metal-gate oxides. This technique, based on the simultaneous simulations of I-V, C-V and G-V curves at different frequencies, allows profiling the distribution of interfacial and bulk defects inside the gate oxide and investigating the composition of the high-κ stacks on III-V materials. The proposed technique is applied to investigate the properties of high-κ stacks of InGaAs MOSFETs.

2015 - Characterization of anomalous Random Telegraph Noise in Resistive Random Access Memory [Relazione in Atti di Convegno]
Puglisi, Francesco Maria; Larcher, Luca; Padovani, Andrea; Pavan, Paolo
abstract

In this paper we explore the features of complex anomalous Random Telegraph Noise (aRTN) in TiN/Ti/HfO2/TiN Resistive Random Access Memory (RRAM) devices. Careful design of experiment, dedicated characterization techniques, and physics-based simulations are exploited to gain insights into the physics of this phenomenon. The RTN parameters (amplitude of the current fluctuations, capture and emission times) observed in the experiments are analyzed in a variety of operating conditions. Anomalous behaviors are examined and their statistical characteristics are analyzed. Physics-based simulations taking into account both the Coulomb interactions among different defects in the device and the possibility for defects to show metastable states are exploited to suggest a possible origin of the aRTN. Results highlight the importance of the electrostatic interactions among individual defects and the trapped charge.

2015 - Cross-correlation of electrical measurements via physics-based device simulations: Linking electrical and structural characteristics [Relazione in Atti di Convegno]
Padovani, A.; Larcher, L.; Vandelli, L.; Bertocchi, M.; Cavicchioli, R.; Veksler, D.; Bersuker, G.
abstract

We present a comprehensive simulation framework to interpret electrical characteristics (I-V, C-V, G-V, Charge-Pumping, BTl, CVS, RVS, ...) commonly used for material characterization and reliability analysis of gate dielectric stacks in modern semiconductor devices. By accounting for the physical processes controlling charge transport through the dielectric (e.g. carrier trapping/de-trapping at the defect sites, defect generation, etc.), which is modeled using a novel approach based of material characteristics, the simulations provide a unique link between the electrical measurements data and specific atomic defects in the dielectric stack. Within this methodology, the software allows an accurate defect spectroscopy by cross-correlating measurements of pre-stress electrical parameters (IV, CV, BTl). These data are then used to project the stack reliability through the simulations of stress-induced leakage current (SILC) and time-dependent dielectric degradation trends, demonstrating the tool capabilities as a technology characterization/optimization benchmark.

2015 - Extraction of interface and border traps in beyond-Si devices by accounting for generation and recombination in the semiconductor [Relazione in Atti di Convegno]
Sereni, G.; Larcher, L.
abstract

In this work we will apply a novel extraction procedure to characterize interfacial states and border traps in InGaAs and Ge MOSFETs. The extraction technique, which will allow profiling the defect distributions in the (E,z) dielectric bandgap, is based on the simultaneous simulation of C-V and G-V characteristic over a wide frequency range. The impact of minority carrier generation mechanisms taking place in the semiconductor will be deeply investigated, as its impact is essential when the technique is applied to direct low-bandgap semiconductors such as InGaAs and Ge. Results will confirm that the minority carrier generation has to carefully consider to avoid overestimating the extracted defect density.

2015 - Microscopic Modeling of HfOₓ RRAM Operations: From Forming to Switching [Articolo su rivista]
Padovani, Andrea; Larcher, Luca; Pirrotta, Onofrio; Vandelli, Luca; Bersuker, Gennadi
abstract

We propose a model describing the operations of hafnium oxide-based resistive random access memory (RRAM) devices at the microscopic level. Charge carrier and ion transport are self-consistently described starting from the leakage current in pristine HfO₂. Material structural modifications occurring during the RRAM operations, such as conductive filament (CF) creation and disruption, are accounted for. The model describes the complex processes leading to a formation of the CF and its dependence on both electrical conditions (e.g., current compliance, voltage stress, and temperature) and device characteristics (e.g., electrodes material and dielectric thickness).

2015 - Reconfigurable RF Energy Harvester with Customized Differential PCB Antenna [Articolo su rivista]
Bertacchini, Alessandro; Larcher, Luca; Maini, Moreno; Vincetti, Luca; Scorcioni, Stefano
abstract

In this work, a RF Energy harvester comprised of a differential RF-DC CMOS converter realized in ST130nm CMOS technology and a customized broadband PCB antenna with inductive coupling feeding is presented. Experimental results show that the system can work with different carrier frequencies and thanks to its reconfigurable architecture the proposed converter is able to provide a regulated output voltage of 2 V over a 14 dB of RF input power range. The conversion efficiency of the whole system peaks at 18% under normal outdoor working conditions.

2015 - SrTiO<inf>x</inf> for sub-20 nm DRAM technology nodes - Characterization and modeling [Articolo su rivista]
Kaczer, B.; Larcher, Luca; Vandelli, Luca; Reisinger, H.; Popovici, M.; Clima, S.; Ji, Z.; Joshi, S.; Swerts, J.; Redolfi, A.; Afanas'Ev, V. V.; Jurczak, M.
abstract

The electrical properties of Ru/SrTiO<inf>x</inf>/Ru capacitors have been investigated. Equivalent Oxide Thickness (EOT) of 0.38 nm at 0 V and current density of 10<sup>-7</sup> A cm<sup>-2</sup> at ±1 V and 25 °C meet the sub-20 nm DRAM requirements. Relaxation measurements were performed, indicating acceptable charge loss. Modeling of charge trapping at defect sites based on multi-phonon trap-assisted-tunneling quantitatively well describes leakage and capacitance behavior.

2015 - Statistical analysis of random telegraph noise in HfO2-based RRAM devices in LRS [Articolo su rivista]
Puglisi, Francesco Maria; Pavan, Paolo; Larcher, Luca; Padovani, Andrea
abstract

In this work, we present a thorough statistical characterization of Random Telegraph Noise (RTN) in HfO2-based Resistive Random Access Memory (RRAM) cells in Low Resistive State (LRS). Devices are tested under a variety of operational conditions. A Factorial Hidden Markov Model (FHMM) analysis is exploited to extrapolate the properties of the traps causing multi-level RTN in LRS. The trapping and de-trapping of charge carriers into/out of defects located in the proximity of the conductive filament results in a shielding effect on a portion of the conductive filament, leading to the observed RTN current fluctuations. It is found that both oxygen vacancies and oxygen ions defects may be responsible for the observed RTN. The variations of the current observed at subsequent set/reset cycles are instead attributed to the stochastic variations in the filament due to oxidation/reduction processes during reset and set operations, respectively.

2015 - Substrate and temperature influence on the trap density distribution in high-k III-V MOSFETs [Relazione in Atti di Convegno]
Sereni, G.; Vandelli, L.; Cavicchioli, R.; Larcher, L.; Veksler, D.; Bersuker, G.
abstract

In this work we apply a new spectroscopic technique based on the simulation of capacitance and conductance measurements to investigate the defect density in high-ț/III-V MOSFETs. This technique exploits the simulation of C-V and GV curves measured over a wide frequency range to extract the defect density map in the energy-position domain. The technique was used to investigate the role of the substrate material and the temperature on the interfacial and bulk defect distributions in the gate stack in InGaAs MOS capacitors grown on both InP and Si substrate. It was found that the substrate material does not affect the defect density in the gate dielectric stack. Applying the technique to C-V and G-V curves measured at different temperatures allows extracting the relaxation energy of defects, an important parameter connected to their atomic nature.

2015 - Ultra low cost triboelectric energy harvesting solutions for embedded sensor systems [Relazione in Atti di Convegno]
Bertacchini, Alessandro; Larcher, Luca; Lasagni, Marco; Pavan, Paolo
abstract

In this paper we present a triboelectric generator realized with ultra-low cost materials, assembled through a very simple in-house fabrication facility. The triboelectric generator is designed to harvest the mechanical energy from shocks. Different combinations of low-cost materials such as acetic, neutral and acrylic silicone was explored to increase device performances. The device prototypes, characterized under various working conditions show a generated output power of 25µW with an applied load of 50 MΩ, which makes this technology solution very attractive for embedded energy-autonomous sensors system solutions.

2014 - A Charge-Trapping Model for the Fast Component of Positive Bias Temperature Instability (PBTI) in High-k Gate-Stacks [Articolo su rivista]
Vandelli, Luca; Larcher, Luca; Veksler, Dmitry; Padovani, Andrea; Bersuker, Gennadi; Matthews, Kenneth
abstract

We propose a physical model for the fast component (<1 s) of the positive bias temperature instability (PBTI) process in SiOx/HfO2 gate-stacks. The model is based on the electron–phonon interaction governing the trapping/emission of injected electrons at the preexisting defects in the dielectric stack. The model successfully reproduces the experimental time dependences of the VTH shift on both stress voltage and temperature. Simulations allow the extraction of the physical characteristics of the defects contributing to PBTI, which are found to match those assisting the leakage current in these stacks (i.e., oxygen vacancies).

2014 - A Compact Model of Program Window in HfOx RRAM Devices for Conductive Filament Characteristics Analysis [Articolo su rivista]
Larcher, Luca; Puglisi, Francesco Maria; Pavan, Paolo; Padovani, Andrea; Vandelli, Luca; Bersuker, Gennadi
abstract

This paper presents a physics-based compact model for the program window in HfOx resistive random access memory devices, defined as the ratio of the resistances in high resistance state (HRS) and low resistance state (LRS). This model allows extracting the characteristics of the conductive filament (CF) in HRS. For a given forming current compliance limit, the program window is shown to be correlated to the thickness of the reoxidized portion of the CF in HRS, which can be modulated by the reset voltage amplitude. On the other hand, the statistical distribution of the memory window depends exponentially on the barrier thickness variations that points to the critical role of reset conditions for the performance optimization of RRAM devices.

2014 - A new method for extracting interface state and border trap densities in high-k/III-V MOSFETs [Relazione in Atti di Convegno]
Sereni, Gabriele; Vandelli, ; L, .; Larcher, Luca; Morassi, ; L, .; Veksler, ; D, .; Bersuker,
abstract

Increased CMOS performance requires the introduction of alternative materials as substrate and gate dielectrics. III-V materials and in particular InGaAs can provide superior electron mobility compared to classic Si substrates. However, such substrate materials have non-optimal dielectric-semiconductor interfaces that can drastically reduce the device performance. Techniques for the extraction of interface and border trap profiles are required for the characterization and optimization of these materials. In this paper we present a new procedure relying on a physical charge-transport model including trap assisted tunneling, lattice relaxation and trap assisted generation and recombination of minority carriers. The procedure allows the extraction of interface and border trap densities from capacitance voltage characteristics measured at different frequencies. The technique is applied to characterize InGaAs MOSFETs Al2O3/ZrO2 stacks of different thicknesses and fabricated with different annealing conditions

2014 - A study on HfO2 RRAM in HRS based on I–V and RTN analysis [Articolo su rivista]
Puglisi, Francesco Maria; Pavan, Paolo; Padovani, Andrea; Larcher, Luca
abstract

This paper presents a statistical characterization of random telegraph noise (RTN) in hafnium-oxide based resistive random access memories (RRAMs) in high resistive state (HRS). Complex RTN signals are analyzed exploiting a Factorial Hidden Markov Model (FHMM) approach, which allows to derive the statistical properties of the RTN signals, directly related to the physical properties of the traps responsible for the multi-level RTN measured in these devices. Noise characteristics in different reset conditions are explored through consecutive switching cycles. Noise spectral analysis is also performed to fully support the investigation. An RRAM compact model is also exploited to estimate the physical properties of the conductive filament and of the dielectric barrier from simple I–V data. These tools are combined together to prove the existence of a direct statistical relation between the reset conditions, the volume of the dielectric barrier created during the reset operation and the average number of active traps contributing to the RTN.

2014 - Active Safety System with RF Energy Harvesting Capabilities for Industrial Applications using Interchangeable Implements [Relazione in Atti di Convegno]
Bertacchini, Alessandro; Napoletano, Giacomantonio; Scorcioni, Stefano; Larcher, Luca; Pavan, Paolo
abstract

In this paper a system for the remote powering of low power electronic devices is presented. The system has been applied to a real industrial application allowing to enhance active safety in industrial vehicles. It is comprised of two main devices: i) the End Device (ED) with an embedded Radio Frequency (RF) energy harvester; ii) the Illuminator-Gateway Device (IGD) with an embedded RF power transmitter. Thanks to the optimization of the customized dual band Planar Inverted Folded Antenna (PIFA) used, the ULP architecture of the ED, the hardware-software co-design approach used and the optimization of the ED firmware, the proposed system is able to provide up to the 63% of the power required by the ED when it is on duty.

2014 - Analysis of Correlated Gate and Drain Random Telegraph Noise in Post-Soft Breakdown TiN/HfLaO/SiOx nMOSFETs [Articolo su rivista]
W. H., Liu; Padovani, Andrea; Larcher, Luca; N., Raghavan; K. L., Pey
abstract

We investigate correlated gate (IG) and drain (ID) random telegraph noise phenomena observed in post breakdown regime on nMOSFET TiN/HfLaO/SiOx gate stacks. We observe two different IG-ID correlation patterns (i.e. of the same and opposite polarities) that we attributed to charge trapping into oxygen vacancy traps of different kinds located in the SiOx close to the Si/SiOx interface. Results reported in this letter provide useful information for improving the understanding of IG/ID RTN phenomena and its impact on the reliability of post-SBD nanometer MOSFETs.

2014 - Analysis of RTN and cycling variability in HfO2 RRAM devices in LRS [Relazione in Atti di Convegno]
Puglisi, Francesco Maria; Pavan, Paolo; Larcher, Luca; Padovani, Andrea
abstract

In this work, we present a thorough statistical characterization of cycling variability and Random Telegraph Noise (RTN) in HfO2-based Resistive Random Access Memory (RRAM) cells in Low Resistive State (LRS). Devices are tested under a variety of operational conditions. A Factorial Hidden Markov Model (FHMM) analysis is exploited to extrapolate the properties of the traps causing multi-level RTN in LRS. The trapping and de-trapping of charge carriers into/out of defects located in the proximity of the conductive filament results in a shielding effect on a portion of the conductive filament, leading to the observed RTN current fluctuations. The variations of the current observed at subsequent set/reset cycles are instead attributed to the stochastic variations in the filament due to oxidation/reduction processes during reset and set operations, respectively. The statistical characterization of RTN and cycling variability does not show correlation between these phenomena.

2014 - Defect density evaluation in a high-k MOSFET gate stack combining experimental and modeling methods [Relazione in Atti di Convegno]
Puglisi, Francesco Maria; Veksler, D.; Matthews, K.; Bersuker, G.; Larcher, Luca; Padovani, Andrea; Vandelli, Luca; Pavan, Paolo
abstract

We report on a methodology to assist fabrication process development using a case study of high thermal budget (HTB) and low thermal budget (LTB) fabrication flows for high- k/metal gate stacks in n-MOSFETs. This methodology is supported by simulations that self-consistently extract defect characteristics by simultaneously considering a set of electrical measurement data, specifically stress-induced leakage current (SILC), threshold voltage shift (PBTI), and multi-frequency charge-pumping (MFCP). The contributions of pre-existing and stress-induced defects in SiO2/HfO2 gate stacks on device performance are examined. Information on defect distributions, extracted in the as-fabricated and post-stress HTB and LTB devices, allow understanding their dependence on the fabrication process, which can provide guidelines for the process optimization.

2014 - Dielectric morphology and RRAM resistive switching characteristics [Relazione in Atti di Convegno]
Bersuker, G.; Butcher, B.; Gilmer, D. C.; Larcher, Luca; Padovani, A; Geer, R.; Kirsch, P. D.
abstract

The connection between the bi-polar hafnia-based resistive-RAM (RRAM) operational characteristics and dielectric structural properties is considered. Specifically, the atomic-level description of RRAM, which operations involve the repeatable rupture/recreation of a localized conductive path, reveals that its performance is determined by the outcome of the initial forming process defining the structural characteristics of the conductive filament and distribution of the oxygen ions released from the filament region. The post-forming ions spatial distribution in the cell is found to be linked to a degree of dielectric oxygen deficiency, which may either assist or suppress the resistive switching processes.

2014 - Instability of HfO2 RRAM devices: Comparing RTN and cycling variability [Relazione in Atti di Convegno]
Puglisi, Francesco Maria; Larcher, Luca; Pavan, Paolo; Padovani, Andrea; Bersuker, G.
abstract

In this study, we present an extensive statistical characterization of the cycling variability and Random Telegraph Noise (RTN) in the HfO2-based Resistive Random Access Memories (RRAM) cells. Devices with different dielectric stacks are tested under a variety of read (sampling times and read voltage magnitudes) and operational (reset voltages) conditions. A Factorial Hidden Markov Model (FHMM) analysis is employed to reveal the properties of the traps causing multi-level RTN in High Resistive State (HRS), while the I-V data are analyzed through the developed compact model to investigate cycling variability. The activation and deactivation of traps assisting the charge transport through a dielectric barrier in HRS is found to be responsible for the observed RTN while the read current variations can be attributed to the stochastic nature of the filament oxidation process during reset, also leading to a variable number of traps formed in the barrier after each switching cycle. The statistical characterization of RTN and cycling variability, which demonstrates the uncorrelated nature of these phenomena, provides guidelines for scaling and optimization of RRAM device operations and reliability.

2014 - Integrated RF-DC converter and PCB antenna for UHF wireless powering applications [Relazione in Atti di Convegno]
Vincetti, Luca; Maini, M.; Scorcioni, Stefano; Larcher, Luca; Bertacchini, Alessandro; Tacchini, Alessandro
abstract

In this work, a broadband differential RF-DC CMOS converter realized in CMOS 130 nm technology with a customized PCB antenna with inductive coupling feeding for RF energy scavenging is presented. Experimental results show that output DC voltage higher than 1V from 800MHz to 970MHz can be obtained with a load of 1kohm.

2014 - Molecular dynamics simulation of amorphous HfO2 for resistive RAM applications [Articolo su rivista]
Broglia, Giulia; G., Ori; Larcher, Luca; Montorsi, Monia
abstract

HfO2 is widely investigated as the favoured material for resistive RAM device implementation. The structural features of HfO2 play a fundamental role in the switching mechanisms governing resistive RAM operations, and a comprehensive understanding of the relation between the atomistic properties and final device behaviour is still missing. In addition, despite the fact that ultra-scaled 10 nm resistive RAM will probably be made of amorphous HfO2, a deeper investigation of the structure is necessary. In this paper, the classical molecular dynamics technique was used to investigate the disordered atomic configuration of amorphous HfO2. The influence of density on both the atomistic structure and the diffusion of O species was carefully analysed. The results achieved showthat the atomistic structure of an amorphous HfO2 system is strongly affected by the density, and the amorphous system is rearranged in an atomic configuration similar to the crystalline configuration at similar densities. The diffusion of oxygen atoms increases with the decrease of the density, consistent with a less-packed atomic structure which allows for easier movement of this species.

2014 - Multi-scale modeling of oxygen vacancies assisted charge transport in sub-stoichiometric TiOx for RRAM application [Relazione in Atti di Convegno]
Pirrotta, Onofrio; Padovani, Andrea; Larcher, Luca; L., Zhao; B., Magyari Köpe; Y., Nishi
abstract

In this work we investigate the charge transport in sub-stoichiometric TiOx for RRAM applications. We explored the atomic defect configurations actively assisting the charge transport in sub-stoichiometric TiOx through a multi-scale approach. We combined density-functional-theory-based non-equilibrium Green's function approach (DFT-NEGF) with physical-based trap assisted tunneling (TAT) modeling to identify the defects dominating the current conduction mechanism and the physical parameters of the defects responsible for the trap-assisted tunneling (TAT). The values of the thermal ionization energy ET and relaxation energy EREL extracted are 0.35-0.4eV and 0.7eV, respectively.

2014 - Optimized Energy-Aware Wireless System for Identification of the Relative Positioning of Articulated Systems in the Free Space [Articolo su rivista]
Bertacchini, Alessandro; Napoletano, Giacomantonio; Dondi, Denis; Larcher, Luca; Pavan, Paolo
abstract

In this paper, a low-cost solution to identify the relative positioning of articulated systems in the free space is presented. To prove the effectiveness of the proposed solution, the system has been applied to a real case study of a tractor connected with a baler. Differently from other solutions, the implemented system can monitor the working conditions of the whole machinery while warning the driver when the machinery gets into a dangerous situation. The system is comprised of two wireless devices called Wireless Master Device (WMD) and Wireless End Device (WED) installed on the tractor and on the baler, respectively. To identify instantaneously the dangerous working conditions, each of the two wireless devices exploits a MEMS inertial sensor measuring 3-D linear accelerations and 3-D magnetic fields components integrated in the devices. Very low power consumption has been obtained by exploiting a hardware–software codesign approach implementing an optimized algorithm combined with a smart task manager. Furthermore, a vibrational energy harvester has been designed and integrated on the WED in order to make the device autonomous from an energetic point of view.

2014 - Progresses in Modeling HfOx RRAM Operations and Variability [Relazione in Atti di Convegno]
Larcher, Luca; Pirrotta, Onofrio; Puglisi, Francesco Maria; Padovani, Andrea; Pavan, Paolo; Vandelli, Luca
abstract

This paper reports on recent progresses in modeling bi-polar RRAM devices based on hafnium oxide. The unique modeling environment adopted for the simulation of device operations accounts self-consistently for the charge and ion transport, and the structural device modification occurring during forming and set/reset operations. Reliability mechanisms as well as the major sources of devices variability are included thanks to a multi-scale approach that connects the electrical device performance to the atomic-level material properties. The modeling methodology can be successfully applied to both improve device performances and fabrication process of state-of-the-art RRAM devices, and devise device solutions for future 3D RRAM architectures.

2014 - Vibrational Energy Harvesting [Capitolo/Saggio]
Larcher, Luca; Roy, Saibal; Mallick, Dhiman; Podder, Pranay; De Vittorio, Massimo; Todaro, Teresa; Guido, Francesco; Bertacchini, Alessandro; Hinchet, Ronan; Keraudy, Julien; Ardila, Gustavo
abstract

Energy harvesting is an attractive way to power electronic systems such as wireless sensors without using batteries or other energy storages with limited lifetime. Among the energy harvesters proposed from different sources (e.g. light, thermal gradient, strain, vibrations, electromagnetic field, air flow and pressure variations), vibrations pervasively available in different environments (indoor and outdoor) represent an attractive option for the development of adequate sources for low power supplying or for extending the autonomy of remote sensors and portable electronics. Power harvested from mechanical vibrations represents a very promising energy source with estimated power in the μW–mW range [ROU 04a]. Vibration-powered generators are typically inertial spring and mass systems (Figure 6.1) which employ three main transduction mechanisms to extract energy from vibrations: piezoelectric, electromagnetic and electrostatic. Piezoelectric generators employ active materials that generate a charge and, therefore, a voltage when mechanically stressed. Electromagnetic generators harvest energy from vibrations by employing the electromagnetic induction arising from the relative motion between a magnetic flux gradient and a conductor. Electrostatic generators use the vibration-induced relative movement between electrically isolated charged capacitor plates against the electrostatic force to generate energy. Other solutions employing either electrets or magnetostrictive materials for the mechanical energy conversion were also proposed [WAN 08, KAR 08a]. Microelectromechanical systems (MEMS) technology was largely investigated to achieve vibration energy harvesters that can be potentially integrated with low power applications such as wireless sensor networks (WSN) nodes [AMM 05,ERI 05, JEO 05, BEE 06, ROU 03]. Piezoelectric transducers have often been proposed to implement easily exploitable energy harvester solutions mainly because of their low-cost manufacturing process and the potential integration with complementary metal–oxide semiconductor (CMOS) technology. Nevertheless, electromagnetic transducers were also explored due to their complementary advantages compared to piezoelectric transducers, and solutions combining both transduction harvesters were proposed to improve the energy density and the conversion efficiency [BEE 07a]. In order to be used in practical applications, the energy harvesters have to deliver a minimum output power and voltage, which are required by power converters to operate with acceptable efficiency. Unfortunately, this is not always the case, and several energy harvesters proposed in the literature have low output power and voltages, were large and bulky, and their efficiency was shown to peak only in a very narrow frequency range, thus making such devices unsuitable to scavenge energy from actual ambient vibrations. For this reason, research activities have been oriented to improve the power efficiency and the output power of the vibration energy harvester, to decrease the size of the transducers, to decrease the operating frequency, to match the low frequency ambient vibrations and to widen their bandwidth to maximize the energy collection (ambient vibrations rarely never occur at exact frequencies) [MUR 09]. This chapter will present a short overview of the MEMS energy harvesters employing both piezoelectric and electromagnetic effects both proposed in the literature and developed in the framework of the Nanofunction project. A short overview of state-of-the-art vibration energy transducers employing the piezoelectric effect is presented in section 6.2, where MEMS prototypes realized in the framework of the Nanofunction project will also be presented. Near-field characterization techniques as well as electromechanical modeling and simulation required for the design of the energy harvesting transducers will be illustrated. Electromagnetic generat

2013 - A Compact Model of Hafnium-Oxide-Based Resistive Random Access Memory [Relazione in Atti di Convegno]
Puglisi, Francesco Maria; Pavan, Paolo; Padovani, Andrea; Larcher, Luca
abstract

In this paper, a compact model of hafnium-oxide-based resistive random access memory (RRAM) cell is developed. The proposed model includes the effect of the temperature and cycle-to-cycle stochastic variations affecting the device operations. Simple I-V measurements are used to extract the model parameters. The model accurately reproduces the I-V curves of the switching cycles in different operating conditions.

2013 - A Reconfigurable Differential CMOS RF Energy Scavenger With 60% Peak Efficiency and -21dBm Sensitivity [Articolo su rivista]
Scorcioni, Stefano; Larcher, Luca; Bertacchini, Alessandro
abstract

A differential RF-DC CMOS converter for RF energy scavenging based on a reconfigurable voltage rectifier topology is presented. The converter efficiency and sensitivity are optimized thanks to the proposed reconfigurable architecture. Prototypes realized in 130nm provide a regulated output voltage of ~2V when working at 868MHz, with a -21dBm sensitivity. The circuit efficiency peaks at 60%, remaining above the 40% for a 18dB input power range.

2013 - A simulation framework for modeling charge transport and degradation in high-k stacks [Articolo su rivista]
Larcher, Luca; Padovani, Andrea; Vandelli, Luca
abstract

In this paper we present a comprehensive physical model that describes charge transport and degradation phenomena in high-k stacks. The physical mechanisms are modeled using a novel material-related approach that includes in a self-consistent fashion the charge transport (dominated by defect-assisted contribution), power dissipation and temperature increase, defect generation, and ion and vacancy diffusion and recombination. The physical properties of defects, which play a crucial role in determining the electrical behavior of the high-k stacks, depend on their atomistic configurations, as calculated using ab-initio methods. This simulation framework represents a powerful tool to interpret electrical characterization measurements. In addition, it can be used to optimize logic and memory device stacks thanks to its predictive statistical capabilities that allow reproducing gate current, threshold voltage increase and time to breakdown (TDDB) statistics. Simulation results performed using this simulation package are shown to reproduce accurately leakage current, Stress-Induced Leakage Current (SILC), threshold voltage shift observed during Positive Bias Temperature Instability (PBTI) stress, TDDB in various dielectric stacks.

2013 - An Empirical Model for RRAM Resistance in Low- and High-Resistance State [Articolo su rivista]
Puglisi, Francesco Maria; Larcher, Luca; G., Bersuker; Padovani, Andrea; Pavan, Paolo
abstract

We present a simple empirical expression describing hafnium-based RRAM resistance at different reset voltages and current compliances. The model that we propose describes filament resistance measured at low (∼0.1 V) reading voltage in both low-resistance state (LRS) and high-resistance state (HRS). The proposed description confirms that conduction in LRS is ohmic (after forming with a sufficiently high current compliance) and is consistent with the earlier description of HRS resistance as controlled by a trap-assisted electron transfer via traps in the oxidized portion of the filament. The length of the nonohmic part of the filament is found to be directly proportional to reset voltage. Moreover, low-frequency noise measurements at different reset voltages evidence a tradeoff between HRS resistance and noise in reading conditions.

2013 - An Integrated RF Energy Harvester for UHF Wireless Powering Applications [Relazione in Atti di Convegno]
Scorcioni, Stefano; Larcher, Luca; Bertacchini, Alessandro; Vincetti, Luca; Maini, Moreno
abstract

A RF energy harvester comprised of an integrated differential RF-DC converter and a differential PCB custom antenna is presented for wireless powering applications in the UHF band. The RF-DC converter, based on a reconfigurable voltage rectifier directly connected to a matched PCB antenna, is designed for high efficiency and sensitivity. Prototypes of the RF-DC converter realized in CMOS 130nm technology show a -16dBm sensitivity and a 60% peak efficiency. The RF-DC converters operate within the -16dBm÷0dBm input power range with an efficiency which remains above the 40% for more than 10dB. The RF energy harvester operates on the 840MHz÷975MHz band thanks to the ad-hoc designed wide-band differential antenna.

2013 - Charge Transport and Degradation in HfO2 and HfOx Dielectrics [Articolo su rivista]
Padovani, Andrea; Larcher, Luca; Gennadi, Bersuker; Pavan, Paolo
abstract

We combine experiments and simulations to investigate leakage current and breakdown (BD) in stoichiometric and sub-stoichiometric hafnium oxides. Using charge-transport simulations based on phonon-assisted carrier tunneling between trap sites, we demonstrate that higher currents generally observed in HfOx are due to a higher density of the as-grown oxygen vacancy defects assisting the charge transport. Reduction of the dielectric breakdown field (EBD) in HfOx is explained by the lower zero-field activation energy (EA,G) of the defect generation process, as extracted from time-dependent dielectric breakdown experiments.

2013 - Compact modeling of TANOS program/erase operations for SPICE-like circuit simulations [Articolo su rivista]
Padovani, Andrea; Larcher, Luca; Pavan, Paolo
abstract

We present an analytical model of TANOS program/erase transients that can be used to implement a compact SPICE-like model of these memory devices. Simulation results obtained from a physics-based TANOS model are used to derive simple analytical formulas relating the program/erase currents and the centroid of the trapped charge distribution to operating conditions and stack composition. The model allows reproducing with a good agreement the experimental program/erase transients, thus providing a valuable tool for IC designers to optimize TANOS memory circuits, especially in the framework of multi-level applications.

2013 - Connecting RRAM Performance to the Properties of the Hafnia-based Dielectrics [Relazione in Atti di Convegno]
G., Bersuker; B., Butcher; D., Gilmer; P., Kirsch; Larcher, Luca; Padovani, Andrea
abstract

The connection between the resistive-RAM (RRAM) operational-mechanism, performance, and utilized-dielectric properties is described. Specifically, the atomic-level description of bi-polar hafnia-based RRAM, which operations involve the repeatable rupture/recreation of a localized conductive path, reveals that its performance is determined by the outcome of the initial forming process; defining the structural characteristics of the conductive filament and distribution of the oxygen ions released from the filament region. The ion distribution, in turn, is found to be linked to the level of dielectric oxygen deficiency, which may either assist or suppress the resistive switching process. With this improved understanding of the connection between RRAM performance and materials properties the optimization of RRAM devices may be more readily achieved.

2013 - Connecting the physical and electrical properties of Hafnia-based RRAM [Relazione in Atti di Convegno]
B., Butcher; G., Bersuker; D., Gilmer; P., Kirsch; Larcher, Luca; Padovani, Andrea; Vandelli, Luca; R., Geer; P. D., Kirsch
abstract

Simulations of the dynamic physical processes involved in HfO2-based resistive-memory-operations are used to identify the dielectric structural properties responsible for device performance, while revealing that repeatable switching and higher HRS resistances are enabled by the oxide substoichiometric composition. These simulations support a conductive-filament-formation physical model which is resulted from metal-oxygen bond breakage and subsequent oxygen ion out-diffusion, thus leaving behind an oxygen vacancy rich region. The subsequent reset process is also shown to be controlled by re-oxidation of the filament tip.

2013 - Exploration of new Materials, Devices and Technologies for Energy Harvesting: the NANOFUNCTION Experience [Abstract in Atti di Convegno]
M. Tartagni, A. G. Nassiopoulou; Costache, M. V.; Svalenzuela, S. Valenzuela; Torres, S.; Wwang, W. Wang; Wang, N.; Rohan, J.; Hayes, M.; Roy, S.; Flandre, D.; Kaminski Cachopo, A.; Ardila, G.; Mouis, M.; Aberg, M.; Parker, E.; Bertacchini, Alessandro; Larcher, Luca; Romani, A.; Fiegna, C.; Zanuccoli, M.
abstract

The concurrent decrease of energy demanded by electronic systems with the increase of the energy stored and/or harvested by generation systems give the opportunity for novel approaches in a large amount of applications. Some of them already reached the market, however, the real limits of the issues should be carefully analyzed on a case-by-case basis. Energy harvesting entails conversion of ambient energy sources into usable energy and will become a critical technology for applications such wireless sensor networks, wireless security systems, in situ monitoring for mobile/moving systems, body area networks, biomedical devices, portable power generation for mobile electronics. This paper shows the recent results achieved by research groups within the NANOFUNCTION FP7 Network of Excellence working in the energyharvesting field. The collaboration provided advanced solutions in energy harvesting systems thanks to the interdisciplinary research performed at the level of materials, devices and architectures. The objective of the joint group is twofold: to investigate the potential of nano-materials and nano-devices capable to scale to micro- and nano-system level the energy harvesting and energy storage functions, and to devise innovative, integrated and efficient systems able to manage and deliver the harvested and stored energy.

2013 - Identifying the First Layer to Fail in Dual Layer SiOx/HfSiON Gate Dielectric Stacks [Articolo su rivista]
Padovani, Andrea; Nagarajan, Raghavan; Larcher, Luca; Kin Leong, Pey
abstract

We use the thermochemical model of bond breakage to investigate the degradation occurring in dual layer SiOx/HfSiON gate dielectric stacks during low compliance soft breakdown experiments, with the ultimate goal of identifying the first layer that degrades. Time dependent dielectric breakdown (TDDB) experiments reveal that the degradation of conventional SiON and SiOx/HfSiON dielectric stacks have the same kinetics, i.e., activation energy and field acceleration factor. This finding, supported by physics-based breakdown simulations, indicates that the degradation in SiOx/HFSiON stacks is governed by the defect generation in the silicon oxide interfacial layer, which is the first that degrades in the multi-layer stack.

2013 - Investigation of the role of electrodes on the retention performance of HfOx based RRAM cells by experiments, atomistic simulations and device physical modeling [Relazione in Atti di Convegno]
B., Traore; K. H., Xue; E., Vianello; G., Molas; Padovani, Andrea; Pirrotta, Onofrio; Larcher, Luca; P., Blaise; L., Fonseca; B., De Salvo; Y., Nishi
abstract

In this work we investigate in detail the effects of metal electrodes on the retention performances of HfOx RRAM devices. Motivated by our experimental data, we employ physics-based RRAM modeling and first-principles calculations to show that during the ON-state the concentration of oxygen interstitial (Oi) ions in the oxide depends significantly on the metal electrodes, being much larger for RRAM devices with Pt electrodes compared with Ti. The lower Oi concentration in HfOx with Ti electrodes, known as a strong oxygen getter material, results in improved retention and thermal stability. The presence of oxygen deficient conductive filaments explains the data.

2013 - Leakage Current - Forming Voltage Relation and Oxygen Gettering in HfOx RRAM Devices [Articolo su rivista]
K. G., Young Fisher; G., Bersuker; B., Butcher; Padovani, Andrea; Larcher, Luca; D., Veksler; D. C., Gilmer
abstract

We observe a trend between initial leakage currents in polycrystalline HfOx resisitive random access memory (RRAM) cells (before forming) and the forming voltages. This trend points to the dominant role played by conduction paths located at grain boundaries, which is promoted by the oxygen deficiency in ${rm HfO}_{rm x}$. One of these paths is then converted into the conductive filament responsible for nonvolatile resistance switching. In addition, we find that by engineering the RRAM stack, the forming voltage can be tuned-up to meet specific RRAM requirements, such as lower power and forming-less operations.

2013 - Leakage current through the poly-crystalline HfO2: trap densities at grains and grain boundaries [Articolo su rivista]
Pirrotta, Onofrio; Larcher, Luca; M., Lanza; Padovani, Andrea; M., Porti; Nafría, M. o.; G., Bersuker
abstract

We investigate the role of grains and grain boundaries (GBs) in the electron transport through poly-crystalline HfO2 by means of conductive atomic force microscopy (CAFM) measurements and trap-assisted tunneling simulations. CAFM experiments demonstrate that the leakage current through a thin dielectric film preferentially flows via the GBs. The current I-V characteristics measured on both types of sites, grains, and GBs are successfully simulated by utilizing the multiphonon trap-assisted tunneling model, which accounts for the inelastic charge transport via the electron traps. The extracted density of electrically active traps, whose energy parameters match those of the positively charged oxygen vacancies in hafnia, is ∼3 × 1019 cm−3 at the grains, whereas a much higher value of (0.9÷2.1) × 1021 cm−3 is required to reproduce the leakage current through the GBs.

2013 - Microscopic Modeling of Electrical Stress -Induced Breakdown in Poly-Crystalline Hafnium Oxide Dielectrics [Articolo su rivista]
Vandelli, Luca; Padovani, Andrea; Larcher, Luca; G., Bersuker
abstract

We present a quantitative physical model describing degradation of poly-crystalline HfO2 dielectrics subjected to electrical stress culminating in the dielectric breakdown (BD). The model accounts for the morphology of the hafnium oxide film and considers the interaction of the injected electrons with the atomic defects supporting the charge transport to calculate the 3D power dissipation and temperature maps across the dielectric. This temperature map, along with that of the electric field, is used to self-consistently calculate the stress-induced defect generation rates in the dielectric during stress. The model quantitatively reproduces the evolution of the currents measured on HfO2 MIM capacitors during constant voltage stress, up to the onset of BD, and the dependencies of the time-dependent dielectric breakdown (TDDB) distributions on stress temperature and voltage. It represents a powerful tool for statistical reliability predictions that can be extended to other high-k materials, multilayer stacks and resistive RAM devices based on transition metal oxides.

2013 - Modeling the Effects of Different Forming Conditions on RRAM Conductive Filament Stability [Relazione in Atti di Convegno]
B., Butcher; G., Bersuker; Vandelli, Luca; Padovani, Andrea; Larcher, Luca; A., Kalantarian; R., Geer; D. C., Gilmer
abstract

In order to identify the factors controlling the filament characteristics, we perform physics-based simulations of the inherently stochastic and difficult-to-control forming process using a statistical Monte-Carlo method to model the Hf-O bond-breakage, oxygen ion diffusion and vacancy-oxygen recombination. Simulation results well reproduce the experimental trends observed for the conductivity of the post-forming low resistance state under different forming conditions. It is shown that the distribution of the oxygen ions in the surrounding oxide during forming as well as local filament temperature and electrical field all affect the filament stability.

2013 - Perimeter and area current components in HfO2 and HfO2-x metal-insulator-metal capacitors [Articolo su rivista]
Puglisi, Francesco Maria; Pavan, Paolo; Padovani, Andrea; Larcher, Luca
abstract

In this paper, the authors present an experimental analysis on current conduction mechanisms in high-k oxides, where two metal–insulator–metal structures with different insulators (HfO2 and HfO2-x) are considered. Current density measurements indicate the existence of a perimeter-related component in the current, sizeable in HfO2, and negligible in HfO2-x samples, which have to be taken into account for a correct analysis of the device behavior and cannot be based only on the area scaling rules. For oxide breakdown, for example, a significant contribution of the perimeter-related current component results in conservative extrapolations of breakdown voltages for scaled devices.

2013 - Random Telegraph Noise analysis to investigate the properties of active traps of HfO2-Based RRAM in HRS [Relazione in Atti di Convegno]
Puglisi, Francesco Maria; Pavan, Paolo; Padovani, Andrea; Larcher, Luca
abstract

This paper presents statistical characterization of Random Telegraph Noise (RTN) in hafnium-oxide-based Resistive Random Access Memories (RRAMs) in High Resistive State (HRS). Complex RTN signals are analyzed exploiting a Factorial Hidden Markov Model (FHMM) approach, allowing to derive the statistical properties of traps responsible of the multi-level RTN measured in these devices. Noise characteristics in different reset conditions are explored to prove the existence of a direct relation between the reset voltage, the volume of the dielectric barrier created during the reset operation and the number of active traps contributing to the RTN.

2013 - Random telegraph noise (RTN) in scaled RRAM devices [Poster]
D., Veksler; G., Bersuker; Vandelli, Luca; Padovani, Andrea; Larcher, Luca; A., Muraviev; B., Chakrabarti; E., Vogel; D. C., Gilmer; P. D., Kirsch
abstract

The random telegraph noise (RTN) related read instability in resistive random access memory (RRAM) is evaluated by employing the RTN peak-to-peak (P-p) amplitude as a figure of merit (FoM). Variation of the FoM value over multiple set/reset cycles is found to follow the log-normal distribution. In HRS, P-p decreases with the reduction of the read current, which allows scaling of the RRAM operating current. The RTN effect is attributed to the mechanism of activation/deactivation of the electron traps in (in HRS) or near (in LRS) the filament that affects the current through the RRAM device.

2013 - Resilience of Ultra-Thin Oxynitride Films to Percolative Wear-Out and Reliability Implications for High-k Stacks at Low Voltage Stress [Articolo su rivista]
N., Raghavan; Padovani, Andrea; X., Li; X., Wu; V. L., Lo; M., Bosman; Larcher, Luca; K. L., Pey
abstract

Localized progressive wear-out and degradation of ultra-thin dielectrics around the oxygen vacancy percolation path formed during accelerated time dependent dielectric breakdown (TDDB) tests is a well-known phenomenon documented for silicon oxynitride (SiON) based gate stacks in metal oxide semiconductor field effect transistors (MOSFET). This progressive or post breakdown stage involves an initial phase characterized by “digital” random telegraph noise (RTN) fluctuations followed by the wear-out of the percolation path, which results in an “analog” increase of the leakage current towards the compliance, culminating in a thermal runaway and hard breakdown (HBD). The relative contribution of the digital and analog phases of degradation at very low voltage stress in ultra-thin SiON (16Ǻ) is yet to be fully investigated, which represents the core of this study. We investigate the wear-out process by combining electrical and physical analysis evidences with modeling and simulation results using Kinetic Monte Carlo (KMC) defect generation and multi-phonon trap assisted tunneling (PTAT) models. We show that the transition from the digital to the analog regime is governed by a critical voltage (VCRIT), which determines the reliability margin in the post breakdown phase. Our results have a significant impact on the post-breakdown operational reliability of SiON and advanced high-κ – SiOx interfacial layer (HK-IL) gate stacks, wherein the SiOx layer seems to be the weakest link for percolation event.

2013 - RTS Noise Characterization of HfOx RRAM in High Resistive State [Articolo su rivista]
Puglisi, Francesco Maria; Pavan, Paolo; Padovani, Andrea; Larcher, Luca; G., Bersuker
abstract

In this paper we analyze Random Telegraph Signal (RTS) noise cand Power Spectral Density (PSD) in hafnium-based RRAMs. RTS measured in HRS exhibits fast and slow multilevel switching events. RTS characteristics are examined through novel color-coded time-lag plots and Hidden Markov Model (HMM) time-series analyses. Results are validated by comparing simulated and experimental PSD. Noise is examined at different reset conditions to provide an insight into the conduction mechanisms in HRS. Higher reset voltages are found to result in greater RTS complexity due to a larger number of active traps as confirmed by PSD.

2013 - Single-Ended Broadband Antenna for Radiofrequency Energy Harvesting [Relazione in Atti di Convegno]
Vincetti, Luca; Maini, M.; Bompani, M.; Larcher, Luca; Scorcioni, Stefano; Bertacchini, Alessandro; Grossi, D.; Tacchini, Alessandro
abstract

A single-ended broadband UHF antenna with high inductive input impedance for radiofrequency energy harvesting is here presented. It consists of a small feeding loop and a conical radiating monopole. A prototype has been fabricated on a FR4 substrate and tested. Experimental results show a -3dB power transmission bandwidth of about 130MHz (860MHz−990MHz).

2013 - Temperature impact (up to 200 °C) on performance and reliability of HfO2-based RRAMs [Relazione in Atti di Convegno]
T., Cabout; L., Perniola; V., Jousseaume; H., Grampeix; J. F., Nodin; A., Toffoli; E., Jalaguier; E., Vianello; G., Molas; G., Reimbold; B., De Salvo; Pirrotta, Onofrio; Padovani, Andrea; Larcher, Luca; T., Diokh; P., Candelier; M., Guillermet; M., Bocquet; C., Muller
abstract

This paper provides an overview of the temperature impact (up to 200 °C) on the electrical behavior of oxide-based RRAM, during forming, low-field resistance reading, SET/RESET, disturb, data retention and endurance. . HfO2-RRAM devices (in a 1T1R configuration) integrated in an advanced 65 nm technology are studied for this aim. We show that forming operation is strongly activated in temperature (i.e. -0.5 V per hundred Celsius degree), being much less for SET and RESET voltages (i.e. < -0.05 V per hundred Celsius degree); disturb of HRS at fixed voltage showed to be independent of temperature; endurance up to 3.106 cycles, with optimized set of stress parameters showed no significant variation; data retention at 150 °C up to 68 days showed stable programming window, after different initial programming algorithms.

2013 - The "Buffering" Role of High-k in Post Breakdown Degradation Immunity of Advanced Dual Layer Dielectric Gate Stacks [Relazione in Atti di Convegno]
N., Raghavan; Padovani, Andrea; X., Wu; K., Shubhakar; M., Bosman; Larcher, Luca; K. L., Pey
abstract

Post breakdown (BD) reliability is an important area of study in ultra-thin gate dielectrics as it has significant implications on the performance degradation, lifetime, reliability margin and power dissipation of advanced sub-22 nm transistors and circuits. A prolonged phase of post-BD can ensure we can live with the circuit with moderate performance and error-free operation, even if the soft breakdown (SBD) events occur early. While analysis of post-BD is simple and straightforward for single layer SiO2 / SiON stacks, the number of possible scenarios of post-BD increases when analyzing high-κ – interfacial layer (HK-IL) based technology. This is because the sequence of BD (whether HK or IL fails first followed by the other one) and the competition between multiple SBD in one of these layers, dilative wear-out of a single SBD spot and the possibility of a successive localized BD above / below the HK/IL BD percolation spot (with or without metal filamentation) are all possible phenomena that can be classified as post-BD. The likelihood of occurrence of these various possibilities will determine the immunity of the stack to post-BD degradation. We will investigate each of these scenarios in detail in this work in order to provide a comprehensive assessment of post-BD reliability of state-of-the-art technology. Our analysis on a HK:IL = 25:12Ǻ stack supported by electrical, physical and modeling results provides clear evidence that circuit failure at operating conditions can only be due to multiple SBD events within the IL layer and that the HK is very robust and resilient to breakdown.

2012 - A study of the leakage current in TiN/HfO2/TiN capacitors [Articolo su rivista]
S., Cimino; Padovani, Andrea; Larcher, Luca; V. V., Afanas’Ev; H. J., Hwang; Y. G., Lee; M., Jurczac; D., Wouters; B. H., Lee; H., Hwang; L., Pantisano
abstract

Physical and electrical characteristics of Metal–Insulator–Metal TiN/HfO2/TiN capacitors have been investigated. A detailed study using internal photoemission and trap assisted transport simulation enabled the extraction of relevant important parameters like barrier height (2.5 eV) for both injecting interfaces, optical energy gap (5.6 eV), as well as trap density and energy position within the bandgap (NT = 3E19 cm-3; rT = 1E14 cm2; ET = 2.0–2.6 eV below the bottom of the HfO2 conduction band). The extracted parameters surprisingly showed striking similarities with HfO2 deposited on a Si surface, i.e., in MOSFET process flow. Additionally, Constant Voltage Stress showed a leakage current increase, preferentially at low voltage. This can be explained by preexisting defect precursors (likely related to oxygen vacancies) or by involvement of hydrogen in creating defects as observed on thermal SiO2 layers.

2012 - A WSN System Powered by Vibrations to Improve Safety of Machinery with Trailer [Relazione in Atti di Convegno]
Dondi, Denis; Napoletano, Giacomantonio; Bertacchini, Alessandro; Larcher, Luca; Pavan, Paolo
abstract

In this paper we present an energetically autonomous wireless sensor system designed to enhance safety in industrial machinery comprising a main vehicle with an attached trailer. The proposed system establishes a wireless link between the vehicle ECU and our sensors to provide motion dynamic data of trailer to the vehicle stability control algorithm. The wireless sensor devices we implemented comprise a 3-axial accelerometer and a 3-axial magnetometer to detect the trailer operating conditions. Such motion data are elaborated using an ultra-low power MCU, which communicates to vehicle’ ECU using an IEEE 802.15.4 channel at 2.4GHz. To enable perpetual operation of the system, we developed a vibrational energy harvesting system, VIBester, capable to gather kinetic energy from trailer natural vibrations and convert such energy in electrical energy for the system power supply. The vibrational energy harvester adopts a piezoelectric (PZT) transducer to convert the kinetic energy and a custom AC/DC converter to supply the wireless sensor device.

2012 - A 868MHz CMOS RF-DC Power Converter With -17dBm Input Power Sensitivity and Efficiency Higher Than 40% Over 14dB Input Power Range [Relazione in Atti di Convegno]
Scorcioni, Stefano; Bertacchini, Alessandro; Larcher, Luca
abstract

In this paper we present a novel CMOS RF-DC converter circuit, operating at 868MHz, for RFID and remote powering applications. The novel reconfigurable architecture of the converter allows the circuit to operate over a very wide input power range with very high efficiency compared with previous art works. Prototypes realized in STM 0.130ìm CMOS technology provide a regulated output voltage ~2V with a -17dBm input power sensitivity. The circuit efficiency, higher than 40% over a 14dB input power range, peaks at 60%.

2012 - An autonomous wireless sensor network device powered by a RF energy harvesting system [Relazione in Atti di Convegno]
Dondi, Denis; Scorcioni, Stefano; Bertacchini, Alessandro; Larcher, Luca; Pavan, Paolo
abstract

In this paper, we present an energetically autonomous wireless sensor network (WSN) device designed to enhance safety in vehicles capable to connect extra gear/equipment to the main chassis. The proposed system allows the vehicle stability control system to automatically recognize the connected trailer or implement through a purposely designed WSN device, which is integrated into trailer /implement and wirelessly sends its identification number. The WSN device we developed integrates also a novel RF energy harvesting circuit which gathers the energy from an 868MHz RF signal source, which is purposely transmitted from the vehicle towards the trailer or implement for remote powering. Measurements performed on fabricated WSN system prototypes show that the RF harvester can gather up to ≈50uW@3m from the RF power source with efficiency higher than 30% over a range of 10dBm. The combination of the RF energy harvesting circuit with the ultra-low power architecture and a custom task manager designed for the WSN system allows to further increase primary battery lifetime, making the wireless system capable to operate autonomously for several years.

2012 - Assessment of self-induced Joule-heating effect in the I − V readout region of polycrystalline Ge2Sb2Te5 Phase-Change Memory [Articolo su rivista]
G., Betti Beneventi; L., Perniola; Q., Hubert; A., Glière; Larcher, Luca; Pavan, Paolo; B., De Salvo
abstract

The physical mechanisms that regulate carrier transportin polycrystalline chalcogenides, such as Ge2Sb2Te5 (GST),are still debated. Recently, self-induced Joule-heating (SJH) effecthas been claimed to be the key factor in explaining the nonlinearityof the I–V characteristics of polycrystalline GST-basedphase-change memory (PCM). In this paper, we carefully investigatethe SJH occurring in the GST material by analyzing theI–V characteristics of PCM cells at low voltages, i.e., in thememory-cell readout region. To accomplish the study, we usead hoc fabricated PCM devices allowing an easier evaluationof SJH occurring in the chalcogenide layer. A novel procedureto test the SJH effect is also proposed. A comparison betweennumerical simulations and compact modeling is discussed as well.Our paper shows that the SJH effect is not sufficient to reproducethe experimental I–V nonlinearity, claiming for new experimentsand theoretical investigations. Therefore, this paper can be considereda step forward toward the comprehension of the transportproperties of polycrystalline GST, which is a key aspect for robustmodeling of PCM devices.

2012 - Broadband Printed Antenna for Radiofrequency Energy Harvesting [Relazione in Atti di Convegno]
Vincetti, Luca; Maini, Moreno; E., Pinotti; Larcher, Luca; S., Scorcioni; Bertacchini, Alessandro; D., Grossi; A., Tacchini
abstract

In this work a broadband UHF antenna with high inductive input impedance for radiofrequency energy harvesting is presented. It consists of a small feeding loop and a biconical radiating dipole. A prototype has been fabricated on a FR4 substrate and tested. Experimental results show a - 3dB power transmission bandwidth of about 135MHz (840MHz−975MHz).

2012 - Controlling uniformity of RRAM characteristics through the forming process [Relazione in Atti di Convegno]
Kalantarian, A.; Bersuker, G.; Gilmer, D. C.; Veksler, D.; Butcher, B.; Padovani, A.; Pirrotta, O.; Larcher, L.; Geer, R.; Nishi, Y.; Kirsch, P.
abstract

The proposed constant voltage forming (CVF) is shown to increase the resistances of the low resistance and high resistance states while reducing their variability. By forcing the forming in all devices to occur at the same predefined voltage, the CVF method eliminates a major cause of the device-to-device variation associated with the randomness of the forming voltage values. Moreover, both experiments and simulations show that CVF at lower voltages suppresses the parasitic overshoot current, resulting in a more controlled and smaller filament cross-section and lower operation currents. © 2012 IEEE.

2012 - Controlling Uniformity of RRAM Characteristics via the Forming Process [Relazione in Atti di Convegno]
A., Kalantarian; G., Bersuker; D. C., Gilmer; D., Veksler; B., Butcher; Padovani, Andrea; Pirrotta, Onofrio; Larcher, Luca; P., Kirsch; Y., Nishi
abstract

The proposed constant voltage forming (CVF) is shown to increase the resistances of the low resistance and high resistance states while reducing their variability. By forcing the forming in all devices to occur at the same predefined voltage,the CVF method eliminates a major cause of the device-to-device variation associated with the randomness of the forming voltage values. Moreover,both experiments and simulations show that CVF at lower voltages suppresses the parasitic overshoot current,resulting in a more controlled and smaller filament cross-section and lower operation currents.

2012 - DENSITY INFLUENCE ON AMORPHOUS HFO2 STRUCTURE: A MOLECULAR DYNAMICS STUDY [Relazione in Atti di Convegno]
Broglia, Giulia; Montorsi, Monia; Larcher, Luca; Padovani, Andrea
abstract

In this scenario, the aim of this work is to analyse systematically the influence of the materialdensity on the structure of amorphous HfO2 (a-HfO2). We will focus on investigating theatomic structure in the short, medium and long range in order to understand which is the preferential atomic structure.The molecular dynamics technique has been chosen for this analysis because it permits to investigate accurately the short and medium structural order of this material.

2012 - Enhancing Safety in Vehicles with Implement or Trailer using an Autonomous Wireless Sensor Network System [Relazione in Atti di Convegno]
Dondi, Denis; Bertacchini, Alessandro; Scorcioni, Stefano; Larcher, Luca; Pavan, Paolo
abstract

In this paper, we present an autonomous wireless sensor network system to enhance safety in vehicles with connected implement or trailer. Today’s vehicle stability control algorithms are used to enhance safety and prevent accidents, but they do not take into account if a trailer or an implement is connected to the front/rear of the vehicle. The proposed system allows overcoming this limitation by advertising to the vehicle’ electronic control unit the presence of the connected implement. This allows the stability control algorithm adjusting the vehicles parameters to current real conditions. The developed system comprises two wireless devices: a Wireless Master Device (WMD), mounted on the vehicle, and an autonomous Wireless End Device (WED), installed on the connected implement or trailer. The WED gathers energy from implement’ or trailer’ natural vibrations by using a vibrational energy harvester and a piezoelectric transducer. Thus, avoiding the need of frequent battery replacement and leading the wireless system to autonomously work for several years.

2012 - Evidences for vertical charge dipole formation in charge-trapping memories and its impact on reliability [Articolo su rivista]
Padovani, Andrea; A., Arreghini; Vandelli, Luca; Larcher, Luca; G., Van den bosch; J., Van Houdt
abstract

We demonstrate the formation of a vertical charge dipole in the nitride layer of TaN/Al2O3/Si3N4/SiO2/Si memories and use dedicated experiments and device simulations to investigate its dependence on program and erase conditions. We show that the polarity of the dipole depends on the program/erase operation sequence and demonstrate that is at the origin of the charge losses observed during retention. This dipole severely affects the retention of mildly programmed and erased states, representing a serious reliability concern especially for multi-level applications.

2012 - Leakage current in HfO2 stacks: from physical to compact modeling [Relazione in Atti di Convegno]
Larcher, Luca; Padovani, Andrea; Pavan, Paolo
abstract

In this paper we discuss the physical mechanisms governing the charge transport inside hafnium based dielectric stack from a modeling perspective. We propose a detailed Monte-Carlo physical model, which describes the charge transport across high-k stacks through the multiphonon trap-assisted-tunneling theory. This model reproduces accurately the voltage and temperature dependencies of the leakage current across HfO2-based stacks. Starting from this physical description, we develop an analytical model for the TAT current across high-k stacks, which can be implemented into SPICE-like circuit simulators. Despite the simplifying approximations, this compact model reproduces accurately the measurements, thus representing an effective tool for the investigation of the TAT currents.

2012 - Microscopic understanding and modeling of HfO2 RRAM device physics [Relazione in Atti di Convegno]
Larcher, Luca; Padovani, Andrea; Pirrotta, Onofrio; Vandelli, Luca; G., Bersuker
abstract

In this paper we investigate the physical mechanisms governing operations in HfOx RRAM devices. Forming set and reset processes are studied using a model including power dissipation associated with the charge transport, and the corresponding temperature increase, which assists ion diffusion.

2012 - New Insights into SILC Monitoring During TDDB Stress [Relazione in Atti di Convegno]
C. D., Young; G., Bersuker; M., Jo; K., Matthews; J., Huang; S., Deora; K. W., Ang; T., Ngai; Padovani, Andrea; Larcher, Luca; Chris, Hobbs; P. D., Kirsch
abstract

The breakdown (TDDB/SILC) characteristics of nMOS transistors with hafnium-based gate dielectric stacks of various zirconium content were investigated. It is found that the gate stack composition affects the SILC-voltage dependency while the voltage value chosen for SILC monitoring impacts significantly the SILC-based lifetime projection. For the worst case lifetime evaluation, SILC should be monitored at its maximum value rather than at any pre-defined, fixed voltage.

2012 - New insights into SILC-based life time extraction [Relazione in Atti di Convegno]
Young, C. D.; Bersuker, G.; Jo, M.; Matthews, K.; Huang, J.; Deora, S.; Ang, K. W.; Ngai, T.; Hobbs, C.; Kirsch, P. D.; Padovani, A.; Larcher, Luca
abstract

2012 - Optimized CMOS RF-DC converters for remote wireless powering of RFID applications [Relazione in Atti di Convegno]
Scorcioni, Stefano; Larcher, Luca; Bertacchini, Alessandro
abstract

In this paper, we present for the first time a novel optimization procedure which allows to maximize the efficiency of RF-DC energy harvester converters, taking into account the contributions of the matching network. Thanks to this procedure, we have designed and realized a CMOS RF-DC converter operating in a very wide range of input power -14÷+1dBm with a peak efficiency of 45%. The RF-DC converter provides a constant output voltage ~2V in the whole input power range thanks to a smart voltage regulator integrated with the converter.

2012 - Random Telegraph Signal Noise Properties of HfOx RRAM in High Resistive States [Relazione in Atti di Convegno]
Puglisi, Francesco Maria; Pavan, Paolo; Padovani, Andrea; Larcher, Luca; G., Bersuker
abstract

In this paper we analyze Random Telegraph Signal (RTS) noise in hafnium-based RRAMs. RTS is measured in HRS, showing fast and slow multilevel switching events. RTS characteristics are examined through novel color-coded time-lag plots and Hidden Markov Model (HMM) time-series analyses. Noise is examined at different reset conditions to provide new insights on conduction mechanisms in HRS. Higher reset voltages result in an enhanced complexity in RTS due to a larger number of active traps

2012 - RF to DC CMOS rectifier with high efficiency over a wide input power range for RFID applications [Relazione in Atti di Convegno]
Scorcioni, Stefano; Bertacchini, Alessandro; Larcher, Luca; Ricciardi, Antonio; Dondi, Denis; Pavan, Paolo
abstract

In this paper we present a RF-DC rectifier which operates over a wide range of input power by providing a regulated output DC voltage. The circuit solution we propose is based on a novel active load circuit which adjusts the output current as a function of the incoming RF power. This allows maximizing both the efficiency and sensitivity of the circuit. Circuit prototypes fabricated in 130nm CMOS technology start to operate at -14dBm, providing a regulated output voltage of 1.6÷1.8V in the -14÷1dBm RF input power at 868MHz. Noticeably, the circuit efficiency of the rectifier peaks at 45%, remaining above 30% in the -12÷+1dBm input power range.

2012 - Self Powered Wireless Sensors for Chassis, Powertrain, Working Equipment and Trailed Implements [Relazione in Atti di Convegno]
Carli, Davide; Dondi, Denis; Bertacchini, Alessandro; Larcher, Luca; Ruggeri, Massimiliano
abstract

Performance request, needs for fleet management, vehicle safety and diagnosis control strategies, ask for a continuous increase in vehicle functionalities and work variables observability and controllability. Self-powered wireless sensors, capable of collecting the power they consume directly from the vibrations available on the spot, could play a key role in enabling the sensorization of all those parts of heavy duty machines and trailed vehicles which cannot be connected through wires. This paper investigates the applicability of self-powered wireless sensor nodes to heavy duty machinery by analyzing the energy budget which can be reached by these devices in such operating environment. A survey of possible sensor design is presented, to examine the energy requirements for signal transduction and wireless data transmission, and to highlight most relevant design issues affecting power consumption. Two representative case studies are then considered, to estimate with the support of original experimental data the amount of energy that can be generated by placing state-of-the-art off-the-shelf energy transducers on the gearbox of a baler and on the rear chassis of a tractor. By comparing the amounts of generated and required energy, this paper demonstrates the feasibility of self-powered wireless sensors in the context of heavy duty machines.

2012 - Understanding the Role of the Ti Metal Electrode on the Forming of HfO2-based RRAMs [Relazione in Atti di Convegno]
Padovani, Andrea; Larcher, Luca; Pavan, Paolo; C., Cagli; B., de Salvo
abstract

In this paper we investigate in details the effects of the Ti metal electrode on the forming operation in HfO2 RRAM devices. Starting from electrical data and physico-chemical analysis, we use physics-based RRAM modeling to understand the physics governing the CF formation in RRAM stacks with Ti electrodes. Simulations show that the lower forming voltage typically observed in these devices is due to the Ti-induced formation of a sub-stoichiometric HfOx region in the resistive switching layer. The model allows extracting the characteristics of this sub-stoichiometric region that are crucial for developing future low-voltage RRAM devices.

2012 - Wireless self-powered transponders and multi-sensing unit to enable ISOBUS identification of mechanical implements [Relazione in Atti di Convegno]
Mainardi, Gino; Ferretti, Marco; Marzani, Stefano; Scorcioni, Stefano; Bertacchini, Alessandro; Larcher, Luca; Dondi, Denis; Fantesini, Lorenzo
abstract

In this paper we present an ISOBUS-based system for the automatic identification of implements or trailers without electronics on board. This allows, firstly, to improve the pre-crash prevention because the performances of existing vehicle dynamics control systems can now take into account data coming from the implements or trailers connected to the tractor. Secondly, publishing on the ISOBUS network the information of all the implements, including the ones without electronics, permits a full deployment of precision farming techniques. The absence of electrical or electronic equipment on implements forces the use of wireless solutions, which in turn are limited by their battery lifetime, which is not comparable with the typical lifetime of trailers or implements. To overcome this limiting factor we added energy harvesting capabilities to the developed transponder, which is able to store energy coming from mechanical vibrations occurring during the normal implement working conditions.

2011 - A Comprehensive Understanding of the Erase of TANOS Memories Through Charge Separation Experiments and Simulations [Articolo su rivista]
Padovani, Andrea; A., Arreghini; Vandelli, Luca; Larcher, Luca; G., Van den Bosh; Pavan, Paolo; J., Van Houdt
abstract

We investigate and quantify the role played by electrons and holes during the erase operation of TANOS memories by means of charge separation experiments and physics-based simulations. Results demonstrate that electron emission via trap to-band tunneling dominates the first part of the erase operation, whereas hole injection prevails in the remaining part of the transient. In addition, we show that the efficiency of the erase operation is high and constant mainly because of the high energy offset between nitride and alumina valence bands. Our results clearly identify the physical mechanisms responsible for TANOS erase and allow deriving some important guidelines for the optimization of this operation.

2011 - A Physical model of the temperature dependence of the current through SiO2/HfO2 stacks [Articolo su rivista]
Vandelli, Luca; Padovani, Andrea; Larcher, Luca; R. G., Southwick III; W. B., Knowlton; G., Bersuker
abstract

In this paper, we investigate the characteristics of the defects responsible for the leakage current in the SiO2 and SiO2/HfO2 gate dielectric stacks in a wide temperature range (6 K–400 K). We simulated the temperature dependence of the I–V characteristics both at positive and negative gate voltages by applying the multiphonon trap-assisted tunneling model describing the charge transport through the dielectric. In the depletion/weak inversion regime, the current is limited by the supply of carriers available for tunneling. In strong inversion, the temperature dependence is governed by the charge transport mechanisms through the stacks; in particular, in SiO2/HfO2 dielectric stacks, the coupling of the injected carriers with the dielectric phonons at the trap sites is the dominant mechanism.Matching the simulation results to the measurement data allows extracting important trap parameters, e.g., the trap relaxation and ionization energies, which identify the atomic structure of the electrically active defects in the gate dielectric.

2011 - A Physics-Based Model of the Dielectric Breakdown in HfO2 for Statistical Reliability Prediction [Relazione in Atti di Convegno]
Vandelli, Luca; G., Bersuker; Padovani, Andrea; J. H., Yum; Larcher, Luca; Pavan, Paolo
abstract

We present a quantitative physical model describingthe current evolution due to the formation of a conductivefilament responsible for the HfO2 dielectric breakdown. Bylinking the microscopic properties of the stress-generatedelectrical defects to the local power dissipation and to thecorresponding temperature increase along the conductive paththe model reproduces the rapid current increase observed duringthe breakdown. The model successfully simulates theexperimental time-dependent dielectric breakdown distributionsmeasured in HfO2 MIM capacitors under constant voltage stress,thus providing a statistical reliability prediction capability, whichcan be extended to other high-k materials, multilayer stacks,resistive memories based on transition metal oxides, etc.

2011 - A Vibration-Powered Wireless System to Enhance Safety in Agricultural Machinery [Relazione in Atti di Convegno]
Scorcioni, Stefano; Bertacchini, Alessandro; Dondi, Denis; Larcher, Luca; Pavan, Paolo; G., Mainardi
abstract

In this paper, we present a wireless sensing system capable to enhance safety in agricultural machinery. Modern farm tractors adopt vehicle stability control algorithms to enhance safety and prevent accidents. The main limitation in current approach is that the tractor has no information about the implement connected on the front/rear. The system we propose provides information on the connected implement to the tractor control unit allowing the vehicle characteristics update, which allows enhancing vehicle safety. The system we developed is comprised of two wireless devices. The first one, called Master Device (MD), is mounted on the tractor and receives power supply from on-board electrical system. The second one, called End-Device (ED), is mounted on the implement and gathers the supply energy from implement natural vibrations by using a vibrational energy harvester and a piezoelectric transducer. With this approach, the device can recharge an energy reservoir (e.g. a battery) during the implement usage, thus avoiding the need of frequent battery replacement and leading the wireless system to autonomously work for several years.

2011 - AlN-based MEMS devices for vibrational energy harvesting applications [Relazione in Atti di Convegno]
Bertacchini, Alessandro; Scorcioni, Stefano; Dondi, Denis; Larcher, Luca; Pavan, Paolo; M. T., Todaro; A., Campa; G., Caretto; S., Petroni; A., Passaseo; M., De Vittorio
abstract

This paper presents a new AlN-based MEMS devices suitable for vibrational energy harvesting applications. Due to their particular shape and unlike traditional cantilever which efficiently harvest energy only if subjected to stimulus in the proper direction, the proposed devices have 3D generation capabilities solving the problem of device orientation and placement in real applications. Thanks to their particular shape, the realized devices present more than one fundamental resonance frequencies in a range comprised between 500 Hz and 1.5 kHz, with a voltage generation higher than 300μV and an output power up to 0.4 pW for single MEMS device.

2011 - An Inductor-less 13.5-Gbps 8-mW Analog Equalizer for Multi-Channel Multi-Frequency Operation [Relazione in Atti di Convegno]
Ganzerli, Marcello; Larcher, Luca; S., Erba; D., Sanzogni
abstract

A low-power analog equalizer has been realized in 45nmCMOS technology. Active feedback is employed to avoidinductors and save chip area. Peaking frequency and gain boostcan be finely controlled from 4GHz to 7GHz and from 0dB to24dB to allow multi-channel multi-frequency operation. Thecircuit dissipates 8mW from a 1.1V supply and it occupies0.009mm2. The measured maximum peak-to-peak jitter was 29psfor a 13.5Gb/s data transmission over a 18dB-loss backplane.

2011 - Bent AlN Piezoelectric Resonators for Vibrational Energy Harvesting [Relazione in Atti di Convegno]
M. T., Todaro; G., Caretto; G., Epifani; A., Campa; Bertacchini, Alessandro; Larcher, Luca; A., Passaseo; M., De Vittorio
abstract

Advances in the areas of wireless technology and low-power electronics stimulated extensively research in energy harvesting from environmental vibration sources. Aluminium nitride (AlN) thin films are promising for piezoelectric microsensors/ microactuators as well as for energy harvesting devices, due to their interesting material properties including good piezoelectric and electromechanical coupling coefficients, low permittivity and high Young's modulus. Despite this, few works reported on the AlN employment for the realization of micro power generators. In such cases, devices have a typical flat cantilever shape (M. Marzencki et al. 2008, R. Elfrink et al. 2009) and sizes are larger than that of most typical MEMS devices. This paper reports on the fabrication and analysis of bent AlN cantilever resonators based on a novel and simple fabrication process without additional proof masses for energy harvesting at frequencies around 2 KHz.

2011 - Carbon-doped GeTe: A promising material for Phase-Change Memories [Articolo su rivista]
G., Betti Beneventi; L., Perniola; V., Sousa; E., Gourvest; S., Maitrejean; J. C., Bastien; A., Bastard; B., Hyot; A., Fargeix; C., Jahan; J. F., Nodin; A., Persico; A., Fantini; D., Blachier; A., Toffoli; S., Loubriat; A., Roule; S., Lhostis; H., Feldis; G., Reimbold; T., Billon; B., De Salvo; Larcher, Luca; Pavan, Paolo; D., Bensahel; P., Mazoyer; R., Annunziata; P., Zuliani; F., Boulanger
abstract

This paper investigates Carbon-doped GeTe (GeTeC) as novel material for Phase-Change Memories (PCM). In the first part of the manuscript, a study of GeTeC blanket layers is presented. Focus is on GeTeC amorphous phase stability, which has been studied by means of optical reflectivity and electrical resistivity measurements, and on GeTeC structure and composition, analyzed by XRD and Raman spectroscopy. Then, electrical characterization of GeTeC-based PCM devices is reported: resistance drift, data retention performances, RESET current and power, and SET time have been investigated. Very good data retention properties and reduction of RESET current make GeTeC suitable for both embedded and stand-alone PCM applications, thus suggesting GeTeC as promising candidate to address some of the major issues of today’s PCM technology.

2011 - Charge transport in high-k stacks for charge-trapping memory applications: A modeling perspective (invited) [Articolo su rivista]
Larcher, Luca; Padovani, Andrea; Vandelli, Luca; Pavan, Paolo
abstract

Charge trapping (CT) memories could be a promising technology option for further NAND Flash scaling. The assessment of the scalability limits and ultimate performances of this technology demands for the comprehensive understanding of the physical mechanisms governing device operation and reliability, which requires accurate physics-based models reproducing the electrical device characteristics. The basic features of the models presented in the literature for CT memory devices are reviewed, underlining theirsimilarities and differences, and highlighting their importance in order to achieve a comprehensive understanding of the physical mechanisms responsible for CT device operation and reliability. A physical model describing the charge transport in nitride and high-j stacks is also presented, which allows gaining further insights into reliability issues related to charge localization and high-j tunnel and blocking dielectrics, like the effects of the blocking alumina layer and the band-gap engineered tunnel dielectrics on the TANOS device retention.

2011 - Charge trapping in alumina and its impact on the operation of metal-alumina-nitride-oxide-silicon memories: experiments and simulations [Articolo su rivista]
Padovani, Andrea; Larcher, Luca; DELLA MARCA, Vincenzo; Pavan, Paolo; H., Park; G., Bersuker
abstract

We investigate electron/hole trapping phenomena in alumina blocking oxide and their impact on the program/erase operations and retention of TANOS memory devices. For this purpose, we perform simulations using a physical model reproducing charge injection/trapping in TANOS devices, which is extended to account for the charge trapping phenomena in the blocking layer. We derive the electrical characteristics of both electron and hole traps in Al2O3 by reproducing the measured program, erase and retention transients. Our results show that the amount of electron charge trapped in the alumina during a program operation strongly depends on the stack composition and program voltages and can account for up to 25% of the total threshold voltage shift, whereas hole trapping during erase is negligible. Finally, we investigate the degradation of retention caused by the electron trapping in the alumina blocking layer, which is shown to result in accelerated charge loss.

2011 - Comprehensive physical modeling of forming and switching operations in HfO2 RRAM devices [Relazione in Atti di Convegno]
Vandelli, Luca; Padovani, Andrea; Larcher, Luca; G., Broglia; G., Ori; Montorsi, Monia; G., Bersuker; Pavan, Paolo
abstract

In this work we apply a physical model based on charge transport and molecular mechanics/dynamics simulations to investigate the physical mechanisms governing the RRAM forming and switching operations. The proposed model identifies the major driving forces controlling conductive filament (CF) formation and changes during RRAM switching, thus providing a tool for investigation and optimization of RRAM devices.

2011 - Connecting electrical and structural dielectric characteristics [Relazione in Atti di Convegno]
G., Bersuker; D., Veksler; C. D., Young; H. Park W., Taylor; P., Kirsch; R., Jammy; Morassi, Luca; Padovani, Andrea; Larcher, Luca
abstract

An attempt is made to correlate electrical measurement results to specific defects in the dielectric stacks of high-k/metal gate devices. Defect characteristics extracted from electrical data were compared to those obtained by ab initio calculations of the dielectric structures. It is demonstrated that oxygen vacancies in a variety of charge states and configurations in the interfacial SiO2 layer of the high-k gate stacks contribute to random telegraph noise signal, time-dependent dielectric breakdown, and the flatband voltage roll-off phenomenon.

2011 - Errors affecting split-CV mobility measurements in InGaAs MOS-HEMTs [Relazione in Atti di Convegno]
Morassi, Luca; Verzellesi, Giovanni; Larcher, Luca; H., Zhao; J. C., Lee
abstract

The accuracy of the split-CV mobility extraction method is analyzed in implant-free, buried-channel InGaAs MOS-HEMTs with Al2O3 gate dielectric through a “simulated experiment” procedure. The different error sources affecting the method accuracy are pointed out. As a result of these errors, the split-CV mobility can appreciably underestimate the actual channel mobility under on-state conditions.

2011 - Experimental and Theoretical Study of Electrode Effects in HfO2 based RRAM [Relazione in Atti di Convegno]
C., Cagli; J., Buckley; V., Jousseaume; A., Salaun; H., Grampeix; J. F., Nodin; H., Feldis; A., Persico; J., Cluzel; P., Lorenzi; L., Massari; R., Rao; F., Irrera; T., Cabout; F., Aussenac; C., Carabasse; M., Coue; P., Calka; E., Martinez; L., Perniola; P., Blaise; Z., Fang; Y. H., Yu; G., Ghibaudo; D., Deleruyelle; M., Bocquet; C., Muller; Padovani, Andrea; Pirrotta, Onofrio; Vandelli, Luca; Larcher, Luca; G., Reimbold; B., de Salvo
abstract

In this work, the impact of Ti electrodes on the electricalbehaviour of HfO2-based RRAM devices is conclusivelyclarified. To this aim, devices with Pt, TiN and Ti electrodeshave been fabricated. We first provide severalexperiments to clearly demonstrate that switching is driven bycreation-disruption of a conductive filament. Thus, the role ofTiN/Ti electrodes is explained and modeled based on thepresence of HfOx interfacial layer underneath the electrode. Inaddition, Ti is found responsible to activate bipolar switching.Moreover, it strongly reduces forming and switching voltageswith respect to Pt-Pt devices. Finally, it positively impacts onretention. To support and interpret our results we providephysico-chemical measurements, electrical characterization,ab-initio calculations and modeling.

2011 - Freestanding piezoelectric rings for high efficiency energy harvesting at low frequency [Articolo su rivista]
A., Massaro; S., De Guido; I., Ingrosso; R., Cingolani; M., De Vittorio; M., Cori; Bertacchini, Alessandro; Larcher, Luca; A., Passaseo
abstract

Energy harvesting at low frequency is a challenge for microelectromechanical systems. In this work we present a piezoelectric vibration energy harvester based on freestanding molybdenum (Mo) and aluminum nitride (AlN) ring-microelectromechanical-system (RMEMS) resonators. The freestanding ring layout has high energy efficiency due to the additional torsional modes which are absent in planar cantilevers systems. The realized RMEMS prototypes show very low resonance frequencies without adding proof masses, providing the record high power density of 30.20 μW mm-3 at 64 Hz with an acceleration of 2g. The power density refers to the volume of the vibrating RMEMS layout.

2011 - Grain boundary-driven leakage path formation in HfO2 dielectrics [Articolo su rivista]
G., Bersuker; J., Yum; Vandelli, Luca; Padovani, Andrea; Larcher, Luca; V., Iglesias; M., Porti; M., Nafría; K., Mckenna; A., Shluger; P., Kirsch; R., Jammy
abstract

The evolution over time of the leakage current in HfO2-based MIM capacitors under continuous or periodic constant voltage stress (CVS) was studied for a range of stress voltages and temperatures. The data were analyzed based on the results of conductive atomic force microscopy (AFM) measurements demonstrating preferential current flow along grain boundaries (GBs) in the HfO2 dielectric and ab initio calculations, which show the formation of a conductive sub-band due to the precipitation of oxygen vacancies at the GBs. The simulations using the statistical multi-phonon trap-assisted tunneling (TAT) current description successfully reproduced the experimental leakage current stress time dependency by using the calculated energy characteristics of the O-vacancies. The proposed model suggests that the observed reversible increase in the stress current is caused by segregation of the oxygen vacancies at the GBs and their conversion to the TAT-active charge state caused by reversible electron trapping during CVS.

2011 - Low Power RRAM with Improved HRS/LRS Uniformity through Efficient Filament Control Using CVS Forming [Relazione in Atti di Convegno]
A., Kalantarian; G., Bersuker; D. C., Gilmer; B., Butcher; Padovani, Andrea; Vandelli, Luca; Larcher, Luca; R., Geer; Y., Nishi; P., Kirsch
abstract

Resistance change memory (RRAM) based on transition metal oxides (TMO), whose operation is based on the change in resistivity of a conductive filament in the oxide material, has attracted a lot of attention in recent years due to its promise of high density, speed, and retention. However, achieving a low power operation and high device-to-device uniformity of the cell resistance states are the major challenges for practical applications of the RRAM technology. While some progress has been made on the understanding of the switching mechanism of TMO memory devices [1], lack of precise control over the filament formation, perceived to be a random process, which inturn introduces randomness into the switching characteristics ofthis class of devices, complicates further progress. This studydemonstrates a forming methodology, which addresses the abovediscussed issues by performing a forming operation under theconstant voltage stress (CVS) condition at lower voltages ratherthan by the conventionally used fast voltage ramp method. Thisapproach is shown to lower the reset current, increase resistivityof the low and high resistive states (LRS, HRS) and improvedevice to device uniformity in the HfO2-based RRAM devices.

2011 - Metal oxide resistive memory switching mechanism based on conductive filament properties [Articolo su rivista]
G., Bersuker; D. C., Gilmer; D., Veksler; P., Kirsch; Vandelli, Luca; Padovani, Andrea; Larcher, Luca; K., Mckenna; A., Shluger; V., Iglesias; M., Porti; M., Nafría
abstract

By combining electrical, physical, and transport/atomistic modeling results, this study identifies critical conductive filament (CF) features controlling TiN/HfO2/TiN resistive memory operations. The leakage current through the dielectric is found to be supported by the oxygen vacancies, which tend to segregate at hafnia grain boundaries. We simulate the evolution of a current path during the forming operation employing the multi-phonon trap-assisted tunneling (TAT) electron transport model. The forming process is analyzed within the concept of dielectric breakdown, which exhibits much shorter characteristic times than that of the electroforming process conventionally employed to describe the formation of the conductive filament. The resulting conductive filament is calculated to produce a non-uniform temperature profile along its length during the reset operation, promoting preferential oxidation of the filament tip. A thin dielectric barrier resulting from the CF tip oxidation is found to control filament resistance in the high resistance state. Field-driven dielectric breakdown of this barrier during the set operation restores the filament to its initial low resistive state. These findings point to the critical importance of controlling the filament resistance characteristics (cross section, stoichiometry) during forming to achieve low power RRAM cell switching.

2011 - Modeling of the forming operation in HfO2-base resistive switching memories [Relazione in Atti di Convegno]
Vandelli, Luca; Padovani, Andrea; Larcher, Luca; G., Bersuker; D., Gilmer; Pavan, Paolo
abstract

This paper presents a novel physical description of the forming process in HfO2-based resistive switching memory devices (RRAM). By taking into consideration a grain boundary-driven trap-assisted electron transport and accounting for the local power dissipation and the associated local temperature increase, which assists defect generation, the model reproduces quantitatively the evolution of the leakage current observed during the forming operation in the RRAM devices. The model statistical capabilities allow reproducing the statistical distribution of the forming voltage, thus providing a powerful tool for the assessment of the feasibility of these devices for high-capacity non-volatile memory mass storage applications

2011 - Modeling strategies for flash memory devices [Relazione in Atti di Convegno]
Padovani, Andrea; Larcher, Luca; Pavan, Paolo
abstract

In this paper, we will review the modeling strategies for standard and advanced Flash memory devices based on Floating Gate devices developed by our research group in the last ten years. We will show a complete compact model that includes program/erase and leakage currents that can be used to simulate memory cells in both DC (read operation) and transient conditions (Program/Erase). The same model can be used also for reliability simulations by providing good descriptions of the degradation mechanisms. We will also show the extended model for circuit simulation of NAND strings, modified to account for capacitive coupling effects. Finally, we will show how the same framework can be used to develop a compact model for operations of advanced planar charge-trapping memory devices.

2011 - Modeling the charge transport and degradation in HfO 2 dielectric for reliability improvement and life-time predictions in logic and memory devices [Relazione in Atti di Convegno]
Padovani, Andrea; Larcher, Luca; Vandelli, Luca; Pirrotta, Onofrio; Pavan, Paolo
abstract

HfO2 is currently used in the gate stacks of CMOS logic devices and is widely investigated for its potential application in advanced non-volatile memories such as resistive switching devices (RRAMs). In both applications, the understanding of the physical mechanisms governing the charge transport and the degradation/breakdown (BD) of the dielectric is fundamental to optimize device operation and reliability, and represents the first step toward accurate lifetime predictions. These goals can be achieved through the development of accurate physics-based models linking the microscopic properties of HfO2 to the electrical behavior of the device. We show the model we developed for the charge transport and degradation in HfO2 and its application to logic and memory devices.

2011 - Physical modeling of charge transport and degradation in HfO 2 stacks for logic device and memory applications [Relazione in Atti di Convegno]
Larcher, Luca; Padovani, Andrea; Vandelli, Luca; G., Bersuker
abstract

The understanding of the physical mechanisms responsible of charge transport and degradation in high-k stacks is fundamental for the optimization of advanced logic (MOSFETs) and memory (RRAM, DRAM) devices. In this paper, we present a comprehensive physical model describing the charge transport and the degradation/breakdown processes in the HfO2 layer. This model allows gaining quantitative insights into the physics governing leakage current and degradation processes in HfO2 stacks, reproducing gate current and TDDB statistics

2011 - Threshold Shift Observed in Resistive Switching in Metal-Oxide-Semiconductor Transistors and the Effect of Forming Gas Anneal [Articolo su rivista]
W. H., Liu; K. L., Pey; X., Wu; N., Raghavan; Padovani, Andrea; Larcher, Luca; Vandelli, Luca; M., Bosman; T., Kauerauf
abstract

In this paper the resistive switching mechanism, which is crucial for the operations of RRAM devices, is investigated using HfO2 based MOSFETs. After the SET operation, MOSFETs exhibit a threshold voltage (VT) shift that is found to be closely related to the formation of conductive filaments in the gate oxide. The RESET operation performed through a forming gas anneal treatment is found to have the same effect of applying a reverse polarity gate voltage sweep, as usually done in bipolar switching RRAM devices. After RESET, the gate current and VT measured shift back to their pristine levels, indicating the passivation of oxygen vacancies (forming the conductive path) as the most likely physical mechanism responsible for RRAM’s RESET operation. TEM analysis and physical simulations support these conclusions.

2010 - A Micro Fuel Cell Power Supply Module for Low Power Portable Applications [Relazione in Atti di Convegno]
Bertacchini, Alessandro; Scorcioni, Stefano; Cori, Marco Maria; Larcher, Luca; Pavan, Paolo; J. P., Esquivel; N., Torres Herrero; N., Sabaté; J., Santander
abstract

This paper presents a power supply module targeted for low-power applications in the sub-mW range incorporating a passive µDMFC fuel cell (µFC) acting as energy source and a customized boost converter for the energy conversion. The low power budget of such energy sources mandates the adoption of efficient circuit solutions, hence the design of the boost converter is challenging. To maximize the lifetime of the micro fuel cell, the module has been designed to keep constant the µFC operating point and it has been optimized to work with µFC providing a voltage in the range 280mV-320mV, reaching a maximum efficiency higher than 65% with an output power of 350 μW.

2010 - A novel Algorithm for the Solution of Charge Transport Equations in MANOS Devices Including Charge Trapping in Alumina and Temperature Effects [Relazione in Atti di Convegno]
Padovani, Andrea; Larcher, Luca
abstract

We present a new algorithm for the exact solutionof the system of equations describing charge trappingand transport across the dielectric stack of nitridebasedcharge trapping memories. The algorithm is implementedin a physical MANOS model accounting fortemperature effects and charge trapping into the Al2O3blocking layer. The model reproduces threshold voltageshifts measured at different temperatures on differentMANOS stacks.

2010 - A Physical Model for Post-Breakdown Digital Gate Current Noise [Articolo su rivista]
Padovani, Andrea; Morassi, Luca; N., Raghavan; Larcher, Luca; L., Wenhu; K. L., Pey; G., Bersuker
abstract

We present a new physical model that enables us to reproduce the digital gate current Random Telegraph Noise (RTN) fluctuations observed in ultra-thin SiON dielectrics in the early stages of post breakdown (BD). Gate current (IG) fluctuations are modeled assuming that some traps in the BD path switch between two unstable configurations, corresponding to neutral and negatively charged O vacancies. Energy levels of the trap considered in simulations here are consistent with values calculated from atomistic simulations. The model allows to reproduce accurately the mean and variation in the IG fluctuations observed on 16Å and 22Å thick SiON gate dielectric at different gate voltages.

2010 - Analysis of interface-trap effects in inversion-type InGaAs/ZrO2 MOSFETs [Relazione in Atti di Convegno]
Morassi, Luca; Verzellesi, Giovanni; Padovani, Andrea; Larcher, Luca; Pavan, Paolo; D., Veksler; Injo, Ok; G., Bersuker
abstract

Interface-trap effects are analyzed in inversion-type, self-aligned In0.53Ga0.47As and In0.53Ga0.47As/In0.2Ga0.8As MOSFETs with ALD ZrO2 gate dielectric. Interface-trap densities in the order of 1e13 cm-2 eV-1 are required to explain the measured subthreshold slopes. For these Dit values, donor-like interface traps are compatible with threshold-voltage values in the 0-0.15 V range as those observed in these devices. Moreover, the presence of donor-like interface traps can explain the negative threshold-voltage shift induced by the inclusion of the In0.2Ga0.8As cap layer, as the result of the influence of interface traps located at the In0.2Ga0.8As/ZrO2 interface on the inversion channel forming at the In0.53Ga0.47As/In0.2Ga0.8As interface.

2010 - Carbon-doped GeTe Phase-Change Memory featuring remarkable RESET current reduction [Relazione in Atti di Convegno]
G., Betti Beneventi; L., Perniola; A., Fantini; D., Blachier; A., Toffoli; E., Gourvest; S., Maitrejean; V., Sousa; C., Jahan; J. F., Nodin; A., Persico; S., Loubriat; A., Roule; S., Lhostis; H., Feldis; G., Reimbold; T., Billon; B., De Salvo; Larcher, Luca; Pavan, Paolo; D., Bensahel; P., Mazoyer; R., Annunziata; F., Boulanger
abstract

In this paper we present a study of Phase-Change non-volatile Memory (PCM) devices integrating carbon-doped GeTe as chalcogenide material. Carbon-doped GeTe, named GeTeC, remarkably lowers the RESET current and features very good data retention properties as well. In particular, GeTe PCM with 10% carbon inclusions (named GeTeC10%) yields about 30% of RESET current reduction with respect to pure GeTe and GST. Furthermore, our GeTeC10% memory cells are expected to guarantee a 10-years-lifetime-temperature of about 127°C, which is one of the highest ever reported for PCM. The outstanding properties of GeTeC make this material promising for non-volatile memory technologies.

2010 - Charge loss in TANOS devices caused by Vt sensing measurements during retention [Relazione in Atti di Convegno]
H., Park; G., Bersuker; D., Gilmer; K. Y., Lim; M., Jo; H., Hwang; Padovani, Andrea; Larcher, Luca; Pavan, Paolo; W., Taylor; P. D., Kirsch
abstract

In TANOS stuctures in retention, the major decrease in theprogrammed threshold voltage is found to be caused by the Vtsensing (IdVg measurements) rather than by intrinsic charge loss(when no bias is applied). This Vt decrease can be understoodwithin the process of the temperature-activated charge transportthrough the Al2O3 blocking oxide. The charge loss can beminimized when Vt sensing time is decreased down to microseconds. Blocking oxides engineered by adding a thin SiO2 layerat the SiN/AlO interface demonstrate significant suppression of thecharge loss.

2010 - Current status of EUV lithography development in Japan [Relazione in Atti di Convegno]
Larcher, L.
abstract

Extreme Ultraviolet Lithography (EUVL) is a promising candidate for the device fabrication at feature sizes of a half pitch of 32 nm and below. An EUV lithography system (Fig. 1) is composed of various subsystems, such as the source, the optics, the exposure system, a mask, and the resist. Currently various consortia, private companies, universities, and research institutes are working on the development of EUV lithography in Japan. EUVA and Selete are involved in an EUVL-related NEDO project under MIRAI Scheme. The recent activities of those projects are described in this presentation. © 2010 IEEE.

2010 - Experimental assessment of electrons and holes in erase transient of TANOS and TANVaS memories [Articolo su rivista]
A., Suhane; A., Arreghini; G., Van den bosch; Vandelli, Luca; Padovani, Andrea; L., Breuil; Larcher, Luca; K., De Meyer; J., Van Houdt
abstract

We present carrier separation experiments based on direct charge measurement to assess the contributions of electronsand holes to the erase transient of TANOS-like nonvolatilememories. The role of the different carrier species is analyzed asa function of the erase voltage and of the charge configurationat the initial programmed state. We extend the analysis toBand Engineered tunneling barriers, demonstrating that theperformance improvement in these devices lays more in anenhancement of the hole current rather than of the electron one.

2010 - Fundamental reliability issues of advanced charge-trapping Flash memory devices [Relazione in Atti di Convegno]
Larcher, Luca; Padovani, Andrea
abstract

The basic reliability issues of Charge Trapping (CT) Flash memory devices will be discussed from a physical perspective, highlighting the reliability implications of process and technology innovations introduced to sustain the uninterrupted device scaling down. We will focus on the reliability issues related to the charge localization inside the trapping layer and the high-κ band-gap engineered stacks introduced to implement both tunnel and blocking dielectrics. We will describe the physical mechanisms responsible of reliability degradation (data retention, array disturbs, endurance), discussing briefly the issues related to ultra-scaled and vertically stacked 3D Flash memory devices.

2010 - Gate Leakage Current Reduction in Two-Step Processed High-k Dielectrics for Low Power Applications [Relazione in Atti di Convegno]
G., Bersuker; D., Heh; J., Huang; C. S., Park; Padovani, Andrea; Larcher, Luca; P., Kirsch; R., Jammy
abstract

Reduction of the gate leakage current in nMOS high-k devices is demonstrated by an engineered two-step deposited Hf-based high-k dielectric film. The electrical characteristics and reliability of the devices fabricated using the proposed two-step and conventional one-step high-k gate stacks are shown to be comparable. The lower leakage current is attributed to the misalignment of the grain boundaries in the multi-layer high-k dielectrics.

2010 - High-k related reliability issues in advanced Non-Volatile Memories [Articolo su rivista]
Larcher, Luca; Padovani, Andrea
abstract

In the last decade, important technology solutions have been proposed to scale down Flash memory devices beyond the 30nm node. The most important innovations are the introduction of charge trapping layer and high- materials in both bottom and top dielectric stacks, which allows reducing both the bottom dielectric thickness and the Program/Erase voltages, while maintaining the P/E performances and (theoretically) without degrading the memory device reliability. Theoretical advantages and reliability issues of these important innovations will be reviewed by addressing physical mechanisms responsible of reliability degradation. In particular, charge trapping layers introduced in place of the poly-silicon FG will be discussed highlighting the reliability consequences of the discrete charge storage. Similarly, theoretical advantages and reliability issues of bottom and top dielectric stacks incorporating high- materials (used mainly also to implement band-gap engineered barriers) will be carefully analyzed, relating high- material properties to memory device performances and reliability.

2010 - Investigation of the impact of H-related defects in Al2O3 blocking layer of charge-trap memories by atomistic simulations and device physical modeling [Relazione in Atti di Convegno]
G., Molas; L., Masoero; P., Blaise; Padovani, Andrea; J. P., Colonna; E., Vianello; M., Bocquet; E., Nowak; M., Gasulla; O., Cueto; H., Grampeix; F., Martin; R., Kies; P., Brianceau; M., Gély; A. M., Papon; D., Lafond; J. P., Barnes; C., Licitra; G., Ghibaudo; Larcher, Luca; S., Deleonibus; B., De Salvo
abstract

In this work, we use atomistic simulation, consolidated by a detailed Al2O3 physico-chemical material analysis, to investigate the origin of traps in Al2O3 (in particular, Al- or O-vacancies and H-interstitials). It is shown that the leakage currents through Al2O3 layers, with different post-deposition anneals, are strictly correlated to the H content. Then, for the first time at our knowledge, the hydrogen-based trap features estimated by quantum simulations are introduced in a TANOS device simulator. A very good agreement is obtained between model and device experimental data, allowing for a clear understanding of the role of alumina H content on the retention characteristics of charge-trap memories.

2010 - Investigation of the role of H-related defects in Al2O3 blocking layer on charge-trap memory retention by atomistic simulations and device physical modelling [Relazione in Atti di Convegno]
Molas, G.; Masoero, L.; Blaise, P.; Padovani, A.; Colonna, J. P.; Vianello, E.; Bocquet, M.; Nowak, E.; Gasulla, M.; Cueto, O.; Grampeix, H.; Martin, F.; Kies, R.; Brianceau, P.; Gely, M.; Papon, A. M.; Lafond, D.; Barnes, J. P.; Licitra, C.; Ghibaudo, G.; Larcher, L.; Deleonibus, S.; De Salvo, B.
abstract

2010 - Investigation of trapping/detrapping mechanisms in Al2O3 electron/hole traps and their influence on TANOS memory operations [Relazione in Atti di Convegno]
Larcher, Luca; Padovani, Andrea; Vincenzo della, Marca; Pavan, Paolo; Bertacchini, Alessandro
abstract

The purpose of this work is to investigate the physics of electron/hole trapping/detrapping mechanisms in Al2O3. Combining I-V and C-V measurements with a physical model we derive the energy levels of electron/hole traps and the location of electron/hole charge. The influence of electron/hole alumina traps on TANOS operations and reliability is investigated.

2010 - Leakage Current in TiN/HfO2/TiN MIM Capacitors and Degradation due to Electrical Stress [Relazione in Atti di Convegno]
S., Cimino; Padovani, Andrea; Larcher, Luca; V. V., Afanas’Ev; H. J., Hwang; Y. G., Lee; M., Jurczac; D., Wouters; B. H., Lee; H., Hwang; L., Pantisano
abstract

Metal Insulator Metal (MIM) capacitors are widely used in the electronic industry for DRAM as well as for analog applications. Defects in dielectric structures are very important as they control not only gate leakage and power consumption but, also, device noise and lifetime. Physical and electrical characteristics of TiN/HfO2/TiN capacitors have been investigated aiming at the study of defects and defect energy position in HfO2 on TiN.

2010 - Leakage current in TiN/HfO2/TiN MIM capacitors and degradation due to electrical stress [Relazione in Atti di Convegno]
S., Cimino; Padovani, Andrea; Larcher, Luca; V. V., Afanas’Ev; H. J., Hwang; Y. G., Lee; M., Jurczac; D., Wouters; B. H., Lee; H., Hwang; L., Pantisano
abstract

Electrical characteristics of TiN/HfO2/TiN capacitors have been investigated by means of leakage current and Random Telegraph Noise measurements. Trap assisted transport simulation allowed the extraction of relevant parameters like trap density and trap energy position. The extracted parameters show striking similarities with those reported for HfO2 deposited on a Si surface (i.e., MOSFET applications). Additionally, even low bias Constant Voltage Stress was found to induce leakage current degradation on current vs voltage characteristics, preferentially at low voltage. The leakage current degradation is explained by preexisting defect precursors or by involvement of hydrogen in creating defects as observed on thermal SiO2 layers.

2010 - Mechanism of high-k dielectric-induced breakdown of interfacial SiO2 layer [Relazione in Atti di Convegno]
G., Bersuker; D., Heh; C. D., Young; Morassi, Luca; Padovani, Andrea; Larcher, Luca; K. S., Yew; Y. C., Ong; D. S., Ang; K. L., Pey; W., Taylor
abstract

A mechanism of degradation and breakdown in highk/metal gate transistors was investigated. Based on the electricaltest, physical analysis, and modeling results, we propose that thebreakdown path formation/evolution in the interfacial SiO2 layeris associated with the growth of an oxygen-deficient filamentfacilitated by the grain boundaries of the overlaying high-k film.The model allows reproducing SILC temperature dependencyand its exponential increase from the fresh through soft andprogressive breakdown phases.

2010 - Metal oxide RRAM switching mechanism based on conductive filament microscopic properties [Relazione in Atti di Convegno]
G., Bersuker; D. C., Gilmer; D., Veksler; J., Yum; H., Park; S., Lian; Vandelli, Luca; Padovani, Andrea; Larcher, Luca; K., Mckenna; A., Shluger; V., Iglesias; M., Porti; M., Nafría; W., Taylor; P. D., Kirsch; R., Jammy
abstract

By combining electrical, physical, and transport/atomistic modeling results, this study identifies critical conductive filament features controlling TiN/HfO2/TiN resistive memory operations. The forming process is found to define the filament geometry, which in turn determines the temperature profile and, consequently, the switching characteristics. The findings point to the critical importance of controlling filament dimensions during the forming process (polarity, max current/voltage, etc.).

2010 - Millimeter-Wave 14dBm CMOS Power Amplifier with Input-Output Distributed Transformers [Relazione in Atti di Convegno]
A., Pallotta; W., Eyssa; Larcher, Luca; R., Brama
abstract

We present a novel fully differential input/output distributed transformer topology used for the design of millimeter-wave power amplifiers. Input/output distributed transformers are used to feed the input signal to four differential couples and to combine their output power. This topology improves the stability and the efficiency of the power amplifier, minimizing the chip area. The PA prototype realized in a standard 65nm CMOS technology, supplied with 1.2V, achieves a 1dB single-ended output power compression point of 14dBm at 56GHz with a PAE of 8.3%, occupying only 0.2mm2.

2010 - Modeling Temperature Dependency (6 - 400K) of the Leakage Current Through the SiO2/High-K Stacks [Relazione in Atti di Convegno]
Vandelli, Luca; Padovani, Andrea; Larcher, Luca; G., Bersuker; R. G., Southwick III; W. B., Knowlton
abstract

We investigate the mechanism of the gate leakagecurrent in the Si/SiO2/HfO2/TiN stacks in a wide temperaturerange (6 – 400 K) by simulating the electron transport using amulti-phonon trap assisted tunneling model. Good agreementbetween simulations and measurements allows indentifying thedominant physical processes controlling the temperaturedependency of the gate current. In depletion/weak inversion, thecurrent is limited by the supply of carrier. In strong inversion,the electron-phonon interaction is found to be the dominantfactor determining the current voltage and temperaturedependencies. These simulations allowed to extract importantdefect parameters, e.g. the trap relaxation energy and phononeffective energy, which defines the defect atomic structure.

2010 - On Carbon doping to improve GeTe-based Phase-Change Memory data retention at high temperature [Relazione in Atti di Convegno]
G., Betti Beneventi; E., Gourvestzk; A., Fantini; L., Perniola; V., Sousa; S., Maitrejean; J. C., Bastien; A., Bastard; A., Fargeix; B., Hyot; C., Jahan; J. F., Nodin; A., Persico; D., Blachier; A., Toffoli; S., Loubriat; A., Roule; S., Lhostis; H., Feldis; G., Reimbold; T., Billon; B., De Salvo; Larcher, Luca; Pavan, Paolo; D., Bensahel; P., Mazoyer; R., Annunziata; F., Boulanger
abstract

This paper investigates material and electrical propertiesof a new chalcogenide alloy for Phase-Change Memories(PCM): Carbon-doped GeTe (named GeTeC). First, severalphysico-chemical, optical and electrical analyses have beenperformed on full-sheet chalcogenide depositions in order tounderstand the intrinsic GeTeC phase-change behavior, andto characterize structure and composition of amorphous andcrystalline states. Then, GeTeC with two different Carbon doping(4% and 10%) has been integrated in pillar-type analytical PCMcells. Physico-chemical and electrical data indicate that GeTeC ischaracterized by a much more stable amorphous phase comparedto undoped GeTe. Thus, GeTeC offers a slower programmingspeed versus GeTe, but an improved data retention at hightemperature. Finally, we argue that GeTeC alloy is a promisingcandidate for future developments of PCM technologies forembedded applications.

2010 - Role of Holes and Electrons During Erase of TANOS Memories: Evidences for Dipole Formation and its Impact on Reliability [Relazione in Atti di Convegno]
Vandelli, Luca; Padovani, Andrea; Larcher, Luca; Antonio, Arreghini; Geert Van den, Bosch; Malgorzata, Jurczak; Jan Van, Houdt; Vincenzo Della, Marca; Pavan, Paolo
abstract

The systematic investigation of the role played by electrons and holes during the erase operation of TANOS memories by means of charge separation experiments and physics-based simulations is reported for the first time. We determined a dominance of electrons back-tunneling in the first part of the transient, and dominance of holes in the second part. Good agreement is reached between experimental and simulated data. In addition we demonstrate for the first time the formation of a vertical charge dipole in TANOS devices, whose polarity depends on the P/E operation sequence. This dipole severely affects the program and erase performances and the retention of mild programmed and erased states, which is a concern especially for multilevel applications.

2010 - SET switching effects on PCM endurance [Relazione in Atti di Convegno]
V., Della Marca; F., Carboni; Larcher, Luca; Padovani, Andrea; Pavan, Paolo
abstract

In this paper we report results on PCM endurance failure characterization. We show that endurance failure is related to SET pulse features and we analyze and model SET operation to obtain a better understanding and improve endurance performance. Results give interesting insights on the crystallization process of GST material. SET obeys to a constant energy law. Fast SET pulses require high power; slow SET pulses can be implemented in low power applications. Results may be used for optimized SET/RESET operation to achieve better endurance.

2010 - Temperature Effects on Metal-Alumina-Nitride-Oxide-Silicon Memory Operations [Articolo su rivista]
Padovani, Andrea; Larcher, Luca; D., Heh; G., Bersuker; V., Dellamarca; Pavan, Paolo
abstract

We present a detailed investigation of temperature effects on the operation of TaN/Al2O3 / Si3N4 /SiO2 / Si (TANOS) memory devices. We show that not only retention but also program and erase operations are affected significantly by temperature. Using a large set of experimental data and simulations on a variety of TANOS stacks, we show that the temperature dependence of TANOS program and erase operations can be explained by accounting for that the alumina dielectric constant increases by 20%–25% over a 125 K temperature range.

2010 - 250mV Input Boost Converter for Low Power Applications [Relazione in Atti di Convegno]
Bertacchini, Alessandro; Scorcioni, Stefano; Cori, Marco Maria; Larcher, Luca; Pavan, Paolo
abstract

This paper presents a novel low power boost converter designed and optimized to operate with a minimum input voltage as low as 250mV, which is the typical voltage range of novel micro energy sources. The low power budget of such energy sources (at most few hundreds of μWs) mandates the adoption of very efficient circuit solutions. The realized PCB prototype provides a regulated maximum output voltage 3.3V with 70% maximum efficiency, thus making the proposed converter topology very attractive for low power application power supply. Noticeably the converter operates without any external power supply of the control logic.

2009 - A MEMS Reconfigurable Quad-Band Class-E Power Amplifier for GSM Standard [Relazione in Atti di Convegno]
Larcher, Luca; R., Brama; Ganzerli, Marcello; J., Iannacci; B., Margesin; M., Bedani; A., Gnudi
abstract

In this work we present a reconfigurable mid-power class-E power amplifier (PA) operating at ~900 MHz and ~1800 MHz (GSM standard) realized hybridizing one chip manufactured in AMS 0.35 mum CMOS technology and one MEMS sub-network. The CMOS chip realizes the active part of the circuit, whereas the MEMS block (realized in FBK technology) implements a reconfigurable impedance matching network (MN) that transforms the 50 Omega antenna load to the 12 Omega impedance required by the PA in order to deliver 20 dBm output power in both the GSM operating frequency bands. The prototype of the MEMS/CMOS PA we realized delivers 20 dBm with 38% and 26% drain efficiencies at 900 MHz and 1800 MHz, respectively, demonstrating to be a feasible option compared to standard commercial solutions.

2009 - A MEMS reconfigurable quad-band Class-E Power Amplifier for GSM standard [Relazione in Atti di Convegno]
Larcher, Luca; Brama, Riccardo; Ganzerli, Marcello; J., Iannacci; M., Bedani; A., Gnudi
abstract

In this paper we present a reconfigurable Class-E Power Amplifier (PA) whose operation frequency covers all uplink bands of GSM standard. We describe the circuit design strategy to reconfigure PA operation frequency maximizing the efficiency. Two dies, manufactured using CMOS and MEMS technologies, are assembled through bondwires in a SiP fashion. Prototypes deliver 20dBm output power with 38% and 26% drain efficiencies at lower and upper bands, respectively. MEMS technological issues degrading performance are also discussed.

2009 - A Novel Fluorine Incorporated Band Engineered (BE) Tunnel (SiO2/ HfSiO/ SiO2) TANOS with Excellent Program/Erase & Endurance to 10^5 Cycles [Relazione in Atti di Convegno]
S., Verma; G., Bersuker; D. C., Gilmer; Padovani, Andrea; P., Hokyung; A., Nainani; D., Heh; J., Huang; J., Jiang; K., Parat; P. D., Kirsch; Larcher, Luca; Hsing Huang, Tseng; K. C., Saraswat; R., Jammy
abstract

We demonstrate for the first time a fluorine incorporated band- engineered (BE) tunnel oxide (SiO2/HfSiO/SiO2) TANOS with excellent program / erase (P/E) characteristics and endurance to 105 cycles. Incorporating fluorine in the tunnel dielectric improves Si/SiO2 interface resulting in excellent endurance of nearly constant over 3 V P/E window for at least 105 cycles. Fluorine also reduces interface state generation during retention by ~20%. Furthermore, Fluorine passivates bulk traps leading to as much as ~10times higher charge to breakdown (Qbd) and ~10-50times lower interface state density (Dit). Fluorine passivation for BE-TANOS is significant because it improves reliability assisting implementation of TANOS flash NVM beyond the 20 nm node.

2009 - A technique to extract high-k IPD stack layer thicknesses from C-V measurements [Articolo su rivista]
Larcher, Luca; Pavan, Paolo; Padovani, Andrea; G., Ghidini
abstract

We propose in this letter a simple technique based on C-V measurements which allows to estimate the thicknesses of SiOX and high-k layers of IPD stacks. We apply this technique to IPD Al2O3-based stacks for floating gate memory applications, finding a good agreement with TEM measurements. In addition, simulation results are provided to demonstrate the correctness of the basic assumption of this technique.

2009 - Advanced high-k materials and electrical analysis for memories: the role of SiO2-high-k dielectric intermixing [Relazione in Atti di Convegno]
Morassi, Luca; Larcher, Luca; L., Pantisano; Padovani, Andrea; R., Degreave; M. B., Zahid; B. J., O'Sullivan
abstract

This paper presents an original approach for material studies for memory devices where the degree of intermixing between the high-k and interfacial SiO2 is explicitly quantified experimentally. Using calibrated leakage simulation the importance of intermixing is verified independently together with the conduction mechanism. The implication for NVM reliability are profound and will be discussed toward retention mechanisms and used to optimize retention margins for NVM memories.

2009 - Analytical model for low-frequency noise in amorphous chalcogenide-based phase-change memory devices [Articolo su rivista]
G., Betti Beneventi; A., Calderoni; P., Fantini; Larcher, Luca; Pavan, Paolo
abstract

Low-frequency noise has been experimentally characterized in the disordered insulating phase of chalcogenide-based phase-change memory (PCM) devices. An analytical model of noise based on the two-level systems (TLS) theory has been developed. In this framework we suggest that the origin of the 1/fγ noise in the conductivity of amorphous chalcogenides has to be ascribed to the TLS-induced fluctuations of the mean trap energy in the material. The model allows to quantitatively account for noise magnitude dependence on both voltage and temperature in the readout region of the memory device. Besides, our equations well describe the noise behavior as a function of the drift phenomenon, coherently with existing structural relaxation theories. Measurements and model results show that the noise-to-signal ratio (N/S) in the readout region of the cell is constant with respect to bias; hence there is no particular readout voltage that minimizes N/S. Furthermore, the analysis of noise data with cell scaling confirms that noise in PCMs is mainly due to the bulk properties of the chalcogenide employed rather than to interfacial effects.

2009 - Connecting electrical and structural dielectric characteristics [Relazione in Atti di Convegno]
G., Bersuker; D., Veksler; C. D., Young; H., Park; Morassi, Luca; Padovani, Andrea; Larcher, Luca; W., Taylor; P. D., Kirsch; R., Jammy
abstract

unknown

2009 - Load optimization of an inductive power link for remote powering of biomedical implants [Relazione in Atti di Convegno]
K. M., Silay; Dondi, Denis; Larcher, Luca; M., Declercq; L., Benini; Y., Leblebici; C., Dehollain
abstract

This article presents the analysis of the power efficiency of the inductive links used for remote powering of the biomedical implants by considering the effect of the load resistance on the efficiency. The optimum load condition for the inductive links is calculated from the analysis and the coils are optimized accordingly. A remote powering link topology with a matching network between the inductive link and the rectifier has been proposed to operate the inductive link near its optimum load condition to improve overall efficiency. Simulation and measurement results are presented and compared for different configurations. It is shown that, the overall efficiency of the remote powering link can be increased from 9.84% to 20.85% for 6 mW and from 13.16% to 18.85% for 10 mW power delivered to the regulator, respectively.

2009 - Modeling TANOS Memory Program Transients to Investigate Charge Trapping Dynamics [Articolo su rivista]
Padovani, Andrea; Larcher, Luca; D., Heh; G., Bersuker
abstract

A novel physics-based drift-diffusion model of TANOS program transients is employed to investigate electron trapping and detrapping dynamics in the nitride trapping layer. Trapping process is found to be independent from the energy of injected electrons, while detrapping is dominated by trap-to-band tunneling. Modeling of the trapped charge evolution during program transients allows to extract physical characteristics of the traps and provides useful information for the optimization of TANOS memories.

2009 - Photovoltaic scavenging systems: Modeling and optimization [Articolo su rivista]
Brunelli, D.; Dondi, Denis; Bertacchini, Alessandro; Larcher, Luca; Pavan, Paolo; Benini, L.
abstract

The interest in embedded portable systems and wireless sensor networks (WSNs) that scavenge energyfrom the environment has been increasing over the last years. Thanks to the progress in the design oflow-power circuits, such devices consume less and less power and are promising candidates to performcontinued operation by the use of renewable energy sources. The adoption of maximum power pointtracking (MPPT) techniques in photovoltaic scavengers increases the energy harvesting efficiency andleads to several benefits such as the possibility to shrink the size of photovoltaic modules and energyreservoirs. Unfortunately, the optimization of this process under non-stationary light conditions is still akey design challenge and the development of a photovoltaic harvester has to be preceded by extensivesimulations. We propose a detailed model of the solar cell that predicts the instantaneous powercollected by the panel and improves the simulation of harvester systems. Furthermore, the paperfocuses on a methodology for optimizing the design of MPPT solar harvesters for self-poweredembedded systems and presents improvements in the circuit architecture with respect to our previousimplementation. Experimental results show that the proposed design guidelines allow to incrementglobal efficiency and to reduce the power consumption of the scavenger.

2009 - Understanding endurance degradation in Flash memory through transconductance measurement [Relazione in Atti di Convegno]
S., Verma; G., Bersuker; D. C., Gilmer; Padovani, Andrea; H., Park; A., Nainani; J., Huang; K., Parat; P. D., Kirsch; Larcher, Luca; H. H., Tseng; K. C. Saraswat R., Jammy
abstract

Endurance degradation is a limitation for implementing futurescaled flash memory devices. This degradation is mainly attributableto Si/SiO2 interface traps generated during program/erase (P/E) stress rather than fixed charges in the bulk oxide. In this work, we use Gm (transconductance) to monitor the interface degradation. Wereport that interface defect generation is highest during erase operation. In addition to the interface, hole & electron tunnelingprobability seem crucial to degradation during erase. Fluorine incorporation in tunnel stack is found to reduce Gm degradationsuggesting improved interface.

2008 - A Solar Energy Harvesting Circuit for Low Power Applications [Relazione in Atti di Convegno]
Dondi, Denis; Bertacchini, Alessandro; Larcher, Luca; Pavan, Paolo; D., Brunelli; L., Benini
abstract

In this paper we present a solar energy harvesting circuit for low-power applications describing circuit architecture and guidelines for an optimal design. We evaluate the performance of two implemented prototypes intended to power a wireless embedded system under different light intensities and different switching frequencies. Measurements show that higher switching frequencies allow reaching the maximum efficiency (90%) at higher light intensities, whereas lower operating frequencies perform better under lower irradiance. Experimental results show that circuit optimization depends on light conditions and the proposed solar energy harvester can autonomously supply the nodes of a wireless sensor network WSN.

2008 - A 30.5 dBm 48% PAE CMOS class-E PA with integrated balun for RF applications [Relazione in Atti di Convegno]
R., Brama; Larcher, Luca; Mazzanti, Andrea; F., Svelto
abstract

Integration of the power amplifier together with signal processing in a transmitter is still missing in demanding RF commercial products. Issues preventing PA integration include LO pulling phenomena, thermal dissipation, and power efficiency. In this work we investigate high efficiency watt range Class-E PAs and integrated baluns. In particular, insights in the design of a fully differential cascode topology for high efficiency and reliable operation are provided and a narrowband lumped element balun, employing minimum number of integrated inductors for minimum power loss, is introduced. Two versions have been manufacturedusing a 0.13 m CMOS technology. The first comprises the driver, and a differential PA connected to an external low-loss commercial balun. Experiments prove 31 dBm delivered output power, with 58% PAE and 67% drain efficiency, at 1.7 GHz. The second version adopts the same driver and PA and also integrates the balun. Experiments prove 30.5 dBm delivered output power, with 48% PAE and 55% drain efficiency, at 1.6 GHz.

2008 - Breakdown in the metal/high-k gate stack: Identifying the “weak link” in the multilayer dielectric [Relazione in Atti di Convegno]
G., Bersuker; D., Heh; C., Young; H., Park; P., Khanal; Larcher, Luca; Padovani, Andrea; P., Lenahan; J., Ryan; B. H., Lee; H., Tseng; R., Jammy
abstract

We apply a systematic approach to identify a high-k/metal gate stack degradation mechanism. Our results demonstrate that the SiO2 interfacial layer controls the overall degradation and breakdown of the high-k gate stacks stressed in inversion. Defects contributing to the gate stack degradation are associated with the high-k/metal-induced oxygen vacancies in the interfacial layer.

2008 - Characterization and modelling of low-frequency noise in PCM devices [Relazione in Atti di Convegno]
P., Fantini; G., Betti Beneventi; A., Calderoni; Larcher, Luca; Pavan, Paolo; F., Pellizzer
abstract

Low-frequency noise in PCM devices is experimentally investigated providing a new physical model for the amorphous GST (Ge2Sb2Te5) material. Noise intensity is characterized and modelled as a function of bias, temperature and size. Findings from 1/f noise analysis are used to understand the drift mechanism of the amorphous state resistance.

2008 - Feasibility of SIO2/Al2O3 tunnel dielectric for future Flash memories generations [Relazione in Atti di Convegno]
Padovani, Andrea; Larcher, Luca; S., Verma; Pavan, Paolo; P., Majhi; P., Kapur; K., Parat; G., Bersuker; K., Saraswat
abstract

In this paper, we investigate the feasibility of SiO2/Al2O3 stack tunnel dielectric for future Flash memory generations using statistical leakage current simulations. We show that the statistical Monte Carlo (MC) simulator we employed reproduces accurately leakage currents measured on SiO2/Al2O3 dielectric capacitors. Exploiting its statistical capabilities, we calculate leakage current distributions in Flash memory retention conditions. We show that the high defectiveness of AI2O3 stacks strongly reduces the potential improvement of Flash retention due to the introduction of AI2O3 tunnel dielectric.

2008 - Hole Distributions in Erased NROM Devices: profiling method and effects on reliability [Articolo su rivista]
Padovani, Andrea; Larcher, Luca; Pavan, Paolo
abstract

The NROM-cell concept has been introduced as a promising technology to replace Flash nonvolatile memory devices also in embedded products, owing to its intrinsic 2-b/cell operation and better endurance. However, the presence of physically sepa- rated electron and hole distributions generated by program and erase operations is reported to be one of the main causes of the device’s retention degradation. Therefore, a deep knowledge of the features and evolution of the nitride-storage charge is crucial for reliability, cell optimization, future scalability, and multilevel oper- ation. In this scenario, the purpose of this paper is twofold, which is as follows: 1) to introduce a combined simulative experimental method allowing proﬁling hole distribution in devices erased with different bias conditions and 2) to monitor through this technique the evolution of the nitride charge with cycling, correlating it to the degradation of memory reliability after cycling.

2008 - Modeling and Optimization of a Solar Energy Harvester System for Self Powered Wireless Sensor Networks [Articolo su rivista]
Dondi, Denis; Bertacchini, Alessandro; D., Brunelli; Larcher, Luca; L., Benini
abstract

In this paper we propose a methodology for optimizing a solar harvester with maximum-power point tracking for self powered wireless sensors networks (WSN) nodes. We focus on maximizing the harvester’s efficiency in transferring energy from the solar panel to the energy storing device.A photovoltaic panel analytical model, based on a simplified parameter extraction procedure is adopted. This model predicts the instantaneous power collected by the panel helping the harvester design and optimization procedure. Moreover, a detailed modeling of the harvester is proposed to understand basic harvester behavior and optimize the circuit. Experimental results based on the presented design guidelines demonstrate the effectiveness of the adopted methodology. This design procedure helps in boosting efficiency, allowing to reach a maximum efficiency of 85% with discrete components. The application field of this circuit is not limited to self powered WSN nodes, but it can be easily extended also in embedded portable applications to extend battery life.

2008 - Modeling NAND Flash Memories for IC Design [Articolo su rivista]
Larcher, Luca; Padovani, Andrea; Pavan, Paolo; P., Fantini; A., Calderoni; A., Mauri; A., Benvenuti
abstract

In this letter, we present a compact model of NAND Flash memory strings for circuit simulation purposes. This model is modular and easy to be implemented, and its parameters can be extracted through a simple procedure. It allows accurate simula- tion of NAND Flash memories with a limited computational effort, taking into account capacitive coupling effects which will become extremely important in future technology generations. This model is a very valuable tool for IC designers to optimize NVM circuits, particularly in multilevel applications.

2008 - On the RESET-SET transition in Phase Change Memories [Relazione in Atti di Convegno]
G., Puzzilli; F., Irrera; Padovani, Andrea; Pavan, Paolo; Larcher, Luca; A., Arya; DELLA MARCA, Vincenzo; A., Pirovano
abstract

We characterize SET operation in Phase Change Memories. A measurement procedure aiming to investigate resistance transition from amorphous to crystalline states is shown. Results give interesting insights on the crystallization process of GST material and a simple model is introduced. Crystallization process obeys to a constant energy law. Fast SET pulses require high power; slow SET pulses can be implemented in low power applications. Results may be used for an optimized design of memory cell operating conditions.

2008 - Performance Analysis of Solar Energy Harvesting Circuits for Autonomous Sensors [Relazione in Atti di Convegno]
Bertacchini, Alessandro; D., Dondi; Larcher, Luca; Pavan, Paolo
abstract

In this paper we present two different energy harvesting circuits for solar powered autonomous sensors.Both circuits are able to supply several types of sensor nodes. Performance of these circuits under different light conditions and different loads have been evaluated by experimental results conducted on implemented prototypes.Moreover, starting from the estimated working conditions of the sensor in terms of power requirements and light irradiance, the comparison between these circuits provides useful insights to help a designer in choosing of the optimal harvesting implementation.

2008 - Solar Energy Harvesting: applicazioni a bassa potenza [Altro]
Dondi, Denis; Bertacchini, Alessandro; Larcher, Luca; Pavan, Paolo; D. Brunelli e. L., Benini
abstract

Negli ultimi anni si sono sempre più utilizzate reti wireless a elevata velocità per scambiare dati su larga scala con centinaia e migliaia di nodi. I nodi di queste reti sono dei sistemi embedded, che devono essere in grado di connettersi utilizzando solo la quantità di energia fornita dalle batterie. Proprio questo è uno tra i maggiori limiti al loro funzionamento, dato che tali batterie sono in grado di alimentare questi sistemi solo per poco tempo e non continuativamente. Per questo, l’obiettivo dei recenti lavori di ricerca sia aziendale che accademico è quello di ridurre la dissipazione di potenza per aumentare la vita operativa dei nodi. In questo senso, sono state studiate architetture a bassissimo consumo di potenza e applicazioni che necessitano di bassissimo duty cycle di alimentazione. Sfortunatamente, queste tecniche non sono adatte per le applicazioni con requisiti più severi, come, ad esempio, quelle che richiedono elaborazioni di dati intensive e trasmissione di dati a una distanza e velocità superiori. Inoltre, più di recente, la ricerca scientifica sta studiando la possibilità di estrarre l’energia necessaria al funzionamento dei nodi della rete di sensori wireless da fonti alternative, ad esempio sotto forma di luce solare, vento o vibrazioni. Questo ha spinto i ricercatori a progettare sistemi di energy harvesting. L’energy harvesting non è una novità, tuttavia, mediante una metodologia efficace di progetto, la realizzazione e l’integrazione di energy harvester efficienti nei moderni sistemi embedded rimane tuttora un argomento molto stimolante. In questo scenario, questo articolo presenta un sistema di harvesting da energia solare progettato per alimentare sistemi embedded wireless a bassa potenza. Saranno analizzate l’efficienza del circuito realizzato per valori realistici di duty cycle e l’intensità luminosa del sensore. L’analisi dimostra che la scelta migliore per la configurazione operativa del sistema di energy harvesting non è unica. In particolare, una configurazione con dispositivi a bassissimo consumo di potenza (ULP) è più efficiente in condizioni di bassa intensità luminosa, mentre architetture ad alta frequenza sono ideali in condizioni di luminosità elevata.

2008 - Solar harvesting per reti di sensori wireless [Altro]
Bertacchini, Alessandro; Dondi, Denis; Larcher, Luca; Pavan, Paolo; D., Brunelli; L., Benini
abstract

Lo sviluppo di sistemi ad alimentazione perpetua, in grado di evitare la frequente sostituzione e/o ricarica delle batterie, è uno degli obiettivi di ricerca più importanti nella progettazione di sistemi embedded distribuiti e delle reti wireless di sensori. Nonostante i progressi tecnologici nella progettazione di dispositivi low-power, che possono contribuire a prolungare la durata delle batterie, la ridotta capacità degli accumulatori di energia (batterie, supercapacitor, ecc.) limita severamente l’autonomia di sistemi quali le Wireless Sensor Networks (WSN). Recentemente circuiti che convertono fonti energetiche ambientali,come l'energia solare o termica, in energia elettrica si stanno diffondendo nella comunità dei progettisti di sistemi embedded e solitamente assumono il nome di energy scavenger o energy harvester. In particolare scavengerche utilizzano piccole celle solari sono stati proposti per alimentare perpetuamente le reti di sensori. Rispetto ai tradizionali sistemi fotovoltaici su larga scala (decine o centinaia di kW), che riescono agevolmente a operare nel punto di massima potenza delle celle (maximum power point - MPP), i circuiti di gestione dell’energia elettrica generata da pannelli fotovoltaici di piccole dimensioni devono affrontare problemi aggiuntivi: (a) una ridotta quantità di energia dovuta alla dimensione delle celle; (b) la realizzazione di circuiti efficienti in grado di operare nel punto di massima potenza (MPP) di norma richiede un’alta percentuale dell’energia fornita dalla cella stessa (riducendo in pratica la potenza disponibile al sistema);(c) è presente un’interazione sostanziale fra circuiti di conversione e immagazzinamento dell’energia e i dispositivi alimentati. La progettazione deve quindi essere supportata da simulazioni, in particolare quando l’efficienza della conversione deve essere ottimizzata per basse intensità di radiazione solare. La definizione di un flusso di progettazione chiaro è perciò fondamentale per lo sviluppo di soluzioni circuitali con buone prestazioni. Simulazioni circuitali dell’intero sistema elettronico di gestione dell’energia generata dai pannelli fotovoltaici sono tipicamente disponibili per sistemi fotovoltaici a larga scala, ma si rendono fondamentali e necessarie per l’efficienza di reti di sensori wireless e sistemi low-power. Per ottenere simulazioni affidabili, è opportuno sviluppare modelli compatti e accurati e la validazione di un modello delle caratteristiche I-V non lineari dei moduli PV diventano cruciali per la progettazione dei sistemi di energy harvesting efficienti.

2008 - Statistical modeling of leakage currents through SiO2/high- κ dielectrics stacks for non-volatile memory applications [Relazione in Atti di Convegno]
Padovani, A.; Larcher, L.; Verma, S.; Pavan, P.; Majhi, P.; Kapur, P.; Parat, K.; Bersuker, G.; Saraswat, K.
abstract

We present here a statistical Monte Carlo (MC) simulator modeling leakage currents across SiO2/high-κ dielectric stacks. We show that simulations accurately reproduce experimental currents measured at various temperatures on capacitors with different high-k dielectric stacks. We exploit statistical simulations to investigate the impact of high-κ's traps on leakage current distribution for Flash memory applications. We demonstrate that the high defectiveness typical of high-k materials strongly reduces the potential improvement due to the introduction of band-gap engineered high-κ tunnel dielectric stacks. In this regard, the simulator is a useful tool to optimize high-κ tunnel stacks and to improve technology- reliability issues related to Flash memory applications. © 2008 IEEE.

2008 - Statistical modeling of leakage currents through SiO2/high-k dielectric stacks for non-volatile memory applications [Relazione in Atti di Convegno]
Padovani, Andrea; Larcher, Luca; S., Verma; Pavan, Paolo; P., Majhi; P., Kapur; K., Parat; G., Bersuker; K., Saraswat
abstract

We present here a statistical Monte Carlo (MC) simulator modeling leakage currents across SiO2/high-kappa dielectric stacks. We show that simulations accurately reproduce experimental currents measured at various temperatures on capacitors with different high-k dielectric stacks. We exploit statistical simulations to investigate the impact of high-kappapsilas traps on leakage current distribution for flash memory applications. We demonstrate that the high defectiveness typical of high-k materials strongly reduces the potential improvement due to the introduction of band-gap engineered high-kappa tunnel dielectric stacks. In this regard, the simulator is a useful tool to optimize high-kappa tunnel stacks and to improve technology reliability issues related to flash memory applications.

2007 - A 1.7GHz 31dBm differential CMOS Class-E Power Amplifier with 58% PAE [Relazione in Atti di Convegno]
Brama, Riccardo; Larcher, Luca; Mazzanti, Andrea; F., Svelto
abstract

This paper shows that CMOS Class-E PAs are capable of high Power-Added Efficiency (PAE), even when delivering large output powers at Radio Frequency (RF). In particular, a cascode device is used to obtain high efficiency while assuring reliable operation. A differential solution has been adopted to maximize 2nd harmonic suppression and minimize potential on-chip interference. Prototypes realized in 0.13μm CMOS technology using thick oxide devices show the following performances: 31dBm maximum output power at 1.7GHz with 67% drain efficiency and 58% PAE, -51dBc and -39.5dBc suppression for 2nd and 3rd harmonics, respectively.

2007 - CMOS Balanced Regenerative Frequency Dividers for Wide Band Quadrature LO Generation [Articolo su rivista]
Mazzanti, Andrea; Larcher, Luca; F., Svelto
abstract

CMOS regenerative frequency dividers, based on a fully balanced Gilbert cell, are analyzed in this paper for quadrature local oscillator (LO) signal generation. Driven in opposite phase by double frequency signals, they provide quadrature waveforms while simultaneously driving large mixers LO input capacitances, thereby avoiding power hungry buffers typically required. Experimental results, carried out on 0.18 mm CMOS prototypes, show 68% bandwidth around 2GHz center frequency, with a quadrature accuracy better than 11, making them suitable for multi-standard wireless receivers. To keep the output amplitude constant while simultaneously minimizing the average power consumption, a digital calibration loop regulates each divider biasing current.

2007 - Dielectric Reliability for Future Logic and Non-Volatile Memory Applications: a Statistical Simulation Analysis Approach [Relazione in Atti di Convegno]
Padovani, Andrea; Larcher, Luca; A., Chimenton; Pavan, Paolo; P., Olivo
abstract

In this paper, we present a physically-based Monte-Carlo (MC) model reproducing the leakage current flowing across typical dielectric layers (SiO2, high-k) used in ULSI technologies. Simulations will be shown to predict accurately currents measured on MOSFETs, large area MOS capacitor, and tunnel oxides of Flash memories after electrical and radiation stresses. Statistical aspects related to leakage current and threshold voltage are reproduced correctly, allowing worst case corner prediction, necessary to assess dielectric damaging effects on logic circuits and non-volatile memory operation.

2007 - Hole Distributions in NROM Devices: Profiling Technique and Correlation to Memory Retention [Relazione in Atti di Convegno]
Padovani, Andrea; Larcher, Luca; Pavan, Paolo
abstract

In this work, we presented a new technique to profile hole distribution in NROM devices. The evolution of the nitride charge in cycled cells was monitored. The key role played by holes in NROM retention degradation was identified. Electron injection far from the junction and VT drift in erased NROM cells are successfully explained.

2007 - ID-VGS Based Tools to Profile Charge Distributions on NROMTM Memory Devices [Articolo su rivista]
Padovani, Andrea; Larcher, Luca; Pavan, Paolo; L., Avital; I., Bloom; B., Eitan
abstract

The NROM cell concept has been introduced as a promising technology to replace Flash non-volatile memory devices also in embedded products, thanks to its intrinsic two-bits/cell operation and better endurance. However, the presence of physically separated electron and hole distributions generated by program and erase operations is reported to be one of the main causes of device’s retention degradation. Therefore, a deep knowledge of the features and evolution of the nitride storage charge is crucial for reliability, cell optimization, future scalability and multi-level operation. In this scenario, the purpose of this paper is twofold: 1) to introduce a combined simulative-experimental method allowing profiling hole distribution in devices erased with different bias conditions; 2) to monitor through this technique the evolution of the nitride charge with cycling, correlating it to the degradation of memory reliability after cycling.

2007 - Modeling NAND Flash memories for circuit simulations [Relazione in Atti di Convegno]
Larcher, Luca; Padovani, Andrea; I., Rimmaudo; Pavan, Paolo; A., Calderoni; G., Molteni; F., Gattel; P., Fantini
abstract

In this paper, we will present the basic structure and the parameter extraction procedure for a compact model of a NAND Flash memory string working in Spice-like circuit simulators. To the author knowledge, this is the first Spice-like model of a NAND Flash memory string. This model is modular and simple to be implemented. It will allow accurately reproducing both DC and transient behavior of NAND Flash memories without increasing computational effort, thus becoming an indispensable tool for designers to optimize circuits especially in multi-level applications.

2007 - Monte-Carlo Simulations of Flash Memory Array Retention [Relazione in Atti di Convegno]
Padovani, Andrea; Larcher, Luca; A., Chimenton; Pavan, Paolo
abstract

One of the major scalability limitations of flash memories is anomalous SILC, which strongly endangers device reliability and data retention. Therefore, an accurate evaluation of SILC statistics on large arrays is crucial for reliability predictions and new Flash technology development. In the last years, oxide leakage currents were deeply investigated and modeled, neglecting SILC statistics and effects on large Flash arrays. More recently, analytical models relating Flash statistical threshold voltage (VT) distributions to defect statistics and leakage current were proposed. However, these models rely on several simplifying assumptions such as the equivalent cell concept and an uniform defect population. Still, these models do not account for the initial VT distribution and neglect the role played by trap energy and effective field. In this scenario, the purpose of this paper is to present a Monte-Carlo (MC) simulator reproducing flash VT distribution, which overcomes the above model limitations. We will show that this model can be used to 1) investigate effects of defect features and technology parameters on VT distribution, and 2) analyze the impact of temperature and voltage accelerated stresses on final VT distribution.

2007 - Photovoltaic Cell Modeling for Solar Energy Powered Sensor Networks [Relazione in Atti di Convegno]
Dondi, Denis; D., Brunelli; L., Benini; Pavan, Paolo; Bertacchini, Alessandro; Larcher, Luca
abstract

Photovoltaic scavenging circuits have been presentedto reduce installation and maintenance costs of wirelesssensor networks. When small-size photovoltaic modules areadopted, optimizing the efficiency of the harvesting process andtracking theMaximum Power Point (MPP) becomes very difficult,and the development of a photovoltaic harvester has to bepreceded by extensive simulations. The paper focuses on thedefinition of the model for a small PV cell allowing the simulationof harvester systems. The model is validated on a case study ofMPPT circuit for sensor networks.

2006 - Analysis of Reliability and Power Efficiency in Cascode Class-E PAs [Articolo su rivista]
Mazzanti, Andrea; Larcher, Luca; R., Brama; F., Svelto
abstract

Power efficiency in switched common source class-E amplifiers is usually obtained at the expense of device stress. Device stacking is a viable way to reduce voltage drops across a single device, improving long-term reliability. In this paper, we focus on cascode-based topologies, analyzing the loss mechanisms and giving direction to optimize the design. In particular, a new dissipative mechanism, peculiar of the cascode implementation, is identified and a circuit solution to minimize its effect is proposed. Prototypes, realized in a 0.13-mu m CMOS technology demonstrate 67% PAE while delivering 23 dBm peak power at 1.7 GHz. Good bandwidth was also realized with greater than 60% PAE over the frequency range of 1.4-2 GHz.

2006 - CMOS and interconnect reliability-non-volatile memory reliability [Relazione in Atti di Convegno]
Visconti, A.; Larcher, L.
abstract

2006 - Impact of Scaling on CMOS Radio Frequency Class-E Power Amplifiers [Relazione in Atti di Convegno]
Brama, Riccardo; Larcher, Luca; Mazzanti, Andrea; F., Svelto
abstract

Although technological scaling is benefical for CMOS ICs for digital applications, it impacts seriously on analog and radio frequency circuit performances. In this scenario, this paper investigates power loss mechanisms in Class-E power amplifiers highlighting their depedencies on technological scaling. An analytical model describing PA power added efficiency is obtained, and its results are validated against circuit simulations. Scaling effects on efficiency are modeled and discussed, showing that Class-E PA performance are not significantly affected by scaling when delivering low-medium output power, contrarily to what is commonly believed.

2006 - Oxide Breakdown After RF Stress: Experimental Analysis and Effects on Power Amplifier Operation [Relazione in Atti di Convegno]
Larcher, Luca; D., Sanzogni; Brama, Riccardo; Mazzanti, Andrea; F., Svelto
abstract

The target in the design of CMOS radio-frequency (RF) transceivers for wireless application is the highest integration level, despite reliability issues of conventional submicron MOSFETs, due to high RF voltage and current peaks. In this scenario, this paper investigates gate-oxide breakdown under RF stress by using a class-E power amplifier (PA) for experiments. We showed that maximum RF voltage peaks for safe device operation are much larger than usual DC limits, and that the physical mechanism of oxide degradation is triggered by the rms value of oxide field, and not by its maximum, as generally believed. This finding has a strong impact on RF circuit designs, especially in MOSFET scaling perspectives. Finally, breakdown effects on PA operations are discussed.

2006 - Profiling charge distribution in NROM devices [Relazione in Atti di Convegno]
Padovani, Andrea; Larcher, Luca; Pavan, Paolo
abstract

NROM memory cells are proposed as one of the most promising non-volatile memories. Issues on scaling and endurance have risen due to the presence of both electrons and holes, for the control of their relative position and spread in the charge trapping material. Therefore, a deeper analysis of the injected-charge distribution region is very important for program/erase bias optimization, reliability prediction and future scaling. In this paper, we introduce and discuss two tools, based on subthreshold slope and temperature effects, able to correctly estimate program charge distribution features from simple ID - VGS measurements

2006 - Single event leakage current in Flash memory [Relazione in Atti di Convegno]
G., Cellere; Larcher, Luca; A., Paccagnella; A., Visconti; M., Bonanomi
abstract

Flash memories are the leader among nonvolatile memory technologies. Ionizing radiation impact their reliability both on the control circuitry and on the memory array itself. In particular, in FGs hit by ions the tracks of defects generated by ions in the tunnel oxide may result in a radiation induced leakage current (RILC), which can leads to retention problems in hit FGs. We are demonstrating and modeling this phenomenon in a state-of-the-art Floating Gate memory technology. We are also showing that RILC has a peculiar erratic behavior

2006 - Subattoampere current induced by single ions in silicon oxide layers of nonvolatile memory cells [Articolo su rivista]
Cellere, G; Paccagnella, A; Larcher, Luca; Visconti, A; Bonanomi, M.
abstract

A single ion impinging on a thin silicon dioxide layer generates a number of electron/hole pairs proportional to its linear energy transfer coefficient. Defects generated by recombination can act as a conductive path for electrons that cross the oxide barrier, thanks to a multitrap-assisted mechanism. We present data on the dependence of this phenomenon on the oxide thickness by using floating gate memory arrays. The tiny number of excess electrons stored in these devices allows for extremely high sensitivity, impossible with any direct measurement of oxide leakage current. Results are of particular interest for next generation devices.

2006 - Temperature Monitor: a New Tool to Profile Charge Distribution in NROMTM Memory Devices [Relazione in Atti di Convegno]
L., Avital; Padovani, Andrea; Larcher, Luca; I., Bloom; R., Arie; Pavan, Paolo; B., Eitan
abstract

NROM memory cells are proposed as one of the most promising non-volatile memories. Issues on scaling and endurance have risen due to the presence of both electrons and holes for the control of their relative position and spread in the charge trapping material. In this paper, we present a new characterization tool able to sense charge distribution features in different program/erase conditions that can be efficiently used for program/erase bias optimization and reliability predictions. This new tool exploits temperature effects on ID-VGS current measurements

2005 - A 1.4GHz – 2GHz Wideband CMOS Class-E Power Amplifier Delivering 22dBm peak with 67% PAE [Relazione in Atti di Convegno]
Mazzanti, Andrea; Larcher, Luca; Brama, Riccardo; F., Svelto
abstract

The design of CMOS power amplifiers (PA) is still a challenging issue. Efficiency is one of the key requirements, but it is usually obtained at the expense of large device stress. The latter can be reduced by introducing a cascode solution, which features an efficiency penalty due to dissipative mechanisms associated with MOS capacitive parasitics, overlooked up to date. A class-E PA is proposed which allows simultaneously high efficiency and reduced stress by means of an integrated inductor tuning out the parasitic. Prototypes, realized in a 0.13 μm CMOS technology, demonstrate 67% PAE while delivering 23 dBm peak power at 1.7 GHz. PAE is still above 60% within the range 1.4-2 GHz.

2005 - Balanced CMOS LC-tank Analog Frequency Dividersfor Quadrature LO Generation [Relazione in Atti di Convegno]
Mazzanti, Andrea; Larcher, Luca; F., Svelto
abstract

A regenerative circuit based on an LC-tank balanced divider is proposed in this paper for quadrature LO signal generation. For given tank quality factor, it provides larger operation bandwidth and improved quadrature accuracy when compared with conventional injection locked frequency dividers. The operation bandwidth can be adjusted dynamically simply regulating the biasing current. Experimental results, carried on 0.18 μm CMOS prototypes, show 42% range with a tank quality factor of 13.5 while driving 1.5pF output capacitance. At the band edge, biasing current is 8mA. Quadrature accuracy has been verified through the achievable image rejection of a purposely developed single side band up-converter. Measurements, performed on five samples, show a minimum image rejection of 44dB.

2005 - Flash memories for SoC: an overview on system constraints and technology issues [Relazione in Atti di Convegno]
Larcher, Luca; Pavan, Paolo; A., Maurelli
abstract

Flash memories are today one of the fundamental building blocks in modern electronic systems. Their performance (speed, consumption, alterability, nonvolatility) and the increasing importance of system reconfigurability push for flash memory integration in SoC. Unfortunately, flash integration introduces new issues both at system and at circuit/technology levels that need to be deeply investigated. From the system point of view, several aspects are involved in the choice of the flash memory type to be integrated in SoC: the most important ones, depending on the specific applications and requirements (cost, power consumption, reliability and performance requirements), are illustrated. Also circuit-technology issues specific to flash integration with high-speed logic are discussed in depth by analyzing the real case of an embedded 1-T NOR flash memory.

2005 - Radiation induced leakage current in floating gate memory cells [Articolo su rivista]
G., Cellere; Larcher, Luca; A., Paccagnella; A., Visconti; M., Bonanomi
abstract

Single ions impacting on SiO2 layers generate tracks of defects which may result in a Radiation Induced Leakage Current (RILC). This current is usually studied as the cumulative effect of ion-induced defects in capacitors with ultra-thin oxides. We are demonstrating and modeling this phenomenon in 10 nm oxides by using Floating Gate memories. The impact of a single, high-LET ion can result in severe retention problems, due to several electrically active defects, which cooperate to slowly discharge the FG. We are also proposing innovative simulation tools to reproduce this phenomenon. Results from simulations fully explain our results, and also agree with existing data on thinner (4 nm) oxides.

2005 - Statistical Simulations of Oxide Leakage Current in MOS Transistors and Floating Gate Devices [Relazione in Atti di Convegno]
Larcher, Luca; Pavan, Paolo
abstract

The purpose of this paper is to illustrate a physically-based model allowing the statistical simulations of oxide leakage currents in MOS transistors and Floating Gate memories. This model computes the leakage current through defects randomly generated in the oxide, in case accounting for the formation of percolation paths. Furthermore, a calculation procedure has been developed to calculate the threshold voltage of FG memories from simulated oxide leakage currents in some reliability conditions, thus allowing to investigate their actual Flash data retention issues and their future trends. To this regards, it will be shown how this simulation model can be used to investigate threshold voltage shift occurring in retention conditions in FG memories after both Program/erase cycles, i.e. electrical stress and radiation exposure.

2004 - A wireless network system for low data rata and multimedia data communications on trains [Relazione in Atti di Convegno]
F., Bonizzi; U., Manzoli; Larcher, Luca; Pavan, Paolo
abstract

This paper presents a wireless network system implemented on trains which is suitable for both low and high (multimedia) data rate communication. The network system is comprised of a low rate data communication section, which provides communication services useful for system management, information transmission and train monitoring. A separate section is provided with high data rate communication capabilities, which enables multimedia infotainment applications. The proposed network system is based on wireless technologies in the ISM band, i.e. Bluetooth and Wi-Fi, exploiting their characteristics to optimize the performance of the two sections. Every network node includes an embedded system required to manage the wireless network interface. The embedded system hardware architecture has been developed “ad-hoc”, as well as software procedures and algorithms needed for communication system management.

2004 - An embedded system for wireless data communications on trains [Relazione in Atti di Convegno]
F., Bonizzi; U., MANZOLI U; Larcher, Luca; Pavan, Paolo; D., Liziero
abstract

This paper presents a new wireless networksy.wm implemented on trains. This nehvork system providescommunicatio~i sewices ,for data tran.sfir and systemmanugement roefiil Jbr infornrotion tran.~mi.ssiona nd trainmonitoring. It i . h~u sed on the Bliietrioth technolos. ~vhichi . ~ iised to inte~connecf the varions train cars to thelocomotive. Every nehvork node is composed hy a powe~firlembedded sjsrem reqriired to control the Blnrtooth nehvorkinrerfirce. We developed hoth hardware architecture andsojtx.are desigti of this emhedded sy.stem. which allows to'achieve a,f& mid @cient implementation of'the protocolsand a1gorith;is drsigned for the commiinication systemmanagement

2004 - Data retention of irradiated FG memories [Relazione in Atti di Convegno]
Cellere, G; Larcher, Luca; Paccagnella, A; Modelli, A; Candelori, A.
abstract

Floating Gate (FG) memories, and in particular Flash, are the most important player in nowadays nonvolatile memory arena [1]. The basic structure of all FG memories relies on an insulated polysilicon layer (that is, the FG) interposed between the substrate and the Control Gate, CG (see Fig.1). By accumulating a net charge in the FG we can modify the threshold voltage VTH of the FG transistor, thus storing a bit of information or more. Electrons or holes in the insulated FG cannot escape from it (at least in principle), hence granting a permanent storage of information. Electrons and holes can be injected (emitted) in (from) the FG by using Channel Hot Electron (CHE) injection or Fowler-Nordheim (F-N) tunneling [1][2]. Flash memories feature extremely high density, while maintaining good speed and reliability, but also a complex control circuitry. Single Event Effect (SEE) performances of Flash memories has been studied by several authors [3][4][5]. The most radiation sensitive part of commercial Flash memories is the complex circuitry external to the FG memory cell array [3]. For converse, the loss of the charge stored in the floating gate of a programmed cell and the consequent threshold voltage shift ÄVTH have been less frequently investigated in literature [6][7][8]. By using specially designed instrumentation and devices, we have recently shown that FG charge loss upon heavy ion irradiation is not negligible [7][8], even when it does not lead to a read error at the circuit output. The charge loss subsequent to a single heavy ion strike appears to be due to two parallel mechanisms. The first is a prompt one, taking place in times shorter than those elapsed between irradiation and measurement, and it appears as the responsible for the main part of the charge loss [8]. The second mechanism, which is the main subject of this paper, is active over long times (days and weeks) and is responsible for the slow discharge of some of the hit FGs [8]. In this context, the aim of this paper is to investigate the long-term retention issues in two advanced Flash memory technologies submitted to heavy ion irradiation.

2004 - Floating Gate devices: operation and compact modeling [Relazione in Atti di Convegno]
Larcher, Luca; Pavan, Paolo
abstract

This paper describes a possible approach to Compact Modeling of Floating Gate devices. Floating Gate devices are the basic building blocks of Semiconductor Nonvolatile Memories (EPROM, EEPROM, Flash). Among these, Flash are the most innovative and complex devices. The strategy followed developing this new model allows to cover a wide range of simulation conditions, making it very appealing for device physicists and circuit designers.

2004 - Floating gate devices: operations and compact modeling [Monografia/Trattato scientifico]
Pavan, Paolo; Larcher, Luca; A., Marmiroli
abstract

The goal of this book is twofold. First, it explains the principles and physical mechanisms of Floating Gate device operations. Second, starting from a general overview on Compact Modeling issues, it illustrates features and details of a complete Compact Model of a Floating Gate device, the building block of Flash Memories, one of the “hottest” products in the semiconductor industry. Flash Memories are one of the most innovative and complex types of high-tech, nonvolatile memories in use today [see, for example, Proceedings of the IEEE, Special Issue on: Flash Memory Technology, April 2003]. Since their introduction in the early 1990s, these products have experienced a continuous evolution from the simple first products to emulate EPROM memories, to the extreme flexibility of design application in today products. This is an enabling technology: future limits are beyond our current expectations and limited only by our imagination.In the memory arena, Flash memory is the demonstration of the pervasive use of new electronic applications in our lives. Every new application can exploit this flexible and powerful memory technology, either as a stand-alone component or integrated as the enabling feature of the whole silicon integration.Flash are not just memories, they are “complex systems on silicon”: they are challenging to design, because a wide range of knowledge in electronics is required (both digital and analog), and they are difficult to manufacture. Physics, chemistry, and other fields must be integrated; and conditions must be carefully monitored and controlled in the manufacturing process.Memories demand massive investments in R&D, but they also reward with enormous potential market values. Flash memory market (considered the most important market segment among nonvolatile memories) is expected to progress at a very fast pace, and to gain the second place in the overall memory market. This is due to the optimization of cost/performance tradeoffs, and in particular to the inherent flexibility and versatility of this memory, which brings benefits in many applications.The leading application is in multimedia systems, which require memories that are increasingly larger in size, and demand ever-increasing performance characteristics. Telecommunications, computers, automotive and consumer electronics are some additional areas where these memories make possible numerous emerging applications.Moreover, the Flash memory integration is one of the irreplaceable requirements for further technological innovations, and particularly to realize the so-called system on silicon.Compact Model (CM) means an analytic model of the electrical behavior of a circuit element. Modeling is usually aimed at providing means to simulate the behavior of a device or a circuit by quantitative calculation. CM allows to highlight basic properties of a device, thus making easier the understanding and the synthesis of robust circuits. Therefore, the main intent of modeling is to forecast the behavior of a system. This holds for all integrated devices (resistors, capacitors, inductors, transistors, and also the device subject of this book: the floating gate device) and circuits. Compact Models of Floating Gate devices have the same purpose of all compact models: to be used within a program for circuit simulation. The Floating Gate transistor is the building block of a full array of memory cells and a memory chip. In a first approximation, the reading operation of a FG device, and for some cases also programming and erasing, can be considered a single-cell operation. Nevertheless, CMs are fundamental to simulate the effects of the cells not directly involved in the operation under investigation and the effects of the parasitic elements. Furthermore, they allow the simulation of the interaction with the rest of the device, and hence they are useful to check the design of the circuitry around the memory array:

2004 - On the physical mechanism of the NROM memory erase [Articolo su rivista]
Larcher, Luca; Pavan, Paolo; B., Eitan
abstract

The purpose of this paper is to investigate the physical mechanism of NROM memory erase. Three conduction mechanisms potentially responsible of NROM erase will be analyzed (tunneling and emission of electrons through both bottom and top oxide, tunneling and injection of holes over the bottom oxide barrier) by means of standard two-dimensional simulations and ad-hoc models reproducing hole and electron transport mechanisms across the oxide not included in standard device simulators. Hot-hole injection will be identified as the actual conduction mechanism of NROM erase, and two compact models capable to describe the main characteristics of NROM erase current will be developed.

2004 - Statistical simulations for flash memory reliability analysis and prediction [Articolo su rivista]
Larcher, Luca; Pavan, Paolo
abstract

In this paper, through the use of a recently proposed statistical model of stress-induced leakage current, we will investigate the reliability of actual Flash memory technologies and predict future trends. We investigate either program disturbs (namely gate and drain disturbs) and data retention of state-of-the-art Flash memory cells and use this model to correlate the induced threshold voltage shift to the typical outputs coming from oxide characterization, that are density, cross section, and energy level of defects. Physical mechanisms inducing the largest threshold voltage (V-T) degradation will be identified and explained. Furthermore, we predict the effects of tunnel oxide scaling on Flash memory data retention, giving a rule of thumb to scale the tunnel oxide while maintaining the same retention requirements.

2003 - A complete model of (EPROM)-P-2 memory cells for circuit simulations [Articolo su rivista]
Pavan, Paolo; Larcher, Luca; M., Cuozzo; P., Zuliani; A., Conte
abstract

(EPROM)-P-2 memory devices are widely used in embedded applications. For an efficient design flow, a correct modeling of these memory cells in every operation condition becomes more and more important, especially due to power consumption limitations. Although (EPROM)-P-2 cells are being used for a long time, very few compact models have been developed. Here, we present a complete compact model based on an original procedure to calculate the floating gate Potential in dc conditions, without the need of any capacitive coupling coefficient. This model is designed as a modular structure, so to simplify program/erase and reliability simulations. Program/erase and leakage currents are included by means of simple voltage-controlled current sources implementing their analytical expression. It can be used to simulate memory cells both during read operation (dc conditions) and during program and erase (transient conditions) giving always very accurate results. We will show also that, provided good descriptions of degradation mechanisms, the same model can be used also for reliability simulations, predicting charge loss due to tunnel oxide degradation.

2003 - Data retention after heavy ion exposure of Floating Gate memories: analysis and simulation [Articolo su rivista]
Larcher, Luca; G., Cellere; A., Paccagnella; A., Chimenton; A., Candelori; A., Modelli
abstract

Floating Gate (FG) memories are the most important of nowadays nonvolatile memory technologies. We are investigating the long-term retention issues in advanced Flash memory technologies submitted to heavy ion irradiation. Long tails appears in threshold voltage distribution of cells hit by ions after they have been reprogrammed. This phenomenon is more pronounced in devices with smaller gate area. Results are explained by a new physics-based model of the leakage current flowing through the damaged oxides of FG memory cells. The model calculates the trap assisted tunneling current through a statistically distributed set of defects by using electron coupling to oxide phonons. The model is used to fit experimental data and to discuss retention properties after heavy ions exposure of future devices, featuring thinner tunnel oxide.

2003 - Statistical Simulation of Leakage Currents in MOS and Flash Memory Devices with a new multi-Phonon Trap-Assisted Tunneling Model [Articolo su rivista]
Larcher, Luca
abstract

A new physics-based model of leakage current suitable for MOS and Flash memory gate oxide is presented in this paper. This model, which assumes the multi-Phonon Trap-Assisted Tunneling as conduction mechanism, calculates the total leakage current summing the contributions of the percolation paths formed by one or more aligned traps. Spatial positions and energetic levels of traps have been randomly generated within the oxide by a random number generator which has been integrated into the model. Using this model, statistical simulations of leakage currents measured from both MOS and Flash EEPROM memory tunnel oxides have been carried out. In this way, experimental leakage current distributions can be directly reproduced, thus opening a wide range of useful applications in MOS and Flash EEPROM memory reliability prediction.

2003 - Statistical simulations to inspect and predict data retention and program disturbs in Flash memories [Relazione in Atti di Convegno]
Larcher, Luca; Pavan, Paolo
abstract

A new statistical model of stress-induced leakage current (SILC) is implemented and used to predict data retention and program disturbs of state-of-the-art flash memories, and to correlate oxide characterization outputs (density, cross section, energy level of defects) to flash memory reliability. Physical mechanisms inducing the largest threshold voltage (VT) degradation are explained, and tunnel oxide scaling effects on flash reliability are predicted.

2002 - A Complete Study of SILC Effects on EEPROM Reliability [Relazione in Atti di Convegno]
Larcher, Luca; S., Bertulu; Pavan, Paolo
abstract

In this paper, we investigate SILC effects on E2PROM reliability: the influence of Program/Erase bias and cycle number, of oxide thickness scaling and quality, and of storage field on retention properties of E2PROM memory cell. To accomplish this task, we use a recently proposed compact E2PROM model, extended to include SILC, thus bridging the gap between oxide quality characterization activity performed on MOS test structures, and its actual impact on E2PROM memories.

2002 - A complete study of SILC effects on E2PROM reliability [Relazione in Atti di Convegno]
Larcher, L.; Bertulu, S.; Pavan, P.
abstract

2002 - A New Analytical Model of Channel Hot Electron (CHE) and CHannel Initiated Secondary ELectron (CHISEL) Current Suitable for Compact Modeling [Relazione in Atti di Convegno]
Larcher, Luca; Pavan, Paolo
abstract

This paper presents for the first time a new approach to hot-carrier phenomena leading to an analytical model of both Channel Hot Electron (CHE) and CHannel Initiated Secondary ELectron (CHISEL) currents. This model can be incorporated in Spice-like models of MOS transistors and Floating Gate (FG) devices to include hot carrier phenomena also in circuit simulations.

2002 - A new compact DC model of floating gate memory cells without capacitive coupling coefficients [Articolo su rivista]
Larcher, Luca; Pavan, Paolo; S., Pietri; L., Albani; A., Marmiroli
abstract

This paper presents for the first time a new compact SPICE model of floating gate nonvolatile memory cells capable to reproduce effectively its complete dc electrical behavior in every bias conditions. This model features many advantages compared to previous ones: it is simple and easy to implement since it uses SPICE circuit elements, is scalable, and its computational time is not excessive. It is based on a new procedure that calculates the floating gate voltage without using fixed capacitive coupling coefficients, thus improving the floating gate voltage estimate that is fundamental for the correct modeling of cell operations. Moreover, this model requires only the usual parameters adopted for SPICE-like models of MOS transistors plus the floating gate-control gate capacitance, making it very attractive to industry as the same parameter extraction procedure used for MOS transistors can be directly applied. The model we propose has been validated on (EPROM)-P-2 and Flash memory cells manufactured in existing technology (0.35 mum and 0.25 mum) by STMicroelectronics.

2002 - Anomalous charge loss from Floating-Gate Memory Cells due to heavy ions irradiation [Articolo su rivista]
G., Cellere; A., Paccagnella; Larcher, Luca; A., Chimenton; J., Wyss; A., Candelori; A., Modelli
abstract

We are presenting new data on the charge loss in large Floating Gate (FG) memory arrays subjected to heavy ion irradiation. Existing models for charge loss from charged FG and generation-recombination after a heavy ion strike are insufficient to justify (or in contrast with) our experimental results. In particular, the charge loss is by far larger than predicted by existing models, it depends on the number of generated holes, not on those surviving recombination, and it is larger for FGs with larger threshold voltage before irradiation. We show that these data can be explained as the effect of two different mechanisms. The first one is a semi-permanent multi Trap Assisted Tunneling (TAT), which closely resembles anomalous SILC in electrically stressed devices. The second mechanism is a transient phenomenon responsible for the largest part of the lost FG charge. Detailed physical modeling of this mechanism is still not available, owing to the limited knowledge of the physical background under these phenomena, but three possible models are explored and discussed.

2002 - Impact of programming charge distribution on threshold voltage and subthreshold slope of NROM memory cells [Articolo su rivista]
Larcher, Luca; Verzellesi, Giovanni; Pavan, Paolo; Lusky, E.; Bloom, I.; Eitan, B.
abstract

The aim of this paper is to achieve a correct description of programming charge distribution in NROM memory devices. This is essential to prove device functionality and to extrapolate scaling limits of devices. To this purpose we use an inverse modeling based methodology using measurements easily performed, such as subthreshold characteristics and threshold voltage measurements. We will show a simple model of programming charge distribution that can be easily implemented in 2D TCAD simulations. Results show good agreement between measured and simulated currents under different bias conditions and for different programming levels.

2002 - Micro breakdown in small-area ultra-thin gate oxide [Articolo su rivista]
Cellere, G.; Larcher, Luca; Valentini, M. G.; Paccagnella, A.
abstract

Purpose of this work was to study the gate oxide leakage current in small area MOSFETs. We stressed about 300 nMOSFETs with an oxide thickness tOX=3.2 nm by using a staircase gate voltage. We detected the oxide breakdown at an early stress stage, by measuring the leakage current at low fields during the stress. The gate leakage of stressed devices is broadly distributed, but two well-defined current regimes appear, corresponding to currents larger than 1mA or smaller than 100pA, respectively. We focused our attention on the small current regime, which shows all the electrical characteristics typical of the soft breakdown, with the noticeable exception of the current intensity that is much smaller than usually reported in literature, being the average leakage around 40pA at VG=+2V. For this reason, we introduce the oxide micro breakdown. The leakage kinetics during stress, the gate-voltage characteristics of stressed devices, and the breakdown statistical distributions are in agreement with the formation of a single conductive path across the oxide formed by few oxide defects. Just two positively charged traps can give rise to a gate leakage comparable to those experimentally found, as evaluated by using an new original model of Double Trap Assisted Tunneling (D-TAT) developed ad hoc.

2002 - Plasma-induced Micro Breakdown in small area MOSFETs [Articolo su rivista]
G., Cellere; Larcher, Luca; M. G., Valentini; A., Paccagnella
abstract

Plasma treatments, indispensable for manufacturing of ULSI integrated circuits, may lead to a latent damage in gate oxides of CMOS components. Latent damage may endanger the device long-term reliability, which is usually tested over large area MOS devices. In this work, we investigated the impact of latent plasma induced damage on the reliability of nMOSFETs with small gate area and gate oxide thickness of 3.2 nm. To this purpose, we stressed 1,500 devices with different antenna areas by using a staircase-like stress voltage, and by monitoring the gate leakage at the gate voltage VG=2V. The stress was always stopped because of an abrupt jump in the gate current. The statistics obtained for the breakdown current is characterized by two different oxide breakdown modes. The first is the well-known HardBreakdown, while the second one, which we called Micro Breakdown, can be modeled as a Double Trap Assisted Tunneling mechanism, and is characterized by a very small leakage current (around 100pA at the gate voltage VG=2V). In devices with large antenna, i.e., more prone to be damaged by plasma processing, the number of micro broken oxides is larger and breakdown occurs at lower voltages than in reference devices (non plasma damaged). For converse, the Hard Breakdown statistics shows only a weak dependence on the gate antenna ratio of plasma damaged devices. This has been explained by considering the intrinsic nature of latent plasma-induced oxide defects, linkedto the different generation mechanisms involved in Micro Breakdown and Hard Breakdown phenomena.

2002 - SILC effects on EEPROM memory cell reliability [Articolo su rivista]
Larcher, Luca; Bertulu, S.; Pavan, Paolo
abstract

In this paper, we will investigate SILC effects on the reliability of E2PROM memories. Particularly, we will analyze the influence on the retention properties of E2PROM memory devices of Program/Erase number of cycles and bias conditions, oxide thickness scaling and quality, and storage field. To accomplish this task, we will use a recently proposed compact E2PROM model, which has been extended to include SILC, thus bridging the gap between the oxide quality characterization activity performed on MOS transistors and capacitors, and the actual impact of SILC on the functioning of E2PROM memories.

2002 - Single Event Charge Loss in EPROMs [Relazione in Atti di Convegno]
G., Cellere; Larcher, Luca; J., Wyss; A., Candelori; P., Caprara; A., Paccagnella
abstract

In this summary we are showing preliminary but important and new results obtained in EPROM arrays after high-LET ion irradiation. These devices are characterized by larger integration density and thicker tunnel oxide than Flash memories [15-16]. Moreover, these device are designed to store information for very long times, not for frequent refresh of information, or for on-site reprogramming. As a consequence, target applications are different from those of Flash, too. Nevertheless, we are showing that, if devices are subjected to heavy ion irradiation (as can happen during long space missions) the information stored in EPROM are as vulnerable as those stored in Flash memories [15].

2001 - A model of the stress induced leakage current in gate oxides [Articolo su rivista]
Larcher, Luca; A., Paccagnella; G., Ghidini
abstract

A new quantitative model of the stress induced leakage current (SILC) in MOS capacitors with thin oxide layers has been developed by assuming the inelastic trap-assisted tunneling as the conduction mechanism. The oxide band structure has been simplified by replacing the trapezoidal barrier with two rectangular barriers. An excellent agreement between simulations and experiments has been found by adopting a trap distribution Gaussian in space and in energy. Only minor variations pf the trap distribution parameters were observed by increasing the injected charge during electrical stress, indicating that oxide neutral defects with similar characteristics are generated at any stage of the stress.

2001 - A New Compact Model of Floating Gate Non-Volatile Memory Cells [Relazione in Atti di Convegno]
Larcher, Luca; Pavan, Paolo; F., Gattel; L., Albani; A., Marmiroli
abstract

This paper presents a new compact model of Floating Gate Non-Volatile Memory Cells using SPICE circuit elements. It features many advantages compared to previous models: it is simple and easy to implement, scalable, and its computational time is not critical, thus making it very attractive to industry. It is based on a new procedure that improves the floating gate voltage estimate. The parameter extraction procedure is the same of a MOS transistor.

2001 - A new compact Spice-like model of E2PROM Memory cells suitable for DC and transient simulations [Relazione in Atti di Convegno]
Larcher, Luca; Pavan, Paolo; Cuozzo, M.; Marmiroli, A.
abstract

This paper presents for the first time a new compact Spice-like model of an E2PROM memory cell suitable for both DC and transient circuit simulations. This model is based on a new Floating Gate voltage calcuation procedure that improves strongly the accuracy of the modeling of the cell. Moreover, this model features many advantages compared to the previous ones: i) it is simple to implement and scale; ii) its computational time is not critical; iii) its parameter extractin procedure is the same of a MOS transistor; iv) it can be easily upgraded tot ake into account leakage current contributions (SILC).

2001 - A new model of gate capacitance as a simple tool to extract MOS parameters [Articolo su rivista]
Larcher, Luca; Pavan, Paolo; F., Pellizzer; G., Ghidini
abstract

This paper tackles the difficult task to extract MOS parameters by a new model of the gate capacitance that takes into account both poly-Si depletion and charge quantization and includes temperature effects. A new fast and iterative procedure, based on this simplified self-consistent model, will be presented to estimate simultaneously the main MOS system parameters (oxide thickness, substrate, and poly-Si doping) and oxide held, surface potentials at the Si/SiO2 and at the poly-Si/SiO2 interfaces. Its effectiveness will be demonstrated by comparing oxide field and oxide thickness to those extracted by other methods proposed in the literature. Moreover, these methods are critically reviewed and we suggest improvements to reduce their errors, The agreement between CV simulation and experimental data is good without the need of any free parameter to improve the fitting quality for several gate and substrate materials combinations. Finally, a simple law to estimate substrate and poly-Si doping in n+/n+ MOS capacitor from CV curves is proposed.

2001 - Bias and W/L dependence of capacitive coupling coefficients in floating gate memory cells [Articolo su rivista]
Larcher, Luca; Pavan, Paolo; L., Albani; T., Ghilardi
abstract

In this paper, a complete study of capacitive coupling coefficients dependence on bias and W/L will be presented, including a review on classic methods to extract their value from electrical characterization. Capacitive coupling ratios have been usually adopted to model floating gate (FG) memory cells, in particular to deduce the value of FG potential. Now they are determined by means of a new model (recently proposed in the literature), starting from a new procedure to evaluate the FG potential. Results obtained with this new model will be compared to classic values. Particularly, their bias dependence (during write/read/erase of Flash memory cells) will be deeply investigated, thus demonstrating the limits of considering them constants, as is usually done. By analyzing their W/L dependence, we will deduce useful information on the effects of scaling and the role played by fringing capacitances. The most important methods reported in the literature to estimate the control gate and drain capacitive coupling ratios will be accurately reviewed, thus showing that such procedures are often cumbersome and inaccurate. It is worth noting that for the first time, alpha (B) and alpha (S) will be studied in detail.

2001 - Gate current in ultrathin MOS capacitors: a new model of tunnel current [Articolo su rivista]
Larcher, Luca; A., Paccagnella; G., Ghidini
abstract

We have deduced the analytical expression of the tunneling current across a thin oxide layer for a MOS capacitor, by introducing a new double-box simplified model of the oxide layer. We have developed this model to study some characteristics of the tunneling current, which are neglected when the usual Fowler-Nordheim description is adopted. Matching between experimental and simulated curves is excellent, and no free parameter is needed to adjust the fitting quality, once the values of the main physical parameters are chosen. The model quantitatively describes the quantum oscillations of the gate current produced by the interference between the coherent incident electron-wave and the electron-wave reflected at the oxide/anode interface, From the period of the quantum oscillations, we have deduced a semiempirical relation useful to evaluate the oxide thickness. The quantum oscillations amplitude is related to the oxide/anode interface roughness, which is accounted for by a rugosity parameter introduced in our model. The temperature dependence of the tunneling current has been taken into account as well in two parameters of the model.

2001 - Impact of the As dose in 0.35 mu m EEPROM technology: characterization and modeling [Articolo su rivista]
N., Galbiati; G., Ghidini; C., Cremonesi; Larcher, Luca
abstract

The core of the EEPROM memory cell is the tunnel oxide grown on an n(+) implant named capacitor implant which also electrically connects the cell drain. Different implant doses have been performed to optimize the cell junction characteristics in a 0.35 mum EEPROM technology. The impact on the quality and electrical characteristics of the tunnel oxide is hereafter analyzed. Furthermore, a modeling is presented, giving estimation of the oxide thickness and substrate superficial doping with a good correlation of these last values with SIMS measurements. As expected, a linear relation between the implant dose and the superficial doping has been found. (C) 2001 Elsevier Science Ltd. All rights reserved.

2001 - Radiation effects on floating-gate memory cells [Relazione in Atti di Convegno]
G., Cellere; P., Pellati; P., Olivo; A., Chimenton; J., Wyss; Larcher, Luca; A., Paccagnella
abstract

We have addressed the problem of threshold voltage (VTH) variation in Flash memory cells after heavy ion irradiation by using specially designed array structures and test instruments. After irradiation, low VTH tails appear in VTH distributions, growing with ion LET and fluence. In particular, high LET ions, such as Iodine used in this work, can produce a bit flip. Since the existing models cannot account for large charge losses from the Floating Gate, we propose a new mechanism, based on the excess of positive charge produced by a single ion, temporarily lowering the tunnel oxide barrier (Positive charge Assisted Leakage Current, PALC) and enhancing the tunneling current. This mechanism fully explains experimental data we present

1999 - A model of radiation induced leakage current (RILC) in ultra-thin gate oxides [Relazione in Atti di Convegno]
Larcher, Luca; A., Paccagnella; M., Ceschia; G., Ghidini
abstract

An analytical model of Radiation Induced Leakage Current (RILC) has been developed for ultra-thin gate oxides submitted to high dose ionizing radiation. The model is based on the solution of the Schrödinger equation for a simplified oxide band structure, where RILC occurs through electron trap-assisted tunneling. The values of the model parameters have been calibrated by comparing the transmission probabilities obtained in this model with those obtained through the WKB method in the actual oxide band structure. No free fitting parameter has been introduced, and all physical constant values have been selected within the values found in literature. Different trap distributions have been considered as candidates, but the comparison between simulated and experimental curves have indicated that a double gaussian distribution in space and in energy grants the best fit of the experimental results for different ionizing particles, oxide fields during irradiation, radiation doses, and oxide thickness. Excellent matching has been found for both positive and negative RILC by using a single trap distribution. The trap density linearly increases with the radiation dose and decreases with the oxide field during irradiation. The trap distribution is spatially symmetrical in the oxide, centered in the middle of the oxide thickness, and is not modified as the cumulative dose increases.

1999 - Temperature dependence of stress induced leakage current in ultra-thin oxides [Relazione in Atti di Convegno]
M., Ceschia; A., Paccagnella; A., Cester; Larcher, Luca; G., Ghidini
abstract

In ultra-thin oxides Stress Induced Leakage Current (SILC) is measured at low oxide fields after electrical stresses [1-5]. Different current components contribute to SILC, including transient and non-reproducible ones. This work is focused on the DC component [6], which is currently attributed to an electron trap assisted tunnelling trough neutral traps, which are created by electrical stresses [1-5]. Most of the works on SILC have considered the room (or high) temperature characteristics, demonstrating the onset of recovery processes as the temperature increases. In this work we have performed new experiments at temperatures < 0 °C, in order to study the SILC dependence on measurement and stress temperature.

1998 - A new model for tunneling conduction in ultra-thin dielectrics [Relazione in Atti di Convegno]
Larcher, L.; Paccagnella, A.; Scarpa, A.; Ghidini, G.
abstract

A new simple model for tunnel electron injection into oxides has been proposed, alterative to the conventionQ! FowlerNordheim expression. The latter fails to predict oscillations in the curren't-voltage curves in ultra-thin oxides, due to the limits of the WKB approximation. Our model nicely fits the experimental I-V results and can be used to investigate ultra-thin film characteristics. Finally, we propose an empirical law correlating oxide thickness to the I-V oscillation period.

1998 - A new model for tunnelling conduction in ultra-thin dielectrics [Relazione in Atti di Convegno]
Larcher, Luca; A., Paccagnella; A., Scarpa; G., Ghidini
abstract

A new simple model for electron tunnel injection into oxides has been proposed, alternative to the conventional Fowler-Nordheim expression. The latter fails to predict oscillations in the current-voltage curves in ultra-thin oxides, due to the limits of the WKB approximation. Our model nicely fits the experimental I-V results and can be used to investigate ultra-thin film characteristics. Finnaly, we propose an empirical law correlating oxide thickness to I-V oscillation period.

1998 - New double-box model for SILC and tunnel current in ultra-thin MOS devices [Relazione in Atti di Convegno]
Larcher, Luca; A., Paccagnella; A., Scarpa; G., Ghidini
abstract

We propose a new double-box model of the oxide conduction band of MOS devices, aiming to quantitatively predict the gate current across ultra-thin oxides. Simulations nicely fit the experimental quantum oscillations of the gate current in the Fowler-Nordheim tunnelling regime. The oscillation period of the gate current has been empirically correlated with the oxide thickness. The low-field stress induced leakage current can be fitted by our model as well, by inserting oxide traps mediating an inelastic trap-assisted tunnelling

1998 - New model of tunnelling current and SILC in ultra-thin oxides [Poster]
Larcher, L.; Paccagnella, A.; Scarpa, A.; Ghidini, G.
abstract

We propose a new double-box model of the oxide conduction band of MOS devices, aiming to quantitatively predict the gate current across ultra-thin oxides. Simulations nicely fit the experimental quantum oscillations of the gate current in the Fowler-Nordheim tunnelling regime. The oscillation period of the gate current has been empirically correlated with the oxide thickness. The low-field Stress Induced Leakage Current can be fitted by our model as well, by inserting oxide traps mediating an inelastic trap-assisted tunnelling.

Università degli studi di Modena e Reggio Emilia

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