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Francesco Maria PUGLISI

Professore Associato presso: Dipartimento di Ingegneria "Enzo Ferrari"


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Pubblicazioni

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, Enrique; 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 - 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.


2018 - Coexistence of volatile and non-volatile resistive switching in 2D h-BN based electronic synapses [Relazione in Atti di Convegno]
Shi, Y.; Pan, C.; Chen, V.; Raghavan, N.; Pey, K. L.; Puglisi, F. M.; Pop, E.; Wong, H. -S. P.; Lanza, M.
abstract

We present the first fabrication of electronic synapses using two dimensional (2D) hexagonal boron nitride (/j-BN) as active switching layer. The main advantage of these devices compared to the transition metal oxide (TMO) based counterparts is that multilayer h-BN stacks show both volatile and non-volatile resistive switching (RS) depending on the programming stresses applied, which allows implementing short-term (STP) and long-term plasticity (LTP) rules using a single device and without the need of complex architectures.


2018 - Energy-efficient logic-in-memory I-bit full adder enabled by a physics-based RRAM compact model [Relazione in Atti di Convegno]
Puglisi, Francesco Maria; Pacchioni, Lorenzo; Zagni, Nicolo; Pavan, Paolo
abstract

In this work, we explore the RRAM-based IMPLY logic by means of circuit simulations. Differently from previous works, we use a physics-based compact model of RRAM devices able to reproduce both the AC and the DC behavior, accounting for the intrinsic variability of the resistive states and the logic state degradation. A new implementation of a 1-bit full adder with unique properties for low-power circuits is proposed, and its performance in terms of energy consumption and execution time is evaluated by simulations. Results are compared against recent experiments, demonstrating a good agreement and indicating the direction for further improvement.


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 - On the impact of channel compositional variations on total threshold voltage variability in nanoscale InGaAs MOSFETs [Relazione in Atti di Convegno]
Zagni, Nicolo; Puglisi, Francesco Maria; Pavan, Paolo; Verzellesi, Giovanni
abstract

In this paper we present an analysis of the impact of channel compositional variations on the total threshold voltage variability in nanoscale III-V MOSFETs. The analysis is carried out on a template Dual-Gate Ultra-Thin Body (DG-UTB) MOSFET through TCAD simulations in Sentaurus by Synopsys. The Impedance Field Method (IFM) is employed to evaluate statistical variability for five different sources: Random Dopant Fluctuation (RDF), Work Function Fluctuation (WFF), Body- and Gate-Line Edge Roughness (B-LER and G-LER) and Band Gap Fluctuation (BGF). BGF arises due to the compositional variations of Indium in the compound semiconductor composing the channel, namely InGaAs. Our analysis shows that, by appropriately modeling band gap fluctuations, it is possible to identify a worst-case total relative Vt variability for different amounts of Indium mole fraction variations, providing technologists with an important reference. Side-effects of channel compositional variations on other variability sources are evaluated as well, and are found to have a non-negligible impact on B-LER only.


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.


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 - 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 - Combined variability/sensitivity analysis in III-V and silicon FETs for future technological nodes [Relazione in Atti di Convegno]
Zagni, Nicolo'; Puglisi, Francesco Maria; Verzellesi, Giovanni; Pavan, Paolo
abstract

In this paper, we present a combined analysis of variability and sensitivity effects on electrical characteristics of In0.53Ga0.47As and Si ultra-scaled devices with LG= 15 nm. Two different structures, namely Dual-Gate and FinFET, are analyzed for both channel materials. Variability sources considered in this work are Random Dopant Fluctuation (RDF), Work Function Fluctuation (WFF), Body-and Gate-Line Edge Roughness (LER). Sensitivity is assessed by varying process parameters, namely gate length, channel thickness, oxide thickness, and channel doping. Results show that variability in lnGaAs is dominated by both WFF and Body-LER, whereas WFF only dominates in Si devices. Moreover, control over gate length and channel thickness in In0.53Ga0.47As technology is fundamental in order to keep variability under reasonable values, with FinFET showing slightly better results than Dual-Gate structure. Variability is a major challenge for the industrial introduction of In0.53Ga0.47As, which could limit the alleged superior performance of In0.53Ga0.47As over Si.


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 - Measuring and analyzing Random Telegraph Noise in Nanoscale Devices: The case of resistive random access memories [Relazione in Atti di Convegno]
Puglisi, Francesco Maria
abstract

In this paper, we discuss some of the measurement and analysis techniques for Random Telegraph Noise (RTN). Due to its detrimental impact on devices, RTN mechanism must be investigated and integrated into device models. However, RTN analysis requires a self-consistent framework in which automated measurement techniques, data analysis procedures, and physics-based modeling are blended together. Here we discuss guidelines to perform corrrect RTN measurements, and statistical techniques to perform advanced data analysis. This allows getting reliable results, which can lead to an unbiased physical interpretation of the phenomenon. The statistical analysis of RTN measured in hafnium oxide RRAM devices allows revealing the mechanism leading to the wide RTN fluctuations in high-resistive state, as well as the physical properties of the defect species involved in this phenomenon.


2017 - Modelling nanoscale n-MOSFETs with III-V compound semiconductor channels: From advanced models for band structures, electrostatics and transport to TCAD [Relazione in Atti di Convegno]
Caruso, E.; Carapezzi, S.; Visciarelli, M.; Gnani, E.; Zagni, N.; Pavan, P.; Palestri, P.; Esseni, D.; Gnudi, A.; Reggiani, S.; Puglisi, F. M.; Verzellesi, G.; Selmi, L
abstract

We review a few state of the art solutions and recent developments to model short channel III-V compound semiconductor n-MOSFETs based on full quantum transport, semiclassical multi-valley / multi-subband transport and TCAD models. The pros and cons of each, and the insights they can deliver, are illustrated with examples from recent technology developments and literature. Areas where improvements and implementations at TCAD level are most necessary are highlighted as well.


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 - Random dopant fluctuation variability in scaled InGaAs dual-gate ultra-thin body MOSFETs: source and drain doping effect [Relazione in Atti di Convegno]
Zagni, Nicolo; Puglisi, Francesco Maria; Pavan, Paolo; Verzellesi, Giovanni
abstract

In this paper, we present simulation results on statistical variability of threshold voltage and the respective sensitivity to process variations in Dual Gate Ultra-Thin Body (DG-UTB) InGaAs nMOSFETs at two technological nodes (with physical gate length Lg = 15 nm and Lg = 10.4 nm). Particularly, we focus on the effect of Random Dopant Fluctuations (RDF) in both the channel and the source/drain regions. While the effect of other variability sources (i.e., workfunction fluctuation, WFF, and line edge roughness, LER) can be controlled by existing technological strategies, RDF can become significant due to the 'source-starvation' effect. From our analysis, we find in fact that RDF is strongly dependent on source/drain doping, while the effect due to channel doping variation is marginal. Moreover, results indicate the possibility of achieving lower RDF variability effects at very high source/ drain doping levels that are beyond the reach of current process technology. Hence, RDF can potentially become the limiting factor to the overall variability in ultra-scaled InGaAs devices due to the difficulties in achieving very high source/drain doping.


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 (< 10×10 nm2) RRAMs and their reliability.


2017 - The impact of interface and border traps on current–voltage, capacitance–voltage, and split‐CV mobility measurements in InGaAs MOSFETs [Articolo su rivista]
Pavan, Paolo; Zagni, Nicolo'; Puglisi, Francesco Maria; Alian, Alireza; Thean, Aaron Voon Yew; Collaert, Nadine; Verzellesi, Giovanni
abstract

In this article, we present coupled experimental/simulated results about the influence of interface and border traps on the electrical characteristics and split-CV mobility extraction in InGaAs MOSFETs. These results show that border traps limit the maximum drain current under on-state conditions, induce a hysteresis in the quasi-static transfer characteristics, as well as affect CV measurements, inducing an increase in the accumulation capacitance even at high frequencies where trap effects are commonly assumed to be negligible. Hysteresis in the transfer characteristics can be used as a sensitive monitor of border traps, as suggested by a sensitivity analysis where either the energetic or the spatial distribution of border traps are varied. Finally, we show that mobility extraction by means of the split-CV method is affected by appreciable errors related to the spurious contributions of interface and border traps to the total gate charge, ultimately resulting in significant channel mobility underestimation. In very narrow channel devices, channel electron spilling over the InP buffer layer can also contribute to mobility measurement inaccuracy.


2017 - Threshold Voltage Statistical Variability and Its Sensitivity to Critical Geometrical Parameters in Ultrascaled InGaAs and Silicon FETs [Articolo su rivista]
Zagni, Nicolo'; Puglisi, Francesco Maria; Verzellesi, Giovanni; Pavan, Paolo
abstract

We investigate the statistical variability of the threshold voltage and its sensitivity to critical geometrical parameters in ultrascaled In0.53Ga0.47As and Si MOSFETs by means of 3-D quantum-corrected drift-diffusion simulations. Dual-gate ultrathin-body and FinFET device structures are analyzed for both channel materials. To assess the variability and sensitivity effects also from the scaling perspective, we consider devices belonging to two technological nodes with gate lengths 15 and 10.4 nm, designed according to International Technology Roadmap for Semiconductors (ITRS) specifications. Variability sources included in our analysis are random-dopant fluctuation, work-function fluctuation (WFF), as well as body- and gate-line-edge roughness (LER). Sensitivity to critical geometrical parameters is assessed by varying gate length, channel thickness, and oxide thickness. Results point out the major detrimental effect of WFF and Body-LER for InGaAs FETs, whereas WFF dominates in Si counterparts. Moreover, the sensitivity analysis shows that control over gate length and channel thickness in the InGaAs technology is fundamental in order to keep variability within tolerable values. Scaling of the InGaAs technology highlights the importance of abiding to ITRS projections regarding LER control improvement. Furthermore, a tight channel thickness control is required in ultrascaled devices due to the large sensitivity of the threshold voltage to the channel thickness combined with increased variability.


2017 - Variability and sensitivity to process parameters variations in InGaAs Dual-Gate Ultra-Thin Body MOSFETS: A scaling perspective [Relazione in Atti di Convegno]
Zagni, Nicolò; Puglisi, Francesco Maria; Verzellesi, Giovanni; Pavan, Paolo
abstract

In this work, we present a combined analysis on the statistical variability of threshold voltage, on-state current, and leakage current of III-V ultra-scaled MOSFETs. In addition, we analyze the sensitivity of threshold voltage to critical geometrical and process parameters variations (i.e, gate length, channel thickness, oxide thickness and channel doping). Our analysis verifies the scaling potential of the InGaAs Technology from the variability/sensitivity standpoint for two technologicaTnodes (Lg = 15 nm, Lg = 10.4 nm), by means of Quantum Drift-Diffusion (QDD) simulations. The structure under investigation is a template Dual-Gate Ultra-Thin Body device realized following ITRS projections. The variability sources under consideration are: Random Dopant Fluctuation (RDF), Work Function Fluctuation (WFF), Body- and Gate-Line Edge Roughness (LER). The sensitivity analysis of threshold voltage is performed by considering also the effects of statistical variability to evaluate their combined effect. The results of the statistical variability analysis highlight the importance of carefully controlling Body-LER, as forecasted in the new IRDS report. Moreover, the combined effect of variability and sensitivity to channel thickness are found to be critical to the scaling process (down to Lg =10.4 nm), as it leads to significant leakage increase or performance reduction, potentially resulting in always-on devices.


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.


2016 - A consistent picture of cycling dispersion of resistive states in HfOx resistive random access memory [Relazione in Atti di Convegno]
Puglisi, Francesco Maria; Pavan, Paolo
abstract

In this paper we present the results of a systematic study of resistive states cycling dispersion in HfOx Resistive Random Access Memory (RRAM). A wide set of experimental data is collected on several RRAM devices in different operating conditions. A compact model is exploited lo link the device electrical response to its physical characteristics, delivering a clear physical picture of cycling dispersion and of its sensitivity to operating conditions. The implications of operating voltage, current compliance, and temperature on the device reliability are clarified. Particularly, the dispersion of both RHRS and RLRS is much worsened at low current compliance, which reduces the worst-case read window establishing a trade-off between device reliability and power consumption.


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 - Effects of Border Traps on Transfer Curve Hysteresis and Split-CV Mobility Measurement in InGaAs Quantum-Well MOSFETs [Relazione in Atti di Convegno]
Pavan, Paolo; Zagni, Nicolo'; Puglisi, Francesco Maria; Alian, Alireza; Thean, Aaron Voon Yew; Collaert, Nadine; Verzellesi, Giovanni
abstract

In this paper we present TCAD simulation and experimental results on the influence of interface and border traps on the electrical characteristics of InGaAs quantum-well MOSFETs. These results show that border traps limit the maximum ION, induce a hysteresis in the quasi-static transfer characteristics, and markedly affect CV measurements, inducing a large increase in the accumulation capacitance even at high frequencies where trap effects are commonly assumed to be negligible. The latter effect is particularly insidious from the technologist's perspective, since it can partially compensate quantum capacitance reduction effects, leading to CV data misinterpretation. Interface traps affect mainly the subthreshold slope of IV characteristics and cause frequency dispersion under depletion conditions. Finally, we show that channel mobility extracted by means of the split-CV method is affected by spurious contributions to the gate charge related to both interface and border traps, resulting in channel mobility underestimation.


2016 - Guidelines for a Reliable Analysis of Random Telegraph Noise in Electronic Devices [Articolo su rivista]
Puglisi, Francesco Maria; Pavan, Paolo
abstract

In this paper, we propose new guidelines for the analysis of random telegraph noise (RTN) in electronic devices. Starting from an in-depth understanding of RTN signal characteristics, we will identify the correct measurement conditions to enable RTN analysis as a characterization tool for electronic devices. The estimate of RTN statistical parameters may indeed strongly depend on the choice of measurement conditions. We will carefully consider both the measurement limits and the extraction process constraints to devise a strategy to identify RTN signals measured in conditions allowing a meaningful estimation of their parameters. The proposed strategy will be tested on a variety of different RTN signals and operating conditions.


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 - 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

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 - 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 - 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 analysis as a tool to link physical device features to electrical reliability in nanoscale devices [Relazione in Atti di Convegno]
Puglisi, Francesco Maria
abstract

In this work, we report a detailed discussion on the techniques and the requirements needed to enable Random Telegraph Noise (RTN) analysis as a tool to investigate device reliability. Starting with the understanding of the RTN signal properties, a set of best practices to perform measurements and data analysis is established to guarantee reliable results and a correct ensuing physical interpretation. It will be shown that combining dedicated and careful experiments with refined data analysis and comprehensive physics simulations is hence required to enable RTN analysis as a safe and innovative investigation tool for electron devices. The effectiveness of RTN analysis as an investigation tool is demonstrated on both FinFET and resistive memory devices: the parameters of RTN as observed in the experiments performed on FinFETs allow understanding the details of the defects generation during stress in such devices; RTN analysis on RRAM allows understanding the physical origin of RTN in these devices and to estimate the physical properties of defects involved in the phenomenon.


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 - 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.


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 Novel Program-Verify Algorithm for Multi-Bit Operation in HfO<sub>2</sub> RRAM [Articolo su rivista]
Puglisi, F. M.; Wenger, C.; Pavan, P.
abstract

In this letter, we propose a dispersion-aware program-verify algorithm to enable reliable multi-bit operations in HfO2-based RRAM. The significant intrinsic dispersion of the resistive states, typically hindering multi-bit operations, is exploited to devise a program-verify scheme which enables the multi-bit operations with unique properties of failure resilience and adaptability to degradation. We show that an appropriate choice of the algorithm parameters can minimize the average number of cycles needed to program the cell, enabling fast and reliable multi-bit operation. This maximizes the bit/cell ratio and minimizes the dispersion of targeted resistive states.


2015 - A microscopic physical description of RTN current fluctuations in HfO<inf>x</inf> 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 - 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 - 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 - Temperature impact on the reset operation in HfO2 RRAM [Articolo su rivista]
Puglisi, Francesco Maria; Qafa, Altin; Pavan, Paolo
abstract

In this letter we report about the impact of temperature on the reset process in HfO2 RRAM devices. I-V analysis of the device during consecutive switching cycles in different operating conditions and temperatures is performed. A compact model is exploited to extrapolate the properties of the conductive filament after the reset operation. The different temperature dependences of the reset process and the charge transport in High Resistive State are taken into account: by extracting the effective activation energy of the charge transport in High Resistive State, we are able to estimate the effect of temperature on the reset process. A linear relation is found between barrier thickness and reset temperature. High temperature switching may improve cycling variability at ultra- low reset voltage.


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 study on HfO2 RRAM in HRS based on I–V and RTN analysis [Articolo su rivista]
Puglisi, Francesco M.; 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 - An investigation on the role of current compliance in HfO2-based RRAM in HRS using RTN and I-V data [Relazione in Atti di Convegno]
Francesco M. Puglisi;Paolo Pavan
abstract

In this paper we investigate the effect of current compliance during forming in HfO2-based Resistive Random Access Memories (RRAMs). We implemented a thorough statistical characterization of Random Telegraph Noise (RTN) in High Resistive State (HRS). Complex RTN signals are analyzed through a Factorial Hidden Markov Model (FHMM) approach, deriving the statistical properties of traps responsible for the multi-level RTN measured in these devices. Noise is explored in devices formed at different current compliances, demonstrating a direct relation between the current compliance, the cross-section of both the CF and the dielectric barrier created during the reset operation, and the number of active traps contributing to the RTN.


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, F. M.; Veksler, D.; Matthews, K.; Bersuker, G.; Larcher, L.; Padovani, A.; Vandelli, L.; Pavan, P.
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 - Factorial Hidden Markov Model analysis of Random Telegraph Noise in Resistive Random Access Memories [Articolo su rivista]
F.M. Puglisi; Paolo Pavan
abstract

This paper presents a new technique to analyze the characteristics of multi-level random telegraph noise (RTN). RTN is dened as an abrupt switching of ei- ther the current or the voltage between discrete values as a result of trapping/de-trapping activity. RTN sig- nal properties are deduced exploiting a factorial hid- den Markov model (FHMM). The proposed method considers the measured multi-level RTN as a super- position of many two-levels RTNs, each represented by a Markov chain and associated to a single trap, and it is used to retrieve the statistical properties of each chain. These properties (i.e. dwell times and amplitude) are directly related to physical properties of each trap.


2014 - Instability of HfO2 RRAM devices: Comparing RTN and cycling variability [Relazione in Atti di Convegno]
Puglisi, F. M.; Larcher, L.; Pavan, P.; Padovani, A.; 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 - Progresses in Modeling HfOx RRAM Operations and Variability [Articolo su rivista]
Larcher, Luca.; Pirrotta, Onofrio.; Puglisi, Francesco M.; 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.


2013 - A Compact Model of Hafnium-Oxide-Based Resistive Random Access Memory [Relazione in Atti di Convegno]
F. M. Puglisi; P. Pavan; A. Padovani; L. Larcher
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 - An Empirical Model for RRAM Resistance in Low- and High-Resistance State [Articolo su rivista]
F. M. Puglisi; L. Larcher; G. Bersuker; A. Padovani; P. Pavan
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 - FHMM analysis for Multi-Defect Spectroscopy in HfOX RRAM [Relazione in Atti di Convegno]
Puglisi, Francesco Maria; Pavan, Paolo
abstract

This paper presents a new technique to analyze the characteristics of multi-level random telegraph noise (RTN) in HfOX RRAM. RTN is characterized by abrupt switching of either the current or the voltage between discrete values as a result of trapping/de-trapping activity while reading the RRAM cell. RTN statistical properties are deduced exploiting a factorial hidden Markov model (FHMM). The proposed method considers the measured multi-level RTN as a superposition of many two-levels RTN, each represented by a Markov chain and associated to a single trap, and it is used to retrieve the statistical properties of each chain. These properties (i.e. dwell times and amplitude) are directly related to physical properties of each trap


2013 - Perimeter and area current components in HfO2 and HfO2-x metal-insulator-metal capacitors [Articolo su rivista]
Francesco Maria Puglisi; Paolo Pavan; Andrea Padovani; Luca Larcher
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 - RTN analysis with FHMM as a tool for multi-trap characterization in HfOx RRAM [Relazione in Atti di Convegno]
Francesco Maria Puglisi; Paolo Pavan
abstract

This paper presents a new technique to analyze the characteristics of multi-level random telegraph noise (RTN) in HfOX RRAM. RTN is characterized by abrupt switching of either the current or the voltage between discrete values as a result of trapping/de-trapping activity while reading the RRAM cell. RTN statistical properties are deduced exploiting a factorial hidden Markov model (FHMM). The proposed method considers the measured multi-level RTN as a superposition of many two-levels RTN, each represented by a Markov chain and associated to a single trap, and it is used to retrieve the statistical properties of each chain. These properties (i.e. dwell times and amplitude) are directly related to physical properties of each trap.


2013 - RTS Noise Characterization of HfOx RRAM in High Resistive State [Articolo su rivista]
F. M. Puglisi; P. Pavan; A. Padovani; L. Larcher; 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 - Random Telegraph Noise analysis to investigate the properties of active traps of HfO2-Based RRAM in HRS [Relazione in Atti di Convegno]
F. M. Puglisi; P. Pavan; A. Padovani; L. Larcher
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.


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