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SARA VECCHI

Dottorando
Dipartimento di Ingegneria "Enzo Ferrari"

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Pubblicazioni

2023 - A Unified Framework to Explain Random Telegraph Noise Complexity in MOSFETs and RRAMs [Relazione in Atti di Convegno]
Vecchi, S.; Pavan, P.; Puglisi, F. M.
abstract

2023 - Linking the Intrinsic Electrical Response of Ferroelectric Devices to Material Properties by means of Impedance Spectroscopy [Articolo su rivista]
Benatti, L.; Vecchi, S.; Puglisi, F. M.
abstract

Ferroelectric devices have gained attention in recent years as a potential solution for ultra-low power computing due to their ability to act as memory units and synaptic weights in brain-inspired architectures. One way to study the behavior of these devices under different conditions, particularly the influence of material composition and charge trapping on ferroelectric switching, is through impedance spectroscopy. However, the parasitic impedance of the metal lines that contact the electrodes of the device can affect the measured response and interpretation of the results. In this study, we examined the frequency response of ferroelectric tunnel junctions (FTJs) with a metal-dielectric-ferroelectric-metal (MDFM) stack at various voltages, starting from the analysis of single layer capacitors (MFM and MDM) to better interpret FTJ’s results. To accurately assess the intrinsic response of the device, we developed a method that estimates and removes the parasitic access impedance contribution, which was validated by means of physics-based simulations. This method allows quantifying the intrinsic device-level variability of FTJs and, for the first time, to investigate the relation between the thickness of the dielectric layer, the equivalent phase composition of the ferroelectric material, and the magnitude of the peak in the frequency response, often assumed to be related to charge trapping only.

2023 - Local electric field perturbations due to trapping mechanisms at defects: What random telegraph noise reveals [Articolo su rivista]
Vecchi, S.; Pavan, P.; Puglisi, F. M.
abstract

As devices scale closer to the atomic size, a complete understanding of the physical mechanisms involving defects in high-kappa dielectrics is essential to improve the performance of electron devices and to mitigate key reliability phenomena, such as Random Telegraph Noise (RTN). In fact, crucial aspects of defects in HfO2 are still under investigation (e.g., the presence of metastable states and their properties), but it is well known that oxygen vacancies (V(+)s) and oxygen ions (O(0)s) are the most abundant defects in HfO2. In this work, we use simulations to gain insights into the RTN that emerges when a constant voltage is applied across a TiN/(4 nm)HfO2/TiN stack. Signals exhibit different RTN properties over bias and, thus, appear to originate from different traps. Yet, we demonstrate that they can be instead promoted by the same O(0)s which change their capture (tau(c)) and emission (tau(e)) time constants with the applied bias, which, in turn, changes the extent of their electrostatic interactions with the traps that assist charge transport (V(+)s). For a certain bias, RTN is given by the modulation of the trap-assisted current at V(+)s induced by trapping/detrapping events at O(0)s, which are, in turn, influenced by the bias itself and by trapped charge at nearby O(0)s. In this work, we demonstrate that accounting for the effect of trapped charge is essential to provide accurate estimation of the RTN parameters, which allow us to retrieve information about traps and to explain key mechanisms behind complex RTN signals.

2023 - The Major Effect of Trapped Charge on Dielectric Breakdown Dynamics and Lifetime Estimation [Relazione in Atti di Convegno]
Vecchi, Sara; Padovani, Andrea; Pavan, Paolo; Puglisi, Francesco Maria
abstract

2023 - The Role of Defects and Interface Degradation on Ferroelectric HZO Capacitors Aging [Relazione in Atti di Convegno]
Benatti, L.; Vecchi, S.; Pesic, M.; Puglisi, F. M.
abstract

2022 - Defects Motion as the Key Source of Random Telegraph Noise Instability in Hafnium Oxide [Relazione in Atti di Convegno]
Vecchi, S.; Pavan, P.; Puglisi, F. M.
abstract

Besides standard two- and multi-level Random Telegraph Noise (RTN), more complex cases of RTN are commonly reported which show peculiar current signal instabilities. The physical origin of such phenomena is typically traced back to the presence of metastable defects states, the Coulomb interaction between traps, and the possible interaction of hydrogen species with oxide defects. However, the effect of the motion of atomic species on RTN phenomena has never been brought to the picture, even though such a mechanism is extremely relevant for oxygen ions in HfO2, e.g., it guarantees resistive switching in HfO2 RRAM. In this paper, we demonstrate that complex RTN signals observed in experiments naturally emerge when considering the combination of the Coulomb field due to the trapped charge at defects together with their field-assisted motion. Strikingly, we demonstrate that multilevel RTN signals with high instability and complex time evolution, which are conventionally though to be caused by an intricate many-bodies problem involving several defects, can in fact result by the

2022 - Impedance Spectroscopy of Ferroelectric Capacitors and Ferroelectric Tunnel Junctions [Relazione in Atti di Convegno]
Benatti, L.; Vecchi, S.; Puglisi, F. M.
abstract

Ferroelectric devices are currently considered as a viable option for ultra-low power computing, thanks to their ability to act as memory units and synaptic weights in brain inspired architectures. A common methodology to assess their response in different conditions (especially the role of material composition and charge trapping in ferroelectric switching) is impedance spectroscopy. However, test devices may be affected by the parasitic impedance of the metal lines contacting the electrodes of the device, which may alter the measured response and the results interpretation. In this work, we investigate the frequency response at different voltages of ferroelectric tunnel junction (FTJ) having a metal-dielectric-ferroelectric-metal (MDFM) stack, starting from the analysis of single layer capacitors (MFM and MDM). A simple but reliable method, validated by physics-based simulations, is proposed to estimate and remove the parasitic access impedance contribution, revealing the intrinsic device response. The method is used to quantify the intrinsic device-level variability of FTJs and to highlight for the first time the relation between the thickness of the dielectric layer, the phase composition of the ferroelectric, and the magnitude of the peak in the frequency response, usually thought as related to charge trapping only.

2022 - The Impact of Electrostatic Interactions between Defects on the Characteristics of Random Telegraph Noise [Articolo su rivista]
Vecchi, S.; Pavan, P.; Puglisi, F. M.
abstract

Random telegraph noise (RTN) is one of the most challenging defect-related reliability concerns in emerging HfO2-based devices due to the higher bulk defect density compared to SiO2. Despite many research efforts, the physical mechanisms determining complex signals (e.g., multilevel, anomalous, temporary RTN) are still unclear and need a deeper investigation. With this driving force, we performed physic-based kinetic Monte Carlo (kMC) simulations in a TiN/HfO2/TiN cell to directly analyze the role of defects which promote RTN, both in steady state and transient regime. The nonmonotonic trends of the ratio of the RTN dwell times with the applied bias frequently found in the literature are found to be caused by changes with the applied voltage of the preferential capture/emission source/destination of traps. The in-depth analysis sheds new light on the conventional methods for defect classification and vertical position estimation. Moreover, such source/destination changes also occur over time due to the dynamics of the local electric field, which varies with the evolution of the surrounding electrostatic landscape. Notably, the local field is given by the overlap of the applied voltage and of the trapped charge contributions, the latter being dominant at low voltages. The analysis of the Markov chains of closely spaced defects shown interdependencies and alterations of the RTN capture and emission times. A new method is proposed to include the impact of electrostatic interactions between defects on RTN.

2022 - The Relevance of Trapped Charge for Leakage and Random Telegraph Noise Phenomena [Relazione in Atti di Convegno]
Vecchi, S.; Pavan, P.; Puglisi, F. M.
abstract

The current understanding of key reliability phenomena such as leakage and Random Telegraph Noise (RTN) is still incomplete. Models exist that explain simple cases (2-level RTN), yet experimental reports showed the occurrence of complex cases (e.g., coupled RTN, anomalous and temporary RTN) that deserve deeper investigation. In this paper, we focus on the often overlooked role of trapped charge in the electrostatic coupling among defects, entailing a multi-body problem, and on the related effects on leakage and RTN. The electric field in the dielectric is found to be usually dominated by the trapped charge rather than by the applied voltage, defying common beliefs and elegantly explaining some of the aforesaid complex scenarios. We demonstrate that such defects interactions are responsible for a strong modulation of the capture and emission time constants over time. Moreover, we highlight how defects capture/emission source/destination can change with the local field and therefore with the applied voltage, which gives rise to non-monotonic trends in c/e vs. applied voltage plot. This last point reveals that the classical formula adopted for the estimation of the defects vertical position within the dielectric is oversimplified and may lead to significant errors. The results of this study advance the understanding of leakage and RTN, and can be useful for the design of applications such as low-power Physical Unclonable Functions and True Random Number Generators.