Ludovic Goux

Microscopic origin of random telegraph noise fluctuations in aggressively scaled RRAM and its impact on read disturb variability

Random telegraph noise (RTN) is an important intrinsic phenomenon of any logic or memory device that is indicative of the reliability and stochastic variability in its performance. In the context of the resistive random access memory (RRAM), RTN becomes a key criterion that determines the read disturb immunity and memory window between the low (LRS) and high resistance states (HRS). With the drive towards ultra-low power memory (low reset current) and aggressive scaling to 10 × 10 nm2 area, contribution of RTN is significantly enhanced by every trap (vacancy) in the dielectric. The underlying mechanisms governing RTN in RRAM are yet to be fully understood. In this study, we aim to decode the role of conductance fluctuations caused by oxygen vacancy transport and inelastic electron trapping and detrapping processes. The influence of resistance state (LRS, shallow and deep HRS), reset depth and reset stop voltage (VRESET-STOP) on the conductance variability is also investigated.

Stochastic variability of vacancy filament configuration in ultra-thin dielectric RRAM and its impact on OFF-state reliability

Considering SET and RESET to be dynamic stochastic processes involving the generation-recombination and drift-diffusion of multiple oxygen ions / vacancies, we examine the microscopic statistical changes in the shape of the filament during multiple switching cycles in ultra-thin low-power HfOx-based RRAM. The effect of forming compliance, dielectric microstructure, multi-layer dielectric films and Al-doping on the variability in the filament geometry is investigated based on the quantum point contact (QPC) formulation [1, 2]. The stability (reliability) of the filament in the OFF-state to non-equilibrium vacancy-induced perturbations is evaluated using the disturb voltage (VDIST). Microscopic changes of the defect arrangement in the OFF-state have a big influence on the filament stability. The final configuration of the filament in the OFF-state is bimodal with a finite non-zero probability of being in the QPC mode or entering the tunnel barrier (TUN) mode.

Origin of the current discretization in deep reset states of an Al2O3/Cu-based conductive-bridging memory, and impact on state level and variability

In this paper, we develop a Quantum-Point-Contact (QPC) model describing the state conduction in a W/Al2O3/TiW/Cu Conductive-Bridging Memory cell (CBRAM). The model allows describing both the voltage- and the temperature-dependence of the conduction. For deep current levels, a resistance component is added in series to the point-contact constriction to account for electron scattering in the residual filament. The fitting of single-particle perturbation also allowed to estimate the number and effective size of the conduction-controlling particles in the QPC constriction. The results clearly point to smaller particles for CBRAM (Cu particles) as compared to oxide-based resistive RAM involving oxygen-vacancy defects, which is discussed as a possible origin of deeper reset level obtained in CBRAM. We also evidence a beneficial impact of this smaller particle size on lower Random-Telegraph-Noise amplitude measured on CBRAM devices.

Modeling the Impact of Reset Depth on Vacancy-Induced Filament Perturbations in

Random telegraph noise in resistive switching memory devices is governed by two distinct mechanisms—oxygen vacancy perturbations in the filament as well as the electron trapping–detrapping phenomenon. In this letter, we focus on the dominant role of vacancies in governing the stability of the filament in the high resistance state and characterize the dependence of the read disturb voltage

{\rm HfO}_{2}

(V_{rm DIST})

RRAM

on the depth of the reset level during switching. Our slow voltage ramp read disturb tests at different reset levels indicate the possibility of filamentary instability even for read voltages lower than the standard value of 0.10 V. These experimental trends can be well explained using the quantum point contact model for conduction in the filament, as deeper reset levels induce very steep potential gradients at the two ends of the constriction that make the filaments highly unstable and susceptible to structural modifications due to vacancy generation and/or transport during memory read operation.

Sub-10 nm low current resistive switching behavior in hafnium oxide stack

In this letter, a tip-induced cell relying on the conductive atomic force microscope is proposed. It is verified as a referable replica of an integrated resistive random access memory (RRAM) device. On the basis of this cell, the functionality of sub-10u2009nm resistive switching is confirmed in hafnium oxide stack. Moreover, the low current switching behavior in the sub-10u2009nm dimension is found to be more pronounced than that of a 50u2009×u200950u2009nm2 device. It shows better ON/OFF ratio and low leakage current. The enhanced memory performance is ascribed to a change in the shape of the conductive filament as the device dimensions are reduced to sub-10u2009nm. Therefore, device downscaling provides a promising approach for the resistance optimization that benefits the RRAM array design.

On the bipolar resistive-switching characteristics of Al2O3- and HfO2-based memory cells operated in the soft-breakdown regime

In this article, we investigate extensively the bipolar-switching properties of Al2O3- and HfO2-based resistive-switching memory cells operated at low current down to 15u2009μA), which we relate as intrinsic to soft-breakdown (SBD) regime. We evidence a larger impact of the used switching-oxide in this current range, due to lower density of oxygen-vacancy (Vo) defects in the SBD regime. In this respect, deep resetting and large memory window may be achieved using the stoichiometric Al2O3 material due to efficient Vo annihilation, although no complete erasure of the conductive-filament (CF) is obtained. We finally emphasize that the conduction may be described by a quantum point-contact (QPC) model down to very low current level where only a few Vo defects compose the QPC constriction. The large switching variability inherent to this latter aspect is mitigated by CF shape tuning throug...

Multimode resistive switching in nanoscale hafnium oxide stack as studied by atomic force microscopy

The nanoscale resistive switching in hafnium oxide stack is investigated by the conductive atomic force microscopy (C-AFM). The initial oxide stack is insulating and electrical stress from the C-AFM tip induces nanometric conductive filaments. Multimode resistive switching can be observed in consecutive operation cycles at one spot. The different modes are interpreted in the framework of a low defect quantum point contact theory. The model implies that the optimization of the conductive filament active region is crucial for the future application of nanoscale resistive switching devices.

Conductive filaments multiplicity as a variability factor in CBRAM

In this work we investigate the origin of the resistance variability for the low resistive state in conductive bridging memory devices (CBRAM). We use C-AFM tomography to enable the three-dimensional observation of the filaments and correlate the presence of double-branched conductive filaments to the variability in the device performance.