Aurelio Mauri

Atomic migration in phase change materials

During normal operation of Phase Change Memory (PCM) cells active materials undergo very high electrical and thermal stresses that cause a motion of the different atoms leading to composition variation which has a fundamental impact on performance and reliability. In order to address this issue we introduce here a comprehensive 3D physical model for mass transport in chalcogenide materials. In addition to the driving force for atom diffusion coming from concentration gradient and electric field, the model also accounts for the effect of temperature gradient and phase segregation. This new diffusion model is coupled with a calibrated electro-thermal-phase change model, thus providing a unified framework for the self-consistent simulation of both the electro-thermal and the phase/material change problems. The model is applied to the study of different types of PCM cells showing good agreement with experiments and demonstrating in particular the fundamental role played by the temperature profile.

Impact of Nonuniform Doping on Random Telegraph Noise in Flash Memory Devices

This paper presents a thorough numerical investigation of the effect of nonuniform doping on random telegraph noise (RTN) in nanoscale Flash memory devices. For a fixed average threshold voltage, the statistical distribution of the RTN fluctuation amplitude is studied with nonconstant doping concentrations in the length, width, or depth direction in the channel, showing that doping increase at the active area corners and retrograde and δ-shape dopings appear as the most promising profiles for RTN suppression. In particular, the improvements offered by retrograde and δ-shape dopings increase the more the high doping regions are pushed far from the channel surface due to a more uniform source-to-drain conduction during read. Finally, the suppression of RTN by engineered doping profiles is correlated with the reduction in cell threshold voltage variability.

Physical Modeling for Programming of TANOS Memories in the Fowler–Nordheim Regime

This paper presents a physics-based model that is able to describe the TANOS memory programming transients in the Fowler-Nordheim uniform tunneling regime across the bottom-oxide layer. The model carefully takes into consideration the trapping/detrapping processes in the nitride, the limited number of traps available for charge storage, and their spatial and energetic distribution. Results are in good agreement with experimental data on TANOS devices with different gate-stack compositions, considering a quite extended range of gate biases and times. The reduced gate-bias sensitivity of the programming transients with respect to the floating-gate cell is explained in terms of a finite number of nitride traps and a thinner extension of the nitride trapping region as the gate bias is increased. The model represents a valid contribution for the investigation of the achievable performances of the TANOS technology.

Three-Dimensional Electrostatics- and Atomistic Doping-Induced Variability of RTN Time Constants in Nanoscale MOS Devices—Part II: Spectroscopic Implications

This paper investigates the impact of 3-D electrostatics and atomistic doping on the spectroscopic analysis of random telegraph noise (RTN) traps in nanoscale MOS devices. Using the numerical model and the template decananometer Flash cell presented in Part I of this paper, the gate bias dependence of the capture and emission time constants of oxide traps is shown to largely depend on the trap position over the channel, both in the subthreshold and in the on-state regime. This compromises the accuracy of any 1-D method for trap spectroscopy based on the time constants analysis and, due to the randomness in trap position and dopant placement in the substrate, calls into question the possibility for any accurate trap spectroscopy in nanoscale devices. Finally, the possibility to extract any information on the trap depth from the fluctuation amplitude of RTN waveforms is shown to be precluded by the large statistical spread of the amplitude itself, resulting in its negligible correlation with the trap position in the oxide and with the waveform time constants.

Impact of atomistic doping and 3D electrostatics on the variability of RTN time constants in flash memories

Referring to a template deca-nanometer Flash cell, we show for the first time that 3D electrostatics and atomistic doping play an essential role in the time constants of random telegraph noise in nanoscale MOS devices, resulting in a several orders-of-magnitude spread in their values and in their negligible correlation with the noise fluctuation amplitude. These results reveal that any 1D method for trap spectroscopy is intrinsically flawed when applied to nanoscale devices, and also question the possibility of correctly extracting the physical trap parameters.

Semi-Analytical Model for the Transient Operation of Gate-All-Around Charge-Trap Memories

We present a detailed semi-analytical investigation of the transient dynamics of gate-all-around (GAA) charge-trap memories. To this aim, the Poisson equation is solved in cylindrical coordinates, and a modification of the well-known Fowler-Nordheim formula is proposed for tunneling through cylindrical dielectric layers. Analytical results are validated by experimental data on devices with different gate stack compositions, considering a quite extended range of gate biases and times. Finally, the model is used for a parametric analysis of the GAA cell, highlighting the effect of device curvature on both program/erase and retention.

Three-Dimensional Electrostatics- and Atomistic Doping-Induced Variability of RTN Time Constants in Nanoscale MOS Devices—Part I: Physical Investigation

This paper presents a detailed simulation analysis of the impact of 3-D electrostatics and atomistic doping on the variability of the random telegraph noise (RTN) time constants in nanoscale MOS devices. Results on a template decananometer Flash cell show that both the effects contribute to a large statistical dispersion of the capture/emission time constants of oxide traps placed at the same distance from the silicon surface, mainly due to nonuniform channel inversion. The statistical dispersion has an orders-of-magnitude increase when moving from the on-state to the subthreshold cell regimes and has major implications on the spectroscopic investigation of RTN traps, as will be discussed in Part II of this work.

Impact of Cell Shape on Random Telegraph Noise in Decananometer Flash Memories

This paper presents a comprehensive numerical study of the impact of cell shape on random telegraph noise (RTN) in nanoscale Flash memory devices. The statistical dispersion of the RTN fluctuation amplitude is computed using both classical and quantum-corrected 3-D TCAD simulations of devices featuring three different active-area shapes (planar, rounded edges, and full rounded), with self-aligned or surrounding floating gate. For both the floating-gate geometries, results show that RTN immunity is enhanced by increasing the rounding of the active-area edges in the width direction, due to a more uniform source-to-drain conduction during read. For this analysis, the importance of quantum-mechanical corrections for the correct evaluation of the RTN distribution of sharp-edge devices is highlighted. Finally, the reduction of RTN by cell shape engineering is shown to be anticorrelated with the reduction of cell threshold-voltage variability.

Current status and future prospects of non-volatile memory modeling

We briefly discuss the evolution of Non-Volatile Memory (NVM) technology in term of macro-trends and their implications for modeling activities in an industrial R&D environment. Some examples of difficult modeling issues for different NVM techologies are mentioned, and finally both present needs and future challanges are critically reviewed.