https://www.iue.tuwien.ac.at/pdf/ib_2011/JB2011_Grasser_5.pdf

Stochastic charge trapping in oxides: From random telegraph noise to bias temperature instabilities

We survey progress in the PCM field over the past five years, ranging from large-scale PCM demonstrations to materials improvements for high–temperature retention and faster switching. Both materials and new cell designs that support lower-power switching are discussed, as well as higher reliability for long cycling endurance. Two paths towards higher density are discussed: through 3D integration by the combination of PCM and 3D-capable access devices, and through multiple bits per cell, by understanding and managing resistance drift caused by structural relaxation of the amorphous phase. We also briefly survey work in the nascent field of brain-inspired neuromorphic systems that use PCM to implement non-Von Neumann computing.

Recent Progress in Phase-Change Memory Technology

The similarity between Random Telegraph Noise and Negative Bias Temperature Instability (NBTI) relaxation is further demonstrated by the observation of exponentially-distributed threshold voltage shifts corresponding to single-carrier discharges in NBTI transients in deeply scaled pFETs. A SPICE-based simplified channel percolation model is devised to confirm this behavior. The overall device-to-device ΔV th distribution following NBTI stress is argued to be a convolution of exponential distributions of uncorrelated individual charged defects Poisson-distributed in number. An analytical description of the total NBTI threshold voltage shift distribution is derived, allowing, among other things, linking its first two moments with the average number of defects per device.

Origin of NBTI variability in deeply scaled pFETs

Phase-change memory (PCM) has undergone significant academic and industrial research in the last 15 years After much development, it is now poised to enter the market as a storage-class memory (SCM), with performance and cost between that of NAND flash and DRAM In this paper, we review the history of phase-transforming chalcogenides leading up to our current understanding of PCM as either a storage-type SCM, with high-density and better than NAND flash endurance, write speeds, and retention, or a memory-type SCM, with fast read/write times to function as a nonvolatile DRAM Several of the key findings from the community relating to device dimensional scaling, cell design, thermal engineering, material exploration, and storing multiple levels per cell will be discussed These areas have dramatically impacted the course of development and understanding of PCM We will highlight the performance gains attained and the future prospects, which will help drive PCM to be as ubiquitous as NAND flash in the upcoming decade

Phase-Change Memory—Towards a Storage-Class Memory

This paper presents a comprehensive investigation of random telegraph noise (RTN) in deca-nanometer Flash memories, considering both the nor and the nand architecture. The statistical distribution of the threshold voltage instability is analyzed in detail, evidencing that the slope of its exponential tails is the critical parameter determining the scaling trend for RTN. By means of 3-D TCAD simulations, the slope is shown to be the result of cell geometry, atomistic substrate doping, and random placement of traps over the cell active area. Finally, the slope dependence on cell geometry (width, length, and oxide thickness), doping, and bias conditions is summarized in a powerful formula that is able to predict the RTN instabilities in deca-nanometer Flash memories.

Comprehensive Analysis of Random Telegraph Noise Instability and Its Scaling in Deca–Nanometer Flash Memories

http://ieeexplore.ieee.org/iel5/16/5170230/05159507.pdf

Comprehensive Analysis of Random Telegraph Noise Instability and Its Scaling in Deca-Nanometer

The magnitude of a random telegraph signal (RTS) in nanoscale floating-gate devices has been experimentally investigated as a function of carrier concentration. Discrete current switching, which is caused by a single trap, has been found to be almost one order of magnitude higher with respect to what was predicted by the classical theory of carrier number and correlated mobility fluctuations. Nevertheless, the trap signature well fits the typical SiO2 trap spectroscopy. In addition, the rigid shift between the transfer curves related to filled- and empty-trap state, together with the normalized current fluctuation dependence on the channel carrier density, suggests that a pure number fluctuation is the correct theoretical interpretative framework. Thus, we propose a possible physical explanation for such a giant RTS on the basis of a quasi-1-D current filamentation.

Giant Random Telegraph Signals in Nanoscale Floating-Gate Devices

This letter presents a numerical investigation of the statistical distribution of the random telegraph noise (RTN) amplitude in nanoscale MOS devices, focusing on the change of its main features when moving from the subthreshold to the on-state conduction regime. Results show that while the distribution can be well approximated by an exponential behavior in subthreshold, large deviations from this behavior appear when moving toward the on-state regime, despite a low probability exponential tail at high RTN amplitudes being preserved. The average value of the distribution is shown to keep an inverse proportionality to channel area, while the slope of the high-amplitude exponential tail changes its dependence on device width, length, and doping when moving from subthreshold to on-state.

Investigation of the RTN Distribution of Nanoscale MOS Devices From Subthreshold to On-State

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.

Atomic migration in phase change materials

For the first time, a comprehensive comparative study of the impact of different sources of statistical variability in nonvolatile memory (NVM) has been carried out using the 3-D numerical simulation of large statistical ensembles and approaches based on the impedance-field method. Results of the threshold voltage variability in a template 32-nm floating-gate NVM subject to random discrete dopants (RDD), line edge roughness, oxide thickness fluctuations, polysilicon granularity, and interface trapped charge (ITC) are presented. The relative impact of each source of statistical variability has been highlighted, with RDD being identified as the dominant source and ITC as the next most dominant source. Based on the simulation of statistical samples of 1000 microscopically different devices, the shape and spread of the statistical distribution associated with each individual and combined sources of variability have been found to significantly be different from a normal distribution, particularly within the tails that may have significant implications for design and yield. Finally, an ensemble of 59 000 devices is used to characterize the combined impact of all sources of variability.

Andrea Ghetti

Papers

Comprehensive Analysis of Random Telegraph Noise Instability and Its Scaling in Deca-Nanometer

Giant Random Telegraph Signals in Nanoscale Floating-Gate Devices

Investigation of the RTN Distribution of Nanoscale MOS Devices From Subthreshold to On-State

Atomic migration in phase change materials

Comparative Simulation Study of the Different Sources of Statistical Variability in Contemporary Floating-Gate Nonvolatile Memory