Non-volatile Storage Technologies Research Articles

Heterogeneous integrated InP/SiC high-performance multilevel RRAM

With the advent of the Age of Big Data, resistive random-access memory (RRAM) shows considerable potential for next generation nonvolatile storage technologies owing to its simplified structure, high switching speed, and low power consumption. However, mainstream prepared materials, such as oxides and halide perovskite, face critical issues for practical applications such as switching uniformity and long-term environmental stability. In this work, we report that high carrier mobility material indium phosphide (InP) is prepared as an RRAM medium and is directly bonded to the high thermal conductivity substrate silicon carbide (SiC) at 200 °C, overcoming large (14.9%) lattice mismatch. Importantly, the bonding strength reaches 9.3 MPa, and this high-performance stable RRAM exhibits nonvolatile and reliable switching characteristics including stable endurance (200 cycles) and long data retention (2000 s). Moreover, multilevel storage is also available by modulating RESET stop voltages. This work provides broad possibilities for high-performance RRAM with structures based on traditional semiconductors in the field of nonvolatile storage.

LazySort: A customized sorting algorithm for non-volatile memory

Due to the development of new non-volatile storage technologies, bridging the access delay gap between storage and memory has been promoted. Therefore, since the volume of data continues to increase, the need for new types of non-volatile storage for high-performance computing and database management systems significantly increases. As a basic algorithm of the database management system, external sorting is widely used but suffers from numerous I/O operations caused by the frequent data exchange between the memory and external memory. Non-volatile memory (NVM) is a promising solution. However, the traditional external sorting algorithm is only designed for disk storage and cannot be used for NVM. To solve this issue, we propose LazySort, which is a novel external sorting algorithm that exploits the NVM byte addressing mechanism and the orderly distribution of data to improve the sorting efficiency. Specifically, LazySort first detects the ordered data segment in the memory and then records the storage location of the data segment as well as its maximum/minimum values in an index table named ITable. Next, to reduce memory usage and accelerate the sorting procedure, LazySort performs an optimization strategy RunMerge to merge non-intersecting data blocks according to the range of ITable records. To verify the performance of LazySort, we built an experimental platform with the NVM-DRAM storage architecture combining NVM and dynamic random access memory (DRAM) and conducted a series of experiments. The experimental results show that LazySort is more efficient than the traditional external sorting algorithm, since the sorting time of LazySort is shortened by 93.08% and the number of NVM write operations is reduced by 49.50%. Moreover, the advantage of LazySort over traditional algorithms is more significant when the data amount increases.

Insights from Adsorption and Electron Modification Studies of Polyoxometalates on Surfaces for Molecular Memory Applications.

This Account highlights recent experimental and theoretical work focusing on the development of polyoxometalates (POMs) as possible active switching units in what may be called "molecule-based memory cells". Herein, we critically discuss how multiply charged vanadium-containing POMs, which exhibit stable metal-oxo bonds and are characterized by the excellent ability to change their redox states without significant structural distortions of the central polyoxoanion core, can be immobilized best and how they may work optimally at appropriate surfaces. Furthermore, we critically discuss important issues and challenges on the long way toward POM-based nanoelectronics. This Account is divided into four sections shedding light on POM interplay in solution and on surfaces, ion soft-landing of mass-selected POMs on surfaces, electronic modification of POMs on surfaces, and computational modeling of POMs on surfaces. The sections showcase the complex nature of far-reaching POM interactions with the chemical surroundings in solution and the properties of POMs in the macroscopic environment of electrode surfaces. Section 2 describes complex relationships of POMs with their counter-cations, solvent molecules, and water impurities, which have been shown to exhibit a direct impact on the resulting surface morphology, where a concentration-dependent formation of micellar structures can be potentially observed. Section 3 gives insights into the ion soft-landing deposition of mass-selected POMs on electrode surfaces, which emerges as an appealing method because the simultaneous deposition of agglomeration-stimulating counter-cations can be avoided. Section 4 provides details of electronic properties of POMs and their modification by external electronic stimuli toward the development of multiple-state resistive (memristive) switches. Section 5 sheds light on issues of the determination of the electronic structure properties of POMs across their interfaces, which is difficult to address by experiment. The studies summarized in these four sections have employed various X-ray-scattering, microscopy, spectroscopy, and computational techniques for imaging of POM interfaces in solution and on surfaces to determine the adsorption type, agglomeration tendency, distribution, and oxidation state of deposited molecules. The presented research findings and conceptual ideas may assist experimentalists and theoreticians to advance the exploration of POM electrical conductivity as a function of metal redox and spin states and to pave the way for a realization of ("brain-inspired") POM-based memory devices, memristive POM-surface device engineering, and energy efficient nonvolatile data storage and processing technologies.

Architecture and Process Integration Overview of 3D NAND Flash Technologies

In the past few decades, NAND flash memory has been one of the most successful nonvolatile storage technologies, and it is commonly used in electronic devices because of its high scalability and reliable switching properties. To overcome the scaling limit of planar NAND flash arrays, various three-dimensional (3D) architectures of NAND flash memory and their process integration methods have been investigated in both industry and academia and adopted in commercial mass production. In this paper, 3D NAND flash technologies are reviewed in terms of their architecture and fabrication methods, and the advantages and disadvantages of the architectures are compared.

An alternative approach to nucleic acid memory

DNA is a compelling alternative to non-volatile information storage technologies due to its information density, stability, and energy efficiency. Previous studies have used artificially synthesized DNA to store data and automated next-generation sequencing to read it back. Here, we report digital Nucleic Acid Memory (dNAM) for applications that require a limited amount of data to have high information density, redundancy, and copy number. In dNAM, data is encoded by selecting combinations of single-stranded DNA with (1) or without (0) docking-site domains. When self-assembled with scaffold DNA, staple strands form DNA origami breadboards. Information encoded into the breadboards is read by monitoring the binding of fluorescent imager probes using DNA-PAINT super-resolution microscopy. To enhance data retention, a multi-layer error correction scheme that combines fountain and bi-level parity codes is used. As a prototype, fifteen origami encoded with ‘Data is in our DNA!\\n’ are analyzed. Each origami encodes unique data-droplet, index, orientation, and error-correction information. The error-correction algorithms fully recover the message when individual docking sites, or entire origami, are missing. Unlike other approaches to DNA-based data storage, reading dNAM does not require sequencing. As such, it offers an additional path to explore the advantages and disadvantages of DNA as an emerging memory material.

Advanced TEM Techniques for Mechanisms and Reliability Analysis in State-of-the-Art OxRAM and PCM Non-Volatile Memory Devices

Memory devices play an essential role in today’s electronics, with increasing requirements in terms of density and reliability. The incremental appearing of new and different applications in the market, open the door to cutting-edge memory technology featuring innovative storing mechanisms and improved performances. In particular, Phase-Change Memory (PCM) and OxRAM (metal oxide resistive memory) are promising candidates for next generation of non-volatile memory (NVM) [1]. They represent a very attractive solution to design downscaled ultra-low power devices for new applications such as neuromorphic systems or in-memory computing. Such NVM are both based on an electrically switchable resistance, however, their operation mechanisms are completely different, exactly like the reliability issues from which they can be affected. High-resolution imaging, i.e. Transmission Electron Microscopy (TEM) coupled with Convergent Beam Electron Diffraction (CBED) and Energy Dispersive X-ray Spectroscopy (EDX), in state-of-the-art ultra-scaled NVMs becomes essential for a complete understanding of mechanisms behind the functionality and the failure of these new technologies. In this work, we considered for our investigation both OxRAM and PCM cells integrated in the Back-End-Of-Line (BEOL) of the fabrication of LETI Memory Advanced Demonstrator (MAD) based on 130 nm CMOS technology.OxRAM operations rely on the formation and dissolution of one or multiple conducting filaments in the active oxide of the device. This intrinsically stochastic phenomenon is considered the main responsible for OxRAM performances variability, thus the understanding of the origin of its evolution and degradation along the cell cycles, can enable new ways for the device engineering and improvement. TEM investigation of HfO2-based OxRAM failures along endurance tests were performed. In order to avoid extrinsic results belonging to experimental design, we considered a population of dozen OxRAM devices (each one composed by a selecting transistor and a resistive memory, or “1T1R”, and based on a Ti/HfO2(10nm)/TiN stack) for each level of endurance. We show here in TEM/EDX analyses, the evolution of the device from its pristine state towards different stages of its lifetime, up to the final failure achieved at 107 cycles. We clearly evidence the switching mechanism, represented by a metallic migration from the electrodes in the active oxide. In particular, EDX cartographies highlight this diffusion phenomenon happening in the entire volume of the insulator. Finally, we show how the failure of the cell is likely related to a gradual titanium enrichment of the oxide layer, hindering the dissolution process of the filaments.PCM operations rely on the reversible phase transition of a chalcogenide material between the amorphous and the crystalline phase, which occurs upon current-induced Joule heating. The reliability of the programming operation and its energy consumption therefore depends on the thermal efficiency of the device [2]. In this perspective, we applied an optimized SiC based encapsulation in PCM devices enabling programming current reduction and improved data retention [3]. We considered in our experiment Ge-rich GeSbTe based devices, featuring high temperature stability required by automotive applications. Thanks to TEM analyses, we demonstrate the higher uniformity of the heating process during the programming operation achieved by optimized SiC encapsulation with respect to standard SiN. EDX analyses highlight the homogeneity of the active volume involved in the phase-change transition. We coupled these results with CBED spectra, to evidence the germanium segregation and its more uniform distribution in the active volume of SiC based PCM cells. This result matches and helps validating the hypothesis based on our TCAD simulations here presented.This work presents the reliability analysis of state-of-the-art NVM devices performed by advanced TEM techniques. We provide demonstration of the OxRAM device failure, to be found in the gradual electrode diffusion in the whole active material. Finally, we show how the higher thermal efficiency of a Ge-rich GeSbTe PCM device, based on an optimized SiC encapsulation, is supported by a higher homogeneity of the active volume, evidenced by both EDX and CBED analyses.[1] Yoshio, Nishi. Advances in Non-Volatile Memory and Storage Technology / Edited by Yoshio Nishi. Elsevier Science, 2014.[2] F. Xiong et al., "Towards ultimate scaling limits of phase-change memory," 2016 IEEE International Electron Devices Meeting (IEDM), pp. 4.1.1-4.1.4.[3] A. L. Serra et al., "Outstanding Improvement in 4Kb Phase-Change Memory of Programming and Retention Performances by Enhanced Thermal Confinement," 2019 IEEE 11th International Memory Workshop (IMW), 2019, pp. 1-4.

Nonvolatile Memory Cell Based on Memristor

Memristor, one of the fundamental circuit elements, has promising applications in non-volatile memory and storage technology as it can theoretically achieve infinite states. Information can be stored independently in these states and retrieved whenever required. In this paper, we have proposed a non volatile memory cell based on memristor emulator. The circuit is able to perform read and write operations. In this memristor based memroy cell, unipolar pulse is used for writing and bipolar pulse is used for reading. Unlike other earlier designs, the circuit does not need external read/write enable switches to switch between read and write operations; the switching is achieved by the zero average bipolar read pulse given after the completion of write cycle. In our proposed memristor based memory cell, single bit can be read and any voltages from 0 to 5 volts can be written. Mathematical analysis and the simulation results of memristor emulator based read write circuit have been presented to confirm its operation.

Variable-Node-Based Belief-Propagation Decoding With Message Pre-Processing for NAND Flash Memory

With the fast development of non-volatile storage technology, NAND flash memory faces more and more challenges such as data reliability and lifetime. To overcome the issue of the reliability, low-density parity-check (LDPC) codes are considered as a main candidate of error-correction-codes (ECCs) for NAND flash storages. However, conventional soft decoding algorithms for LDPC codes suffer from the drawback of slow convergence speed, which increases the decoding latency. For achieving fast convergence speed, a variable-node-based belief-propagation with message pre-processing (VNBP-MP) decoding algorithm for binary LDPC codes and a non-binary VNBP-MP (NVNBP-MP) decoding algorithm for non-binary LDPC codes are proposed. Both of the algorithms utilize the characteristics of the NAND flash channel to perform the message pre-processing operations. In theory, the propagation of unreliable messages can be effectively prevented and the propagation of reliable messages can be speeded up. To further improve the decoding convergence, the treatment for oscillating variable nodes is considered after the message pre-processing operations. The simulation results also show that the proposed algorithms both have a noticeable improvement in convergence speed and latency, without compromising error-correction performance, compared with the existing soft decoding algorithms.

Multi-valued and Fuzzy Logic Realization using TaOx Memristive Devices

Among emerging non-volatile storage technologies, redox-based resistive switching Random Access Memory (ReRAM) is a prominent one. The realization of Boolean logic functionalities using ReRAM adds an extra edge to this technology. Recently, 7-state ReRAM devices were used to realize ternary arithmetic circuits, which opens up the computing space beyond traditional binary values. In this manuscript, we report realization of multi-valued and fuzzy logic operators with a representative application using ReRAM devices. Multi-valued logic (MVL), such as Łukasiewicz logic generalizes Boolean logic by allowing more than two truth values. MVL also permits operations on fuzzy sets, where, in contrast to standard crisp logic, an element is permitted to have a degree of membership to a given set. Fuzzy operations generally model human reasoning better than Boolean logic operations, which is predominant in current computing technologies. When the available information for the modelling of a system is imprecise and incomplete, fuzzy logic provides an excellent framework for the system design. Practical applications of fuzzy logic include, industrial control systems, robotics, and in general, design of expert systems through knowledge-based reasoning. Our experimental results show, for the first time, that it is possible to model fuzzy logic natively using multi-state memristive devices.

Non-volatile memory based on atomic crystals

The space and data-exchanging rate and other factors of storage, calculation and communication in the multi-functional low-power chips have become the bottlenecks of the development of integrated circuits. The ultralow power consumption of single-bit erasing/writing, the ultra-small device size, fast operation time and non-volatility are indispensable for high-density and high-speed storage in the future system architecture to satisfy the needs for big data exchanging in the core. The future storage technology consists of employing devices with new principles, such as controlling of electron transition and ionic migration superseding the electron transport, adopting new materials to lower the power consumption and introducing new 3D-stack technology to increase the density. This review focuses on the application of new materials in the non-volatile storage technology, especially the two-dimensional atomic crystals. Two-dimensional atomic crystals have some prominent properties, such as high electron mobility, ultimate ultrathin channel and ultralow surface states with an intrinsic thickness of 0.6‒1.2 nm and abundant band structure, which provide an excellent solution for future storage technology. And they have enormous potential to further scale down to increase the integration level and improve stability, expand the application scenarios and develop new memory devices and they are a new method to break through the bottlenecks of power consumption and integrated level of memories.

SmartCon

Handling of storage IO in modern operating systems assumes that such devices are slow and CPU cycles are valuable. Consequently, to effectively exploit the underlying hardware resources, for example, CPU cycles, storage bandwidth and the like, whenever an IO request is issued to such device, the requesting thread is switched out in favor of another thread that may be ready to execute. Recent advances in nonvolatile storage technologies and multicore CPUs make both of these assumptions increasingly questionable, and an unconditional context switch is no longer desirable. In this article, we propose a novel mechanism called SmartCon, which intelligently decides whether to service a given IO request in interrupt-driven manner or busy-wait--based manner based on not only the device characteristics but also dynamic parameters such as IO latency, CPU utilization, and IO size. We develop an analytic performance model to project the performance of SmartCon for forthcoming devices. We implement SmartCon mechanism on Linux 2.6 and perform detailed evaluation using three different IO devices: Ramdisk, low-end SSD, and high-end SSD. We find that SmartCon yields up to a 39% performance gain over the mainstream block device approach for Ramdisk, and up to a 45% gain for PCIe-based SSD and SATA-based SSDs. We examine the detailed behavior of TLB, L1, L2 cache and show that SmartCon achieves significant improvement in all cache misbehaviors.

STXM observation and quantitative study of magnetic vortex structure

Magnetic recording has now played an important role in the development of non-volatile information storage technologies, so it becomes essential to quantitatively understand the magnetization distribution in magnetic microstructures. In ferromagnetic disks, squares and triangles with submicron sizes, it is energetically favorable for the magnetization to form a closed in-plane vortex and a perpendicular vortex core at the center. This vortex magnetic structure is a new candidate for future magnetic memory device because both the vortex chirality and the core polarity can be manipulated by applying an external magnetic field or a spin-polarized current. Further development of vortex-based memory devices requires quantitative measurement of vortex domain structures, which is still lacking.In this paper, magnetization configuration in a vortex structure has been quantitatively studied by scanning transmission X-ray microscope (STXM) utilizing X-ray magnetic circular dichroism (XMCD) effect in Shanghai Synchrotron Radiation Facility. Samples have been fabricated on the 100 nm silicon-nitride membranes. The patterns are first transferred to PMMA photoresist using e-beam lithography, then a 50 nm thick Ni80Fe20 film is deposited by e-beam evaporation. Magnetic vortex configurations are characterized with the X-ray energy at Fe L3 absorption edge and Ni L3 absorption edge, respectively. The image taken at Fe edge shows greater contrast than that at Ni edge. Experimental results indicate that the magnetic vortex state remains stable in permalloy circle, square and triangle structures with diameters from 2 to 5 m. The STXM images indicate that the magnetization in circular geometry changes continuously along the concentric circles without clear domain boundaries. In contrast, magnetization in square geometry consists of four distinct domains with clear diagonal domain boundaries. Similarly, three domains can be observed in triangle geometry. In order to quantify the in-plane magnetization configuration in magnetic vortices, we also use micromagnetic simulation to calculate the magnetization distributions of these three geometries. By extracting Mx along the circular profiles in both experimental and simulated vortex images, we find that the experimental magnetic profiles in the STXM images are consistent with the simulation data quantitatively. These magnetic structures are also studied by magnetic force microscopy (MFM). Since MFM is only sensitive to the dipolar magnetic field around the domain boundary, the MFM images show different configurations from the STXM images.

Understanding phase-change behaviors of carbon-doped Ge₂Sb₂Te₅ for phase-change memory application.

Phase-change materials are highly promising for next-generation nonvolatile data storage technology. The pronounced effects of C doping on structural and electrical phase-change behaviors of Ge2Sb2Te5 material are investigated at the atomic level by combining experiments and ab initio molecular dynamics. C dopants are found to fundamentally affect the amorphous structure of Ge2Sb2Te5 by altering the local environments of Ge-Te tetrahedral units with stable C-C chains. The incorporated C increases the amorphous stability due to the enhanced covalent nature of the material with larger tetrahedral Ge sites. The four-membered rings with alternating atoms are reduced greatly with carbon addition, leading to sluggish phase transition and confined crystal grains. The lower RESET power is presented in the PCM cells with carbon-doped material, benefiting from its high resistivity and low thermal conductivity.

Silicon versus the rest

We review the material properties that allowed amorphous silicon to become the dominant large area semiconductor and then point out how amorphous oxide semiconductors could displace a-Si in thin film transistors, and how phase change materials, such as GeSbTe alloys, have provided an optical storage technology and will provide a nonvolatile electrical storage technology based on their unique properties.

Refactor, Reduce, Recycle: Restructuring the I/O Stack for the Future of Storage

Emerging nonvolatile storage technologies promise orders-of-magnitude bandwidth increases and latency reductions, but fully realizing their potential requires minimizing storage software overhead and rethinking the roles of hardware and software in storage systems. The Web extra at http://youtu.be/pAALQ6k-CbE is an audio interview in which guest editor Simon S.Y. Shim interviews Steven Swanson about emerging nonvolatile storage technologies.

Void Formation Induced Electrical Switching in Phase-Change Nanowires

Solid-state structural transformation coupled with an electronic property change is an important mechanism for nonvolatile information storage technologies, such as phase-change memories. Here we exploit phase-change GeTe single-nanowire devices combined with ex situ and in situ transmission electron microscopy to correlate directly nanoscale structural transformations with electrical switching and discover surprising results. Instead of crystalline-amorphous transformation, the dominant switching mechanism during multiple cycling appears to be the opening and closing of voids in the nanowires due to material migration, which offers a new mechanism for memory. During switching, composition change and the formation of banded structural defects are observed in addition to the expected crystal-amorphous transformation. Our method and results are important to phase-change memories specifically, but also to any device whose operation relies on a small scale structural transformation.

Preface

The rapid growth in demand for data storage continues, and we expect that the total capacity of storage systems will exhibit a sixfold increase between 2006 and 2010. This growth, coupled with changing customer requirements and shifts in technology, is changing the composition and design of storage systems. Topics for this special issue on storage technologies and systems are diverse and include such subjects as the emergence of nonvolatile storage technologies; virtualization technologies that reduce the distinction between storage and computing platforms; advances in tape densities; the growing use of commodity and distributed storage; and the increasing importance of error and disaster recovery, autonomic storage management, petascale file and archival storage, and long-term data preservation.

Using MEMS-based storage in computer systems---device modeling and management

MEMS-based storage is an emerging nonvolatile secondary storage technology. It promises high performance, high storage density, and low power consumption. With fundamentally different architectural designs from magnetic disk, MEMS-based storage exhibits unique two-dimensional positioning behaviors and efficient power state transitions. We model these low-level, device-specific properties of MEMS-based storage and present request scheduling algorithms and power management strategies that exploit the full potential of these devices. Our simulations show that MEMS-specific device management policies can significantly improve system performance and reduce power consumption.

Using MEMS-based storage in computer systems---MEMS storage architectures

As an emerging nonvolatile secondary storage technology, MEMS-based storage exhibits several desirable properties including high performance, high storage volumic density, low power consumption, low entry cost, and small form factor. However, MEMS-based storage provides a limited amount of storage per device and is likely to be more expensive than magnetic disk. Systems designers will therefore need to make trade-offs to achieve well-balanced designs. We present an architecture in which MEMS devices are organized into MEMS storage enclosures with online spares. Such enclosures are proven to be highly reliable storage building bricks with no maintenance during their economic lifetimes. We also demonstrate the effectiveness of using MEMS as another layer in the storage hierarchy, bridging the cost and performance gap between MEMS storage and disk. We show that using MEMS as a disk cache can significantly improve system performance and cost-performance ratio.

Colossal magnetoresistivity in manganese-based perovskites (invited) (abstract)

Magnetoresistivity values of the order of 106% (and in some cases even higher) have been obtained in epitaxial AxB1−xMnO3−y (A=La,Nd; B=Ca,Sr,Ba) thin films grown by pulsed laser deposition. Ferromagnetic resonance experiments suggest a granular-type behavior with conducting ferromagnetic regions (Rcond<10 mΩ cm) in a less conducting matrix (Rinsulazting≳100.Rcond). Ion channeling experiments over a range of temperatures clearly reveal the existence of structural distortion at the peak resistivity temperature TP. Systematic studies of samples prepared under a variety of oxygenation conditions show that the resistivity above TP can be modeled with a single functional form: Rcond≊eΔ/kT, where Δ, the activation energy, is of the order of 50–200 meV. This suggests that these different samples represent the same basic material in a semiconducting matrix, with differing volume fractions of the two components which depends on the processing conditions. These ‘‘colossal’’ values of MR have been obtained at temperatures lower than room temperature and at fields of the order of a few Teslas, both of which are impediments to the development of viable MR sensor and nonvolatile storage technologies. We are therefore addressing the critical scientific and technological issues through a variety of materials integration approaches. Using structural chemistry and lattice matching as fundamental guiding principles, we are growing epitaxial heterostructure superlattices consisting of the CMR oxides interleaved with magnetic perovskites such as La–Sr–Co–O (metallic ferromagnet), rare earth–Fe–O (ferromagnetic insulator). We are also exploring the possibility of using the semiconducting properties of these materials in an all-perovskite field effect transistor device. In this presentation, we will describe our progress to date on these studies to enhance the field and temperature dependence of the MR properties and explore new device architectures that utilize the inherently novel properties of these materials.