This review article attempts to provide a comprehensive review of the recent progress in the so-called resistive random access memories (RRAMs) First, a brief introduction is presented to describe the construction and development of RRAMs, their potential for broad applications in the fields of nonvolatile memory, unconventional computing and logic devices, and the focus of research concerning RRAMs over the past decade Second, both inorganic and organic materials used in RRAMs are summarized, and their respective advantages and shortcomings are discussed Third, the important switching mechanisms are discussed in depth and are classified into ion migration, charge trapping/de-trapping, thermochemical reaction, exclusive mechanisms in inorganics, and exclusive mechanisms in organics Fourth, attention is given to the application of RRAMs for data storage, including their current performance, methods for performance enhancement, sneak-path issue and possible solutions, and demonstrations of 2-D and 3-D crossbar arrays Fifth, prospective applications of RRAMs in unconventional computing, as well as logic devices and multi-functionalization of RRAMs, are comprehensively summarized and thoroughly discussed The present review article ends with a short discussion concerning the challenges and future prospects of the RRAMs

Recent progress in resistive random access memories: Materials, switching mechanisms, and performance

This review focuses on electrochemical metallization memory cells (ECM), highlighting their advantages as the next generation memories. In a brief introduction, the basic switching mechanism of ECM cells is described and the historical development is sketched. In a second part, the full spectra of materials and material combinations used for memory device prototypes and for dedicated studies are presented. In a third part, the specific thermodynamics and kinetics of nanosized electrochemical cells are described. The overlapping of the space charge layers is found to be most relevant for the cell properties at rest. The major factors determining the functionality of the ECM cells are the electrode reaction and the transport kinetics. Depending on electrode and/or electrolyte material electron transfer, electro-crystallization or slow diffusion under strong electric fields can be rate determining. In the fourth part, the major device characteristics of ECM cells are explained. Emphasis is placed on switching speed, forming and SET/RESET voltage, R(ON) to R(OFF) ratio, endurance and retention, and scaling potentials. In the last part, circuit design aspects of ECM arrays are discussed, including the pros and cons of active and passive arrays. In the case of passive arrays, the fundamental sneak path problem is described and as well as a possible solution by two anti-serial (complementary) interconnected resistive switches per cell. Furthermore, the prospects of ECM with regard to further scalability and the ability for multi-bit data storage are addressed.

https://iopscience.iop.org/article/10.1088/0957-4484/22/25/254003/pdf

Electrochemical metallization memories—fundamentals, applications, prospects

With the explosive growth of digital data in the era of the Internet of Things (IoT), fast and scalable memory technologies are being researched for data storage and data-driven computation. Among the emerging memories, resistive switching memory (RRAM) raises strong interest due to its high speed, high density as a result of its simple two-terminal structure, and low cost of fabrication. The scaling projection of RRAM, however, requires a detailed understanding of switching mechanisms and there are potential reliability concerns regarding small device sizes. This work provides an overview of the current understanding of bipolar-switching RRAM operation, reliability and scaling. After reviewing the phenomenological and microscopic descriptions of the switching processes, the stability of the low- and high-resistance states will be discussed in terms of conductance fluctuations and evolution in 1D filaments containing only a few atoms. The scaling potential of RRAM will finally be addressed by reviewing the recent breakthroughs in multilevel operation and 3D architecture, making RRAM a strong competitor among future high-density memory solutions.

https://iopscience.iop.org/article/10.1088/0268-1242/31/6/063002/pdf

Resistive switching memories based on metal oxides: mechanisms, reliability and scaling

Redox-based nanoionic resistive memory cells are one of the most promising emerging nanodevices for future information technology with applications for memory, logic and neuromorphic computing. Recently, the serendipitous discovery of the link between redox-based nanoionic-resistive memory cells and memristors and memristive devices has further intensified the research in this field. Here we show on both a theoretical and an experimental level that nanoionic-type memristive elements are inherently controlled by non-equilibrium states resulting in a nanobattery. As a result, the memristor theory must be extended to fit the observed non-zero-crossing I-V characteristics. The initial electromotive force of the nanobattery depends on the chemistry and the transport properties of the materials system but can also be introduced during redox-based nanoionic-resistive memory cell operations. The emf has a strong impact on the dynamic behaviour of nanoscale memories, and thus, its control is one of the key factors for future device development and accurate modelling.

/pdf/nanobatteries-in-redox-based-resistive-switches-require-1s2m5t603y.pdf

Nanobatteries in redox-based resistive switches require extension of memristor theory

This paper will review emerging non-volatile memory (NVM) technologies, with the focus on phase change memory (PCM), spin-transfer-torque random-access-memory (STTRAM), resistive random-access-memory (RRAM), and ferroelectric field-effect-transistor (FeFET) memory. These promising NVM devices are evaluated in terms of their advantages, challenges, and applications. Their performance is compared based on reported parameters of major industrial test chips. Memory selector devices and cell structures are discussed. Changing market trends toward low power ( e.g. , mobile, IoT) and data-centric applications create opportunities for emerging NVMs. High-performance and low-cost emerging NVMs may simplify memory hierarchy, introduce non-volatility in logic gates and circuits, reduce system power, and enable novel architectures. Storage-class memory (SCM) based on high-density NVMs could fill the performance and density gap between memory and storage. Some unique characteristics of emerging NVMs can be utilized for novel applications beyond the memory space, e.g. , neuromorphic computing, hardware security, etc . In the beyond-CMOS era, emerging NVMs have the potential to fulfill more important functions and enable more efficient, intelligent, and secure computing systems.

A review of emerging non-volatile memory (NVM) technologies and applications

One of the promising technologies under development for next generation non-volatile memory is the Conductive Bridging Random Access Memory (CBRAM) which utilizes the reversible switching of an electro-resistive dielectric between two conductive states as means of storing logical data [1–7]  In this paper, we describe the successful integration of CBRAM technology into an industry standard logic process Moreover, we show functional operation of such a fully CMOS integrated CBRAM memory array and highlight its specific fundamental low power characteristics that make it suitable to be used in scaled embedded application as well as discrete devices

Demonstration of Conductive Bridging Random Access Memory (CBRAM) in logic CMOS process

Today's main stream NVM technologies require operational conditions that are incompatible with modern low voltage logic CMOS designs. This characteristic results in complex integration issues as well as costly process and array concept especially for embedded NVM use models. Conductive bridging memory cell (CBRAM) technology is an attractive emerging memory technology that offers simple integration and scalable operational conditions. These unique features make CBRAM technology an ideal candidate for embedded applications. In this paper, we have shown successful integration of CBRAM into Copper and Aluminum back end logic CMOS processes with minimal number of added masks.

Demonstration of Conductive Bridging Random Access Memory (CBRAM) in Logic CMOS Process

Ag/GeS2/W conductive-bridge random access memory (CBRAM) cells are shown to program at room temperature to conductance levels near multiples of the fundamental conductance G0 = 2e2/h. This behavior is not accounted for in the traditional view that the conductance of a CBRAM cell is a continuous variable proportional to the maximum current allowed to flow during programming. For on -state resistances on the order of 1/G0 = 12.9 kΩ or less, quantization implies that the Ag “conductive bridge” typically contains a constriction, or even an extended chain, that can be as narrow as a single atom. Implications for device modeling and commercial applications are discussed.

Quantized Conductance in $\hbox{Ag/GeS}_{2}/\hbox{W}$ Conductive-Bridge Memory Cells

Discrete and embedded nonvolatile memory (NVM) technologies have been an integral part of electronic systems for the past 25 years. In recent years, the proliferation of personal media devices such as multimedia-enabled cell phones, personal music players, and digital cameras has accelerated the adoption of silicon-based solid state storage cards in consumer markets. Despite the expanded use of nonvolatile memory technologies in a variety of integrated systems, little has changed with respect to the core technology and cells that hold the data when power has been turned off. Today, floating gate (FG) or oxide-nitride-oxide trapped charge (ONO) cell structures dominate as the core technology behind all NVM devices and embedded blocks. All of the nonvolatile memory devices in production today based on these technologies require high voltage in excess of 5-8 V to operate primarily due to the fundamental nature of core cells and the physics of charge storage mechanisms. These are huge overvoltage requirements considering that the transistors in the logic block require substantially lower voltages (e.g., sub-65 nm logic CMOS operate at less than 1 V). Integrating such high-voltage operation in advanced logic processes such as 65 nm or below logic CMOS process is yet another challenge limiting the exploitation of NVM for low-power embedded applications. The high voltage requirement for operation of these core cells has put strains on the continued scaling of today's discrete and embedded NVM technologies. Furthermore, future ultralow-power and subthreshold CMOS applications such as energy starved electronics require operations at sub-500 mV which clearly set forth significant challenges in integrating today's NVM technologies as nonvolatile storage elements for such systems. Several emerging technologies are competing to become the building blocks of next-generation nonvolatile memory solutions. Each of these emerging technologies has unique characteristics in terms of physical scaling, voltage scaling, cost, performance, and power features which differ from today's FG and ONO based technologies. This paper reviews the fundamental characteristics of current nonvolatile memory technologies as well as several promising emerging technologies from energy and power perspectives and specifically discusses the suitability of each one for use in ultralow-power and subthreshold CMOS applications.

Power and Energy Perspectives of Nonvolatile Memory Technologies

High-temperature data retention is a critical hurdle for the commercialization of emerging nonvolatile memories. For Conductive-Bridge RAM (CBRAM) [1], we discuss high-temperature retention in terms of the physics of quantum point contacts, and we report on a family of CBRAM cells that achieve excellent retention at temperatures exceeding 200°C.

Shane Hollmer

Papers

Demonstration of Conductive Bridging Random Access Memory (CBRAM) in logic CMOS process

Demonstration of Conductive Bridging Random Access Memory (CBRAM) in Logic CMOS Process

Quantized Conductance in $\hbox{Ag/GeS}_{2}/\hbox{W}$ Conductive-Bridge Memory Cells

Power and Energy Perspectives of Nonvolatile Memory Technologies

Conductive-bridge memory (CBRAM) with excellent high-temperature retention