Resistive random-access memory (ReRAM or RRAM) is a type of non-volatile (NV) random-access (RAM) computer memory that works by changing the resistance across a dielectric solid-state material, often referred to as a memristor. One major advantage of ReRAM over other NVRAM technologies is the ability to scale below 10 nm.
ReRAM bears some similarities to conductive-bridging RAM (CBRAM) and phase-change memory (PCM) in that they change dielectric material properties. CBRAM involves one electrode providing ions that dissolve readily in an electrolyte material, while PCM involves generating sufficient Joule heating to effect amorphous-to-crystalline or crystalline-to-amorphous phase changes. By contrast, ReRAM involves generating defects in a thin oxide layer, known as oxygen vacancies (oxide bond locations where the oxygen has been removed), which can subsequently charge and drift under an electric field. The motion of oxygen ions and vacancies in the oxide would be analogous to the motion of electrons and holes in a semiconductor.
Although ReRAM was initially seen as a replacement technology for flash memory, the cost and performance benefits of ReRAM have not been enough for companies to proceed with the replacement. Apparently, a broad range of materials can be used for ReRAM. However, the discovery that the popular high-κ gate dielectric HfO2 can be used as a low-voltage ReRAM has encouraged researchers to investigate more possibilities.
RRAM is the registered trademark name of Sharp Corporation, a Japanese electronic components manufacturer, in some countries, including members of the European Union.
History In the early 2000s, ReRAMs were under development by a number of companies, some of which filed patent applications claiming various implementations of this technology. ReRAM has entered commercialization on an initially limited KB-capacity scale.
In February 2012, Rambus bought a ReRAM company called Unity Semiconductor for $35 million. Panasonic launched a ReRAM evaluation kit in May 2012, based on a tantalum oxide 1T1R (1 transistor – 1 resistor) memory cell architecture.
In 2013, Crossbar introduced an ReRAM prototype as a chip about the size of a postage stamp that could store 1 TB of data. In August 2013, the company claimed that large-scale production of their ReRAM chips was scheduled for 2015. The memory structure (Ag/a-Si/Si) closely resembles a silver-based CBRAM.
Also in 2013, Hewlett-Packard demonstrated a memristor-based ReRAM wafer, and predicted that 100 TB SSDs based on the technology could be available in 2018 with 1.5 PB capacities available in 2020, just in time for the stop in growth of NAND flash capacities.
Different forms of ReRAM have been disclosed, based on different dielectric materials, spanning from perovskites to transition metal oxides to chalcogenides. Silicon dioxide was shown to exhibit resistive switching as early as May 1966, and has recently been revisited.
In 1963 and 1964, a thin-film resistive memory array was first proposed by members of the University of Nebraska–Lincoln. Further work on this new thin-film resistive memory was reported by J.G. Simmons in 1967. In 1970, members of the Atomic Energy Research Establishment and University of Leeds attempted to explain the mechanism theoretically. In May 1997, a research team from the University of Florida and Honeywell reported a manufacturing method for "magneto-resistive random-access memory" by utilizing electron cyclotron resonance plasma etching.
Leon Chua argued that all two-terminal non-volatile memory devices including ReRAM should be considered memristors. Stan Williams of HP Labs also argued that ReRAM was a memristor. However, others challenged this terminology and the applicability of memristor theory to any physically realizable device is open to question. Whether redox-based resistively switching elements (ReRAM) are covered by the current memristor theory is disputed.
Silicon oxide presents an interesting case of resistance switching. Two distinct modes of intrinsic switching have been reported - surface-based, in which conductive silicon filaments are generated at exposed edges (which may be internal—within pores—or external—on the surface of mesa structures), and bulk switching, in which oxygen vacancy filaments are generated within the bulk of the oxide. The former mode suffers from oxidation of the filaments in air, requiring hermetic sealing to enable switching. The latter requires no sealing. In 2014 researchers from Rice University announced a silicon filament-based device that used a porous silicon oxide dielectric with no external edge structure - rather, filaments were formed at internal edges within pores. Devices can be manufactured at room temperature and have a sub-2V forming voltage, high on-off ratio, low power consumption, nine-bit capacity per cell, high switching speeds and good endurance. Problems with their inoperability in air can be overcome by hermetic sealing of devices. Bulk switching in silicon oxide, pioneered by researchers at UCL (University College London) since 2012, offers low electroforming voltages (2.5V), switching voltages around 1V, switching times in the nanoseconds regime, and more than 10,000,000 cycles without device failure - all in ambient conditions.
Forming
The basic idea is that a dielectric, which is normally insulating, can form a conduction path after application of a sufficiently high voltage. The conduction path can arise from different mechanisms, including vacancy or metal defect migration. Once the conduction path is formed, it may be reset (broken, resulting in high resistance) or set (re-formed, resulting in lower resistance) by another lower voltage. Many current paths, rather than a single filament, are possibly involved. The presence of these current paths in the dielectric can be in situ demonstrated via conductive atomic force microscopy.
The low-resistance path can be either localized (filamentary) or homogeneous. Both effects can occur either throughout the entire distance between the electrodes or only in proximity to one of the electrodes. Filamentary and homogenous conduction path switching effects can be distinguished by measuring the area dependence of the low-resistance state.
Under certain conditions, the forming operation may be bypassed. It is expected that under these conditions, the initial current is already quite high compared to insulating oxide layers. ReRAM cells generally do not require high voltage forming if Cu ions are already present in the dielectric, having already been driven-in by a designed photo-diffusion or annealing process; such cells may also readily return to their initial state. In the absence of such Cu initially being in the dielectric, the voltage applied directly to the electrolyte has a strong possibility of forming.
Operation styles For random-access type memories, a 1T1R (one transistor, one resistor) architecture is preferred because the transistor isolates current to cells that are selected from cells that are not. On the other hand, a cross-point architecture is more compact and may enable vertically stacking memory layers, ideal suited for mass-storage devices. However, in the absence of any transistors, isolation must be provided by a "selector" device, such as a diode, in series with the memory element or by the memory element itself. Such isolation capabilities are inferior to the use of transistors if the on/off ratio for the selector is not sufficient, limiting the ability to operate very large arrays in this architecture. Thin film based threshold switch can work as a selector for bipolar and unipolar ReRAM. Threshold switch-based selector was demonstrated for 64 Mb array.I.V. Karpov, D. Kencke, D. Kau, S. Tang and G. Spadini, MRS Proceedings, Volume 1250, 2010 The cross-point architecture requires BEOL compatible two terminal selectors like punch-through diode for bipolar ReRAMV. S. S. Srinivasan et al., Punchthrough-Diode-Based Bipolar RRAM Selector by Si Epitaxy," Electron Device Letters, IEEE, vol.33, no.10, pp.1396,1398, Oct. 2012 doi: 10.1109/LED.2012.2209394 or PIN diode for unipolar ReRAM.
Polarity can be either binary or unary. Bipolar effects cause polarity to reverse when switching from low to high resistance (reset operation) compared to switching high to low (set operation). Unipolar switching leaves polarity unaffected, but uses different voltages.
Material systems for resistive memory cells Multiple inorganic and organic material systems display thermal or ionic resistive switching effects. These can be grouped into the following categories:
phase-change chalcogenides such as or AgInSbTe binary transition metal oxides such as NiO, Ta2O5, or perovskites such as Sr(Zr) or PCMO solid-state electrolytes such as GeS, GeSe, or organic charge-transfer complexes such as CuTCNQ organic donor–acceptor systems such as Al AIDCN two dimensional (layered) insulating materials like hexagonal boron nitride
RRAM-based on Perovskite