Resistive random-access memory (ReRAM or RRAM) is a type of
non-volatile (NV)
random-access
Random access (more precisely and more generally called direct access) is the ability to access an arbitrary element of a sequence in equal time or any datum from a population of addressable elements roughly as easily and efficiently as any othe ...
(RAM) computer memory that works by changing the resistance across a
dielectric
In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...
solid-state material, often referred to as a
memristor
A memristor (; a portmanteau of ''memory resistor'') is a non-linear two-terminal electrical component relating electric charge and magnetic flux linkage. It was described and named in 1971 by Leon Chua, completing a theoretical quartet of fu ...
.
ReRAM bears some similarities to
conductive-bridging RAM (CBRAM) and
phase-change memory
Phase-change memory (also known as PCM, PCME, PRAM, PCRAM, OUM (ovonic unified memory) and C-RAM or CRAM (chalcogenide RAM)) is a type of non-volatile random-access memory. PRAMs exploit the unique behaviour of chalcogenide glass. In PCM, heat pr ...
(PCM). 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
A trademark (also written trade mark or trade-mark) is a type of intellectual property consisting of a recognizable sign, design, or expression that identifies products or services from a particular source and distinguishes them from othe ...
name of
Sharp Corporation
is a Japanese multinational corporation that designs and manufactures electronic products, headquartered in Sakai-ku, Sakai, Osaka Prefecture. Since 2016 it has been majority owned by the Taiwan-based Foxconn Group. Sharp employs more than 5 ...
, a Japanese electronic components manufacturer, in some countries, including members of the
European Union
The European Union (EU) is a supranational political and economic union of member states that are located primarily in Europe. The union has a total area of and an estimated total population of about 447million. The EU has often been de ...
.
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
Rambus Incorporated, founded in 1990, is an American technology company that designs, develops and licenses chip interface technologies and architectures that are used in digital electronics products. The company is well known for inventing ...
bought a ReRAM company called Unity Semiconductor for $35 million.
Panasonic
formerly between 1935 and 2008 and the first incarnation of between 2008 and 2022, is a major Japanese multinational conglomerate corporation, headquartered in Kadoma, Osaka. It was founded by Kōnosuke Matsushita in 1918 as a lightbulb ...
launched an ReRAM evaluation kit in May 2012, based on a
tantalum oxide 1T1R (1 transistor – 1 resistor) memory cell architecture.
In 2013,
Crossbar
Crossbar may refer to:
Structures
* Latch (hardware), a post barring a door
* Top tube of a bicycle frame
* Crossbar, the horizontal member of various sports goals
* Crossbar, a horizontal member of an electricity pylon
Other
* In electronic ...
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
A wafer is a crisp, often sweet, very thin, flat, light and dry biscuit, often used to decorate ice cream, and also used as a garnish on some sweet dishes. Wafers can also be made into cookies with cream flavoring sandwiched between them. They ...
, 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
perovskite
Perovskite (pronunciation: ) is a calcium titanium oxide mineral composed of calcium titanate (chemical formula ). Its name is also applied to the class of compounds which have the same type of crystal structure as (XIIA2+VIB4+X2−3), known a ...
s to
transition metal oxides
An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...
to
chalcogenide : 220px, Cadmium sulfide, a prototypical metal chalcogenide, is used as a yellow pigment.
A chalcogenide is a chemical compound consisting of at least one chalcogen anion and at least one more electropositive element. Although all group 16 elements ...
s.
Silicon dioxide
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
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
The University of Nebraska–Lincoln (Nebraska, NU, or UNL) is a public land-grant research university in Lincoln, Nebraska. Chartered in 1869 by the Nebraska Legislature as part of the Morrill Act of 1862, the school was known as the Univers ...
.
[
][
] 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
, mottoeng = And knowledge will be increased
, established = 1831 – Leeds School of Medicine1874 – Yorkshire College of Science1884 - Yorkshire College1887 – affiliated to the federal Victoria University1904 – University of Leeds
, ...
attempted to explain the mechanism theoretically.
[
] In May 1997, a research team from the
University of Florida
The University of Florida (Florida or UF) is a public land-grant research university in Gainesville, Florida. It is a senior member of the State University System of Florida, traces its origins to 1853, and has operated continuously on its ...
and
Honeywell
Honeywell International Inc. is an American publicly traded, multinational conglomerate corporation headquartered in Charlotte, North Carolina. It primarily operates in four areas of business: aerospace, building technologies, performance ma ...
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
A memristor (; a portmanteau of ''memory resistor'') is a non-linear two-terminal electrical component relating electric charge and magnetic flux linkage. It was described and named in 1971 by Leon Chua, completing a theoretical quartet of fu ...
.
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 Silicon oxide may refer to either of the following:
*Silicon dioxide or quartz, SiO2, very well characterized
*Silicon monoxide
Silicon monoxide is the chemical compound with the formula SiO where silicon is present in the oxidation state +2. In ...
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
, mottoeng = Let all come who by merit deserve the most reward
, established =
, type = Public research university
, endowment = £143 million (2020)
, budget = ...
) 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
In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...
, which is normally insulating, can be made to conduct through a filament or conduction path formed after application of a sufficiently high voltage.
The conduction path can arise from different mechanisms, including vacancy or metal defect migration. Once the filament is formed, it may be ''reset'' (broken, resulting in high resistance) or ''set'' (re-formed, resulting in lower resistance) by another 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 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.
CBRAM cells generally would not require forming if Cu ions are already present in the electrolyte, 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 electrolyte, the voltage would still be applied directly to the electrolyte, and forming would be a strong possibility.
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, ideally 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. The cross-point architecture requires
BEOL
The back end of line (BEOL) is the second portion of IC fabrication where the individual devices (transistors, capacitors, resistors, etc.) get interconnected with wiring on the wafer, the metalization layer. Common metals are copper and alumi ...
compatible two terminal selectors like punch-through diode for bipolar ReRAM 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 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
ABO3-type inorganic perovskite materials such as BaTiO3, SrRuO3, SrZO3, and SrTiO3 have attracted extensive research interest as the storage media in memristors due to their remarkable resistance switching effects and various functionalities such as ferroelectric, dielectric, and semiconducting physical characteristics. However, the fragile nature and high cost of the fabrication process limit the wide applications of these ABO3-type inorganic perovskite materials for memristors. Recently, ABX3-type lead trihalide perovskites have received extensive research interest for using in optoelectronic devices such as photovoltaics, photodetectors, and light-emitting diodes (LED). In these structures, A is a monovalent organic or inorganic (MA:CH3NH3+, FA: CH(NH2)2+, Cs+, Rb+), B is a divalent metal cation (Pb2+, Sn2+), and X is a halide anion (Cl, Br, I). The A cation resides at the eight corners of the cubic unit and the B cation locates at the center of the octahedral cluster
X6 to form the 3D perovskite structure. According to the different A-site cations, these structures can be classified into organic-inorganic hybrid perovskites and all-inorganic perovskites. Moreover, this type of perovskite can be obtained readily by solution-processable methods at a low cost.
6Nevertheless, owing to the inclusion of organic cations, it was commonly found that the intrinsic thermal instability of methylammonium (MA) and formamidinium (FA) lead trihalide perovskites was really a bottleneck for the development of hybrid perovskite-based electronic devices. Therefore, to resolve this issue, the organic cations must be substituted by other ions such as Cesium (Cs) cations. Interestingly, there are some reports of Cesium/Cesium hybridization solar cells that give us many new clues for the improved stability of all-inorganic perovskite-based electronic devices. More and more publications demonstrate that inorganic Cs cation-based all-inorganic perovskites could be both structurally and thermally stable above 100 °C, while hybrid perovskites thermally degraded to lead iodide above 85 °C. Therefore, it has been implied that all-inorganic perovskites could be excellent candidates for the fabrication of stable and highly efficient resistive switching memory devices using a low-cost process. Considering the CsPbX3 perovskites are usually prepared by solution method, point defects such as vacancies, interstitials, and antisites are possible in the crystals. These defects are essential for the defect drift-dominated resistive switching memory. Thus, these CsPbX3 perovskites have great potential for application in memory devices. Given the fact that resistance switching in halide perovskite-based RRAM is caused by migrations of halide atoms through vacancies, the migration characteristics of a vacancy within the RRAM are one of the most important material properties of the RRAM determining the key features of it. However, despite its importance, the activation energy of halide vacancy in RRAMs has been no serious study topic at all. Obviously, a small activation barrier of halide vacancy expected in halide perovskite-based RRAMs plays a central role in allowing this RRAM to operate at low voltages and thus at low power consumption mode.
Demonstrations
Papers at the IEDM Conference in 2007 suggested for the first time that ReRAM exhibits lower programming currents than
PRAM or
MRAM
Magnetoresistive random-access memory (MRAM) is a type of non-volatile random-access memory which stores data in magnetic domains. Developed in the mid-1980s, proponents have argued that magnetoresistive RAM will eventually surpass competing tec ...
without sacrificing programming performance, retention or endurance. Some commonly cited ReRAM systems are described further below.
HfO2-based ReRAM
At IEDM 2008, the highest-performance ReRAM technology to date was demonstrated by
ITRI
The Industrial Technology Research Institute (ITRI; ) is a technology research and development institution in Taiwan.
Founded in 1973, ITRI has contributed to moving Taiwan's industries from labor-intensive to innovation-driven. ITRI is hea ...
using HfO
2 with a Ti buffer layer, showing switching times less than 10 ns and currents less than 30μA. At IEDM 2010, ITRI again broke the speed record, showing <0.3 ns switching time, while also showing process and operation improvements to allow yield up to 100% and endurance up to 10 billion cycles.
IMEC
Interuniversity Microelectronics Centre (IMEC) is an international research & development organization, active in the fields of nanoelectronics and digital technologies, with headquarters in Belgium. Luc Van den hove has served as President an ...
presented updates of their ReRAM program at the 2012 Symposia on VLSI Technology and Circuits, including a solution with a 500 nA operating current.
ITRI had focused on the Ti/HfO
2 system since its first publication in 2008. ITRI's patent 8362454 has since been sold to TSMC; the number of prior licensees is unknown. On the other hand, IMEC focused mainly on Hf/HfO
2. Winbond had done more recent work toward advancing and commercializing the HfO
2-based ReRAM.
Panasonic
Panasonic revealed its TaO
x-based ReRAM at IEDM 2008. A key requirement was the need for a high work function metal such as Pt or Ir to interface with the TaO
x layer. The change of O content results in resistance change as well as Schottky barrier change. More recently, a Ta
2O
5/TaO
x layer was implemented, which still requires the high work function metal to interface with Ta
2O
5. This system has been associated with high endurance demonstration (trillion cycles), but products are specified at 100K cycles. Filament diameters as large as ~100 nm have been observed. Panasonic released a 4Mb part with Fujitsu, and is developing 40 nm embedded memory with UMC.
HP memristor
On 30 April 2008, HP announced that they had discovered the memristor, originally envisioned as a missing 4th fundamental circuit element by Chua in 1971. On 8 July they announced they would begin prototyping ReRAM using their memristors. HP first demonstrated its memristor using TiO
x, but later migrated to TaO
x, possibly due to improved stability. The TaO
x-based device has some material similarity to Panasonic's ReRAM, but the operation characteristics are different. The Hf/HfOx system was similarly studied.
Adesto Technologies
The
Adesto Technologies ReRAM is based on filaments generated from the electrode metal rather than oxygen vacancies. The original material system was Ag/GeS
2 but eventually migrated to ZrTe/Al
2O
3. The tellurium filament achieved better stability as compared to silver. Adesto has targeted the ultralow power memory for Internet-of-Things (IoT) applications. Adesto has released products manufactured at Altis foundry
and entered into a 45 nm foundry agreement with
TowerJazz/
Panasonic
formerly between 1935 and 2008 and the first incarnation of between 2008 and 2022, is a major Japanese multinational conglomerate corporation, headquartered in Kadoma, Osaka. It was founded by Kōnosuke Matsushita in 1918 as a lightbulb ...
.
Weebit Nano
Weebit Nano
Weebit Nano is a public semiconductor IP company founded in Israel in 2015 and headquartered in Hod Hasharon, Israel. The company develops Resistive Random-Access Memory (ReRAM or RRAM) technologies which is a specialized form of non-volatile mem ...
has been working with
CEA-Leti, one of the largest nanotechnology research institutes in Europe to further ReRAM technology. Beginning in November, 2017, the company has demonstrated the manufacturability in 40 nm SiOx ReRAM cells, followed by demonstrations of working arrays in 2018 and discrete components in 2020. In July 2021, the company taped out its first embedded ReRAM modules. In September 2021, Weebit, together with Leti, produced, tested and characterized a 1 Mb ReRAM array, using a 28 nm FDSOI process on 300mm wafers.
Crossbar
Crossbar
Crossbar may refer to:
Structures
* Latch (hardware), a post barring a door
* Top tube of a bicycle frame
* Crossbar, the horizontal member of various sports goals
* Crossbar, a horizontal member of an electricity pylon
Other
* In electronic ...
implements an Ag filament in amorphous Si along with a threshold switching system to achieve a diode+ReRAM. Their system includes the use of a transistor in 1T1R or 1TNR architecture. Crossbar started producing samples at SMIC on the 40 nm process in 2017. The Ag filament diameter has been visualized on the scale of tens of nanometers.
Programmable metallization cell
Infineon Technologies
Infineon Technologies AG is a German semiconductor manufacturer founded in 1999, when the semiconductor operations of the former parent company Siemens AG were spun off. Infineon has about 50,280 employees and is one of the ten largest semicon ...
calls it conductive-bridging RAM(CBRAM), NEC has a variant called "Nanobridge" and Sony calls their version "electrolytic memory". New research suggests CBRAM can be
3D printed
3D printing or additive manufacturing is the construction of a three-dimensional object from a CAD model or a digital 3D model. It can be done in a variety of processes in which material is deposited, joined or solidified under computer co ...
.
Quantum dot resistive memory device
Quantum dot based non-volatile resistive memory device with a switching speed of 10 ns and ON/OFF ratio of 10 000. The device showed excellent endurance characteristics for 100 000 switching cycles. Retention tests showed good stability and the devices are reproducible. Memory operating mechanism is proposed based on charge trapping in quantum dots with AlOx acting as barrier. This mechanism is supported by marked variation in capacitance value in ON and OFF states.
ReRam test boards
* Panasonic AM13L-STK2 : MN101LR05D 8-bit MCU with built in ReRAM for evaluation, connector
Future applications
Compared to PRAM, ReRAM operates at a faster timescale (switching time can be less than 10 ns), while compared to MRAM, it has a simpler, smaller cell structure (less than 8F² MIM stack). A vertical 1D1R (one diode, one resistive switching device) integration can be used for crossbar memory structure to reduce the unit cell size to 4F² (F is the feature dimension). Compared to flash memory and racetrack memory, a lower voltage is sufficient, and hence it can be used in low-power applications.
ITRI has shown that ReRAM is scalable below 30 nm. The motion of oxygen atoms is a key phenomenon for oxide-based ReRAM; one study indicated that oxygen motion may take place in regions as small as 2 nm. It is believed that if a filament is responsible, it would not exhibit direct scaling with cell size. Instead, the current compliance limit (set by an outside resistor, for example) could define the current-carrying capacity of the filament.
A significant hurdle to realizing the potential of ReRAM is the sneak path problem that occurs in larger passive arrays. In 2010,
complementary resistive switching (CRS) was introduced as a possible solution to sneak-path current interference. In the CRS approach, the information storing states are pairs of high- and low-resistance states (HRS/LRS and LRS/HRS) so that the overall resistance is always high, allowing larger passive crossbar arrays.
A drawback to the initial CRS solution is the requirement for switching endurance caused by conventional destructive readout based on current measurements. A new approach for a nondestructive readout based on capacity measurement potentially lowers the requirements for both material endurance and power consumption. Bi-layer structure is used to produce the nonlinearity in LRS to avoid the sneak path problem. A single-layer device exhibiting a strong nonlinear conduction in LRS was reported. Another bi-layer structure was introduced for bipolar ReRAM to improve the HRS and stability.
Another solution to the sneak current issue is to perform ''read'' and ''reset'' operations in parallel across an entire row of cells, while using ''set'' on selected cells. In this case, for a 3D-ReRAM 1TNR array, with a column of ''N'' ReRAM cells situated above a select transistor, only the intrinsic nonlinearity of the HRS is required to be sufficiently large, since the number of vertical levels ''N'' is limited (e.g., ''N'' = 8–32), and this has been shown possible for a low-current ReRAM system.
Modeling of 2D and 3D caches designed with ReRAM and other non-volatile random access memories such as
MRAM
Magnetoresistive random-access memory (MRAM) is a type of non-volatile random-access memory which stores data in magnetic domains. Developed in the mid-1980s, proponents have argued that magnetoresistive RAM will eventually surpass competing tec ...
and
PCM can be done using DESTINY tool.
Proposed role in artificial intelligence applications
The increasing computational demands necessary for many improvements in
artificial intelligence
Artificial intelligence (AI) is intelligence—perceiving, synthesizing, and inferring information—demonstrated by machines, as opposed to intelligence displayed by animals and humans. Example tasks in which this is done include speech r ...
have led many to speculate that ReRAM implementations could be extremely useful hardware for running
artificial intelligence
Artificial intelligence (AI) is intelligence—perceiving, synthesizing, and inferring information—demonstrated by machines, as opposed to intelligence displayed by animals and humans. Example tasks in which this is done include speech r ...
and
machine learning
Machine learning (ML) is a field of inquiry devoted to understanding and building methods that 'learn', that is, methods that leverage data to improve performance on some set of tasks. It is seen as a part of artificial intelligence.
Machine ...
applications.
Researchers at School of Engineering of Stanford University Havel built up a RRAM that "does the AI processing within the memory itself, thereby eliminating the separation between the compute and memory units." It is twice as energy efficient as state-of-the-art.
References
{{emerging technologies, topics=yes, infocom=yes
Solid-state computer storage media
Computer memory
Non-volatile random-access memory
Emerging technologies
Types of RAM