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The programmable metallization cell, or PMC, is a
non-volatile Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retain stored information even after power is removed. In contrast, volatile memory needs constant power in order to retain data. Non-volatile memory typic ...
computer memory developed at
Arizona State University Arizona State University (Arizona State or ASU) is a public research university in the Phoenix metropolitan area. Founded in 1885 by the 13th Arizona Territorial Legislature, ASU is one of the largest public universities by enrollment in the ...
. PMC, a technology developed to replace the widely used
flash memory Flash memory is an electronic non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for the NOR and NAND logic gates. Both us ...
, providing a combination of longer lifetimes, lower power, and better memory density.
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 semicond ...
, who licensed the technology in 2004, refers to it as conductive-bridging RAM, or CBRAM. CBRAM became a registered trademark of
Adesto Technologies Adesto Technologies is an American corporation founded in 2006 and based in Santa Clara, California. The company provides application-specific semiconductors and embedded systems for the Internet of Things (IoT), and sells its products directly to ...
in 2011. NEC has a variant called "Nanobridge" and Sony calls their version "electrolytic memory".


Description

PMC is a two terminal resistive memory technology developed at
Arizona State University Arizona State University (Arizona State or ASU) is a public research university in the Phoenix metropolitan area. Founded in 1885 by the 13th Arizona Territorial Legislature, ASU is one of the largest public universities by enrollment in the ...
. PMC is an electrochemical metallization memory that relies on redox reactions to form and dissolve a conductive filament. The state of the device is determined by the resistance across the two terminals. The existence of a filament between the terminals produces a low resistance state (LRS) while the absence of a filament results in a high resistance state (HRS). A PMC device is made of two solid metal electrodes, one relatively inert (e.g., tungsten or nickel) the other electrochemically active (e.g., silver or copper), with a thin film of
solid electrolyte In materials science, fast ion conductors are solid conductors with highly mobile ions. These materials are important in the area of solid state ionics, and are also known as solid electrolytes and superionic conductors. These materials are usefu ...
between them.


Device operation

The resistance state of a PMC is controlled by the formation (programming) or dissolution (erasing) of a metallic conductive filament between the two terminals of the cell. A formed filament is a fractal tree like structure.


Filament formation

PMC rely on the formation of a metallic conductive filament to transition to a low resistance state (LRS). The filament is created by applying a positive voltage bias (''V'') to the anode contact (active metal) while grounding the cathode contact (inert metal). The positive bias
oxidizes Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
the active metal (M): :M → M+ + e The applied bias generates an
electric field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
between the two metal contacts. The ionized (oxidized) metal ions migrate along the electric field toward the cathode contact. At the cathode contact, the metal ions are reduced: :M+ + e → M As the active metal deposits on the cathode, the electric field increases between the anode and the deposit. The evolution of the local electric field (''E'') between the growing filament and the anode can be simplistically related to the following: : E = -\frac where ''d'' is the distance between the anode and the top of the growing filament. The filament will grow to connect to the anode within a few nanoseconds. Metal ions will continue to be reduced at the filament until the voltage is removed, broadening the conductive filament and decreasing the resistance of the connection over time. Once the voltage is removed, the conductive filament will remain, leaving the device in a LRS. The conductive filament may not be continuous, but a chain of electrodeposit islands or nanocrystals. This is likely to prevail at low programming currents (less than 1 μ A) whereas higher programming current will lead to a mostly metallic conductor.


Filament dissolution

A PMC can be "erased" into a high resistance state (HRS) by applying a negative voltage bias to the anode. The redox process used to create the conductive filament is reversed and the metal ions migrate along the reversed electric field to reduce at the anode contact. With the filament removed, the PMC is analogous to parallel plate capacitor with a high resistance of several M Ω to G Ω between the contacts.


Device read

An individual PMC can be read by applying a small voltage across the cell. As long as the applied read voltage is less than both the programming and erasing voltage threshold, the direction of the bias is not significant.


Technology comparison


CBRAM vs. metal-oxide ReRAM

CBRAM differs from metal-oxide ReRAM in that for CBRAM metal ions dissolve readily in the material between the two electrodes, while for metal-oxides, the material between the electrodes requires a high electric field causing local damage akin to dielectric breakdown, producing a trail of conducting defects (sometimes called a "filament"). Hence for CBRAM, one electrode must provide the dissolving ions, while for metal-oxide RRAM, a one-time "forming" step is required to generate the local damage.


CBRAM vs. NAND Flash

The primary form of
solid-state Solid state, or solid matter, is one of the four fundamental states of matter. Solid state may also refer to: Electronics * Solid-state electronics, circuits built of solid materials * Solid state ionics, study of ionic conductors and their use ...
non-volatile memory in use is
flash memory Flash memory is an electronic non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for the NOR and NAND logic gates. Both us ...
, which is finding use in most roles formerly filled by hard drives. Flash, however, has problems that led to many efforts to introduce products to replace it. Flash is based on the floating gate concept, essentially a modified transistor. Conventional flash transistors have three connections, the source, drain and gate. The gate is the essential component of the transistor, controlling the resistance between the source and drain, and thereby acting as a switch. In the
floating gate transistor The floating-gate MOSFET (FGMOS), also known as a floating-gate MOS transistor or floating-gate transistor, is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) where the gate is electrically isolated, creating a floating no ...
, the gate is attached to a layer that traps electrons, leaving it switched on (or off) for extended periods of time. The floating gate can be re-written by passing a large current through the emitter-collector circuit. It is this large current that is flash's primary drawback, and for a number of reasons. For one, each application of the current physically degrades the cell, such that the cell will eventually be unwritable. Write cycles on the order of 105 to 106 are typical, limiting flash applications to roles where constant writing is not common. The current also requires an external circuit to generate, using a system known as a charge pump. The pump requires a fairly lengthy charging process so that writing is much slower than reading; the pump also requires much more power. Flash is thus an "asymmetrical" system, much more so than conventional RAM or hard drives. Another problem with flash is that the floating gate suffers leakage that slowly releases the charge. This is countered through the use of powerful surrounding insulators, but these require a certain physical size in order to be useful and also require a specific
physical layout Integrated circuit layout, also known IC layout, IC mask layout, or mask design, is the representation of an integrated circuit in terms of planar geometric shapes which correspond to the patterns of metal, oxide, or semiconductor layers that make ...
, which is different from the more typical
CMOS Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSFE ...
layouts, which required several new fabrication techniques to be introduced. As flash scales rapidly downward in size the charge leakage increasingly becomes a problem, which led to predictions of its demise. However, massive market investment drove development of flash at rates in excess of
Moore's Law Moore's law is the observation that the number of transistors in a dense integrated circuit (IC) doubles about every two years. Moore's law is an observation and projection of a historical trend. Rather than a law of physics, it is an empir ...
, and semiconductor fabrication plants using 30 nm processes were brought online in late 2007. In contrast to flash, PMC writes with relatively low power and at high speed. The speed is inversely related to the power applied (to a point, there are mechanical limits), so the performance can be tuned. PMC, in theory, can scale to sizes much smaller than flash, theoretically as small as a few ion widths wide. Copper ions are about 0.75 angstroms, so line widths on the order of nanometers seem possible. PMC was promoted as simpler in layout than flash.


History

PMC technology was developed by Michael Kozicki, professor of electrical engineering at
Arizona State University Arizona State University (Arizona State or ASU) is a public research university in the Phoenix metropolitan area. Founded in 1885 by the 13th Arizona Territorial Legislature, ASU is one of the largest public universities by enrollment in the ...
in the 1990s. Early experimental PMC systems were based on silver-doped germanium selenide glasses. Work turned to silver-doped germanium sulfide electrolytes and then to the copper-doped germanium sulfide electrolytes. There has been renewed interest in silver-doped germanium selenide devices due to their high, high resistance state. Copper-doped silicon dioxide glass PMC would be compatible with the CMOS
fabrication Fabrication may refer to: * Manufacturing, specifically the crafting of individual parts as a solo product or as part of a larger combined product. Processes in arts, crafts and manufacturing *Semiconductor device fabrication, the process used t ...
process. In 1996, Axon Technologies was founded to commercialize the PMC technology.
Micron Technology Micron Technology, Inc. is an American producer of computer memory and computer data storage including dynamic random-access memory, flash memory, and USB flash drives. It is headquartered in Boise, Idaho. Its consumer products, including ...
announced work with PMC in 2002. Infineon followed in 2004. PMC technology was licensed to Adesto Technologies by 2007. infineon had spun off memory business to its
Qimonda Qimonda AG ( ) was a German memory company split out of Infineon Technologies (itself a spun off business unit of Siemens AG) on 1 May 2006 to form at the time the second largest DRAM company worldwide, according to the industry research firm Ga ...
company, which in turn sold it to Adesto Technologies. A DARPA grant was awarded in 2010 for further research. In 2011, Adesto Technologies allied with the French company
Altis Semiconductor Corbeil-Essonnes () on the River Seine is a commune in the southern suburbs of Paris, France. It is located from the center of Paris. Although neighboring Évry is the official seat of the Arrondissement of Évry, the sub-prefecture buildi ...
for development and manufacturing of CBRAM. In 2013, Adesto introduced a sample CBRAM product in which a 1 megabit part was promoted to replace
EEPROM EEPROM (also called E2PROM) stands for electrically erasable programmable read-only memory and is a type of non-volatile memory used in computers, usually integrated in microcontrollers such as smart cards and remote keyless systems, or as a ...
. NEC developed the so-called nanobridge technology, using Cu2S or tantalumpentoxide as dielectric material. Hereby copper (compatible with copper metallization of the IC) makes the copper to migrate through Cu2S or Ta2O5 making or breaking shorts between the copper and ruthenium electrodes. The dominant use of this type of memory are space applications, since this type of memory is intrinsically radiation hard.


See also

* Static random-access memory


References


External links


Axon Technologies CorporationMichael N. KozickiAdesto Technologies
{{emerging technologies, topics=yes, infocom=yes Computer memory Non-volatile memory Emerging technologies