# Programmable metallization cell

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The programmable metallization cell, or PMC, is a non-volatile computer memory developed at Arizona State University. PMC a technology developed to replace the widely used flash memory, providing a combination of longer lifetimes, lower power, and better memory density. Infineon Technologies, who licensed the technology in 2004, refers to it as conductive-bridging RAM , or CBRAM. CBRAM became a registered trademark of Adesto Technologies in 2011. [1] NEC has a variant called "Nanobridge" and Sony calls their version "electrolytic memory".

Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retrieve stored information even after having been power cycled. In contrast, volatile memory needs constant power in order to retain data. Examples of non-volatile memory include flash memory, read-only memory (ROM), ferroelectric RAM, most types of magnetic computer storage devices, optical discs, and early computer storage methods such as paper tape and punched cards.

In computing, memory refers to the computer hardware integrated circuits that store information for immediate use in a computer; it is synonymous with the term "primary storage". Computer memory operates at a high speed, for example random-access memory (RAM), as a distinction from storage that provides slow-to-access information but offers higher capacities. If needed, contents of the computer memory can be transferred to secondary storage; a very common way of doing this is through a memory management technique called "virtual memory". An archaic synonym for memory is store.

Arizona State University is a public metropolitan research university on five campuses across the Phoenix metropolitan area, and four regional learning centers throughout Arizona.

## Description

PMC is a two terminal resistive memory technology developed at Arizona State University. PMC is an electrochemical metallization memory that relies on redox reactions to form and dissolve a conductive filament. [2] 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 between them. [3]

Redox is a type of chemical reaction in which the oxidation states of atoms are changed. Redox reactions are characterized by the transfer of electrons between chemical species, most often with one species undergoing oxidation while another species undergoes reduction. The chemical species from which the electron is stripped is said to have been oxidized, while the chemical species to which the electron is added is said to have been reduced. In other words:

Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. The name tungsten comes from the former Swedish name for the tungstate mineral scheelite, tung sten or "heavy stone". Tungsten is a rare metal found naturally on Earth almost exclusively combined with other elements in chemical compounds rather than alone. It was identified as a new element in 1781 and first isolated as a metal in 1783. Its important ores include wolframite and scheelite.

Nickel is a chemical element with the symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile. Pure nickel, powdered to maximize the reactive surface area, shows a significant chemical activity, but larger pieces are slow to react with air under standard conditions because an oxide layer forms on the surface and prevents further corrosion (passivation). Even so, pure native nickel is found in Earth's crust only in tiny amounts, usually in ultramafic rocks, and in the interiors of larger nickel–iron meteorites that were not exposed to oxygen when outside Earth's atmosphere.

## 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.

Diffusion-limited aggregation (DLA) is the process whereby particles undergoing a random walk due to Brownian motion cluster together to form aggregates of such particles. This theory, proposed by T.A. Witten Jr. and L.M. Sander in 1981, is applicable to aggregation in any system where diffusion is the primary means of transport in the system. DLA can be observed in many systems such as electrodeposition, Hele-Shaw flow, mineral deposits, and dielectric breakdown.

### 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 the active metal (M):

Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential between two points. The difference in electric potential between two points in a static electric field is defined as the work needed per unit of charge to move a test charge between the two points. In the International System of Units, the derived unit for voltage is named volt. In SI units, work per unit charge is expressed as joules per coulomb, where 1 volt = 1 joule per 1 coulomb. The official SI definition for volt uses power and current, where 1 volt = 1 watt per 1 ampere. This definition is equivalent to the more commonly used 'joules per coulomb'. Voltage or electric potential difference is denoted symbolically by V, but more often simply as V, for instance in the context of Ohm's or Kirchhoff's circuit laws.

An anode is an electrode through which the conventional current enters into a polarized electrical device. This contrasts with a cathode, an electrode through which conventional current leaves an electrical device. A common mnemonic is ACID, for "anode current into device". The direction of conventional current in a circuit is opposite to the direction of electron flow, so electrons flow out the anode into the outside circuit. In a galvanic cell, the anode is the electrode at which the oxidation reaction occurs.

In electrical engineering, ground or earth is the reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the earth.

M → M+ + e

The applied bias generates an electric field 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:

An electric field surrounds an electric charge, and exerts force on other charges in the field, attracting or repelling them. Electric field is sometimes abbreviated as E-field. The electric field is defined mathematically as a vector field that associates to each point in space the force per unit of charge exerted on an infinitesimal positive test charge at rest at that point. The SI unit for electric field strength is volt per meter (V/m). Newtons per coulomb (N/C) is also used as a unit of electric field strength. Electric fields are created by electric charges, or by time-varying magnetic fields. Electric fields are important in many areas of physics, and are exploited practically in electrical technology. On an atomic scale, the electric field is responsible for the attractive force between the atomic nucleus and electrons that holds atoms together, and the forces between atoms that cause chemical bonding. Electric fields and magnetic fields are both manifestations of the electromagnetic force, one of the four fundamental forces of nature.

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:

${\displaystyle E=-{\frac {V}{d}}}$

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. [4] 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. [5] 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.

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 non-volatile memory in use is flash memory, 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 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, which is different from the more typical CMOS 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, 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. [6]

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, [7] so line widths on the order of nanometers seem possible. PMC was promoted as simpler in layout than flash. [6]

## History

PMC technology was developed by Michael Kozicki, professor of electrical engineering at Arizona State University in the 1990s. [8] [9] [10] [11] [12] [13] [14] 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. [4] 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 process.

In 1996, Axon Technologies was founded to commercialize the PMC technology. Micron Technology announced work with PMC in 2002. [15] Infineon followed in 2004. [16] PMC technology was licensed to Adesto Technologies by 2007. [6] infineon had spun off memory business to its Qimonda company, which in turn sold it to Adesto Technologies. A DARPA grant was awarded in 2010 for further research. [17]

In 2011, Adesto Technologies allied with the French company Altis Semiconductor for development and manufacturing of CBRAM. [18] In 2013, Adesto introduced a sample CBRAM product in which a 1 megabit part was promoted to replace EEPROM. [19]

## Related Research Articles

Cathode rays are streams of electrons observed in vacuum tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, glass behind the positive electrode is observed to glow, due to electrons emitted from the cathode. They were first observed in 1869 by German physicist Julius Plücker and Johann Wilhelm Hittorf, and were named in 1876 by Eugen Goldstein Kathodenstrahlen, or cathode rays. In 1897, British physicist J. J. Thomson showed that cathode rays were composed of a previously unknown negatively charged particle, which was later named the electron. Cathode ray tubes (CRTs) use a focused beam of electrons deflected by electric or magnetic fields to render an image on a screen.

A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic CCD for Cathode Current Departs. A conventional current describes the direction in which positive charges move. Electrons have a negative electrical charge, so the movement of electrons is opposite to that of the conventional current flow. Consequently, the mnemonic cathode current departs also means that electrons flow into the device's cathode from the external circuit.

An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit. The word was coined by William Whewell at the request of the scientist Michael Faraday from two Greek words: elektron, meaning amber, and hodos, a way.

A triode is an electronic amplifying vacuum tube consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's 1906 Audion, a partial vacuum tube that added a grid electrode to the thermionic diode, the triode was the first practical electronic amplifier and the ancestor of other types of vacuum tubes such as the tetrode and pentode. Its invention founded the electronics age, making possible amplified radio technology and long-distance telephony. Triodes were widely used in consumer electronics devices such as radios and televisions until the 1970s, when transistors replaced them. Today, their main remaining use is in high-power RF amplifiers in radio transmitters and industrial RF heating devices. In recent years there has been a resurgence in demand for low power triodes due to renewed interest in tube-type audio systems by audiophiles who prefer the sound of tube-based electronics.

In electronics, a vacuum tube, an electron tube, or valve or, colloquially, a tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.

In chemistry and manufacturing, electrolysis is a technique that uses a direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell. The voltage that is needed for electrolysis to occur is called the decomposition potential.

Electroplating is a process that uses an electric current to reduce dissolved metal cations so that they form a thin coherent metal coating on an electrode. The term is also used for electrical oxidation of anions on to a solid substrate, as in the formation of silver chloride on silver wire to make silver/silver-chloride electrodes. Electroplating is primarily used to change the surface properties of an object, but may also be used to build up thickness on undersized parts or to form objects by electroforming.

The Hall–Héroult process is the major industrial process for smelting aluminium. It involves dissolving aluminium oxide (alumina) in molten cryolite, and electrolysing the molten salt bath, typically in a purpose-built cell. The Hall–Héroult process applied at industrial scale happens at 940–980°C and produces 99.5–99.8% pure aluminium. Recycled aluminum requires no electrolysis, thus it does not end up in this process.

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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 original equipment manufacturers (OEMs) and original design manufacturers (ODMs) that manufacture products for its end customers.

## References

1. Valov, Ilia; Waser, Rainer; Jameson, John; Kozicki, Michael (June 2011). "Electrochemical metallization memories-fundamentals, applications, prospects". Nanotechnology. 22 (25): 254003. doi:10.1088/0957-4484/22/25/254003.
2. Michael N. Kozicki, Chakravarthy Gopalan, Murali Balakrishnan, Mira Park, and Maria Mitkova (August 20, 2004). "Non-Volatile Memory Based on Solid Electrolytes" (PDF). Non-Volatile Memory Technology Symposium. IEEE. doi:10.1109/NVMT.2004.1380792 . Retrieved April 13, 2017.CS1 maint: uses authors parameter (link)
3. M.N. Kozicki, M. Balakrishnan, C. Gopalan, C. Ratnakumar and M. Mitkova (November 2005). "Programmable metallization cell memory based on Ag-Ge-S and Cu-Ge-S solid electrolytes". Non-Volatile Memory Technology Symposium. IEEE. doi:10.1109/NVMT.2005.1541405.CS1 maint: uses authors parameter (link)
4. Muralikrishnan Balakrishnan, Sarath Chandran Puthen Thermadam, Maria Mitkova and Michael N. Kozicki (November 2006). "A Low Power Non-Volatile Memory Element Based on Copper in Deposited Silicon Oxide". Non-Volatile Memory Technology Symposium. IEEE: 111–115. doi:10.1109/NVMT.2006.378887.CS1 maint: uses authors parameter (link)
5. Madrigal, Alexis (October 26, 2007). "Terabyte Thumb Drives Made Possible by Nanotech Memory". Wired . Archived from the original on May 11, 2008. Retrieved April 13, 2017.
6. Ion Sizes of Common Elements, compare with Co
7. B. Swaroop, W. C. West, G. Martinez, Michael N. Kozicki and L.A. Akers (May 1998). "Programmable current mode Hebbian learning neural network using programmable metallization cell". International Symposium on Circuits and Systems. IEEE. doi:10.1109/ISCAS.1998.703888.CS1 maint: uses authors parameter (link)
8. "Micron Technology Licenses Axon's Programmable Metallization Cell Technology". Press release. January 18, 2002.
9. "Adesto Technologies Wins DARPA Award to Develop Sub-Threshold Non-Volatile, Embedded CBRAM Memory". Press release. Adesto. November 29, 2010. Retrieved April 13, 2017.
10. "Adesto's CBRAM targets 70 billion dollar market". Nanalyze. July 30, 2013. Retrieved April 13, 2017.