Programmable metallization cell

Programmable metallization cell

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The programmable metallization cell, or PMC, is a new form of non-volatile
Non-volatile memory
Non-volatile memory, nonvolatile memory, NVM or non-volatile storage, in the most basic sense, is computer memory that can retain the stored information even when not powered. Examples of non-volatile memory include read-only memory, flash memory, ferroelectric RAM, most types of magnetic computer...

 computer memory
Computer memory
In computing, memory refers to the physical devices used to store programs or data on a temporary or permanent basis for use in a computer or other digital electronic device. The term primary memory is used for the information in physical systems which are fast In computing, memory refers to the...

 being developed at Arizona State University
Arizona State University
Arizona State University is a public research university located in the Phoenix Metropolitan Area of the State of Arizona...

 and its spinoff, Axon Technologies.
PMC is one of a number of technologies that are being developed to replace the widely used flash memory
Flash memory
Flash memory is a non-volatile computer storage chip that can be electrically erased and reprogrammed. It was developed from EEPROM and must be erased in fairly large blocks before these can be rewritten with new data...

, providing a combination of longer lifetimes, lower power, and better memory density. Infineon Technologies
Infineon Technologies
Infineon Technologies AG is a German semiconductor manufacturer and was founded on April 1, 1999, when the semiconductor operations of the parent company Siemens AG were spun off to form a separate legal entity. , Infineon has 25,149 employees worldwide...

, who licensed the technology in 2004, refers to it as conductive-bridging RAM, or CBRAM. NEC has a variant called “Nanobridge” and Sony calls their version “electrolytic memory”.

Description


PMC is based on the physical re-location of ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s within a solid electrolyte. A PMC memory cell
Memory cell
Memory cell may refer to:* Memory cell, a building block of computer data storage*Memory B cell, an antibody producing cell*Memory T cell, an infection fighting cell...

 is made of two solid metal electrodes, one relatively inert (e.g., tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...

) the other electrochemically active (e.g., silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

 or copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

), with a thin film
Thin film
A thin film is a layer of material ranging from fractions of a nanometer to several micrometers in thickness. Electronic semiconductor devices and optical coatings are the main applications benefiting from thin film construction....

 of the electrolyte between them. A control transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...

 can also be included in each cell.

When a negative bias is applied to the inert electrode, metal ions in the electrolyte, as well as some originating from the now-positive active electrode, flow in the electrolyte and are reduced (converted to atoms) by electrons from the inert electrode. After a short period of time the ions flowing into the filament form a small metallic "nanowire
Nanowire
A nanowire is a nanostructure, with the diameter of the order of a nanometer . Alternatively, nanowires can be defined as structures that have a thickness or diameter constrained to tens of nanometers or less and an unconstrained length. At these scales, quantum mechanical effects are important —...

" between the two electrodes. The "nanowire
Nanowire
A nanowire is a nanostructure, with the diameter of the order of a nanometer . Alternatively, nanowires can be defined as structures that have a thickness or diameter constrained to tens of nanometers or less and an unconstrained length. At these scales, quantum mechanical effects are important —...

" dramatically reduces the resistance along that path, which can be measured to indicate that the "writing" process is complete.

Actually the nanowire may not be continuous but a chain of electrodeposit islands or nanocrystals. This is likely to prevail at low programming currents (less than 1 microampere) whereas higher programming current will lead to a mostly metallic conductor.

Reading the cell simply requires the control transistor to be switched on, and a small voltage applied across the cell. If the nanowire is in place in that cell, the resistance will be low, leading to higher current, and that is read as a "1". If there is no nanowire in the cell, the resistance is higher, leading to low current, and is read as a "0".

Erasing the cell is identical to writing, but uses a positive bias on the inert electrode. The metal ions will migrate away from the filament, back into the electrolyte, and eventually to the negatively-charged active electrode. This breaks the nanowire and increases the resistance again.

PMC is not the only application of this basic concept, which relates to "nanoionics
Nanoionics
Nanoionics is the study and application of phenomena, properties, effects and mechanisms of processes connected with fast ion transport in all-solid-state nanoscale systems. The topics of interest include fundamental properties of oxide ceramics at nanometer length scales, and fast ion conductor...

". Other prospective applications include dynamically-reroutable electronics, optical switches, and microfluidic valves.

Arizona State University was among the first to perform studies of PMC, developed by the university's Center for Applied Nanoionics. The new technology will presumably be used in commercial products. PMC technology has been licensed to Infineon (Qimonda
Qimonda
Qimonda AG, was a memory company split out of Infineon Technologies on 1 May 2006, to form at the time the second largest DRAM company worldwide, according to the industry research firm Gartner Dataquest...

), Micron Technology
Micron Technology
Micron Technology, Inc. is an American multinational corporation based in Boise, Idaho, USA, best known for producing many forms of semiconductor devices. This includes DRAM, SDRAM, flash memory, SSD and CMOS image sensing chips. Consumers may be more familiar with its consumer brand Crucial...

, and Adesto Technologies, and several other large semiconductor companies and OEMs have also shown interest in the new technology .

CBRAM vs. RRAM


CBRAM differs from RRAM in that for CBRAM metal ions dissolve readily in the material between the two electrodes, while for RRAM, 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 RRAM, a one-time "forming" step is required to generate the local damage.

Comparison


The primary form of solid-state
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...

 non-volatile memory in use today is flash memory, which is finding use in most roles that used to be filled by hard drives. Flash, however, has a number of problems that have 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
Charge pump
A charge pump is a kind of DC to DC converter that uses capacitors as energy storage elements to create either a higher or lower voltage power source. Charge pump circuits are capable of high efficiencies, sometimes as high as 90–95% while being electrically simple circuits.Charge pumps use some...

. The pump requires a fairly lengthy charging processes 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
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 up the components of the integrated circuit.When...

, which is different from the more typical CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...

 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 has led to several predictions of flash's ultimate demise. However, massive market investment has driven development of flash at rates in excess of Moore's Law
Moore's Law
Moore's law describes a long-term trend in the history of computing hardware: the number of transistors that can be placed inexpensively on an integrated circuit doubles approximately every two years....

, and semiconductor fabrication plants using 30 nm processes are currently (late 2007) being brought online.

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 for different roles. Additionally, the writing process is "almost infinitely reversible" , making PMC much more universally applicable than flash.

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 is also much simpler in layout than flash, which should lead to simpler construction and lower costs . Whether or not these advantages can be brought to market remains to be seen; the wide variety of other "flash killers" have so far always been behind the technology curve of flash's massive investment. However, as the CEO of one licensee claimed, "No other technology can deliver the orders-of-magnitude improvement in power, performance and cost that this memory can."

Current status


Early experimental PMC systems were based on silver-doped germanium selenide glasses, but these materials were not able to withstand the temperatures used in standard CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...

 fabs. Work then turned to silver-doped germanium sulfide electrolytes and then finally to the current copper-doped germanium sulfide electrolytes.

Axon Technologies has been licensing the basic concept since its formation in 2001. The first licensee was Micron Technology
Micron Technology
Micron Technology, Inc. is an American multinational corporation based in Boise, Idaho, USA, best known for producing many forms of semiconductor devices. This includes DRAM, SDRAM, flash memory, SSD and CMOS image sensing chips. Consumers may be more familiar with its consumer brand Crucial...

, who started work with PMC in 2002. Infineon followed in 2004, and a number of smaller companies have since joined as well.

External links