Programmable metallization cell Totally Explained

Everything about The Programmable Metallization Cell totally explained

The programmable metallization cell, or PMC, is a new form of non-volatile computer memory being developed at Arizona State University and its spinoff, Axon Technologies. PMC is one of a number of technologies that are being 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.

Description

PMC is based on the physical re-location of metallic ions within a glassy solid electrolyte. A PMC memory cell is made of two solid metal electrodes, one tungsten the other silver or (more recently) copper, with a thin film of the electrolyte between them, along with a control transistor. Additional metal ions are deposited within the electrolyte.
   When a negative bias is applied to the tungsten electrode, a current forms within the electrolyte in a thin filament running to the opposite electrode. Ions in the electrolyte, as well as some from the now-positive electrode, are attracted to the current flow and migrate towards the filament. After a short period of time the ions flowing into the filament form a small metallic "nanowire" between the two electrodes. This 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. Such nanocrystal structures may store and release charge, like nanocapacitors, causing the PMC to behave similarly to low-retention flash memory.
   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's read as a "1". If there's 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. The copper ions will migrate away from the current, back into the electrolyte, and eventually to the negatively-charged copper electrode. This breaks the nanowire and increases the resistance again.
   PMC isn't the only application of this basic concept, which relates to "nanoionics". Other prospective applications include dynamically-reroutable electronics, optical switches, and microfluidic valves.
   The Arizona State University was the one to perform the study on the PMC, developed by the college's Center for Applied Nanoionics. The new technology will presumably be used in commercial products. Micron Technology, Samsung, Sony and IBM had already showed their 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 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 transistors have three connections, the emitter, collector and base. The base is the essential component of the transistor, controlling the resistance between the emitter and collector, and thereby acting as a switch. In the floating gate transistor, the base 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's Flash's primary drawback, and for a number of reasons. For one, each application of the current physically degrades the cell, and that'll eventually not be able to be written to. Write cycles on the order of 10⁵ to 10⁶ are typical, limiting its application to roles where constant writing isn't 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 processes so writing is much slower than reading, and requires much more power as well. Flash is thus an "asymmetrical" system, much more so than conventional RAM or hard drives.
   PMC, on the other hand, writes with relatively low power and high speeds. 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 PCM much more universally applicable than Flash.
   Another problem with Flash is that the floating gate suffers leakage that slowly releases the charge. This is countered through the use of powerful insulators surrounding it, but these require a certain physical size in order to be useful. It also requires a specific physical layout, which is different than 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, and 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, and startup semiconductor fabrication plants using 30 nm processes are currently (late 2007) being brought online.
   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. It is also much simpler in layout than Flash, which should lead to simpler construction and lower costs. 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, who started work with PMC in 2002. Infineon followed in 2004, and a number of smaller companies have since joined as well.

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