CBRAM- conductive-bridging RAM

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. NEC have a variant called “Nanobridge” and Sony calls their version “electrolytic memory”.

Description:

PMC is based on the physical re-location of ions within a solid electrolyte. A PMC memory cell is made of two solid metal electrodes, one relatively inert (e.g., tungsten) the other electrochemically active (e.g., silver or copper), with a thin film of the electrolyte between them. A control transistor 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" between the two electrodes. The "nanowire" 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". 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), Micron Technology, and Adesto Technologies, and several other large semiconductor companies and OEMs have also shown interest in the new technology.