Ovonic Unified Memory (OUM) is a type of non-volatile Random Access Memory.
This NRAM is based on a technology that exploits physico-chemical properties of chalcogenide alloy, created in thin layers to store information, with potentially high density and low latency.
The film used in OUM is a chemical compound similar to the material used in CD-RW and DVD-RAM disks. The roots of this technology date back to 1960s and 1970s, when it was first developed by Energy Conversion Devices.
The principle of operation relies on the ability of chalcogenide alloy to change its resistance in different states. Under high temperature (over 600 °C), chalcogenide becomes liquid and by subsequent rapid cooling it is frozen into an amorphous glass-like state and its electrical resistance is high. By heating the chalcogenide to a temperature above its crystallization, but below the melting temperature it will transform into a crystalline state with a much lower resistance. The process of switching its phase can be completed in under 10 ns. A typical OUM memory cell is basically a strip of film connected to a transistor.
Currently, there are commercial prototypes being developed.
Samsung Electronics Co. Ltd. has presented its 64 Mbit phase-transition memory chip in 2004. The chip was fabricated in a 0.18 micrometre process, from a chalcogenide phase-change material with a cell size of 0.504 mm².
Although the silicon giant Intel has been developing OUM memory units for years now, no commercial products have been released as of this date.