Phase-change memory
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Phase-change memory (also known as PRAM and Ovonic Unified Memory) is a type of non-volatile computer memory. PRAM uses the unique behavior of chalcogenide glass. The internal organisation of chalcogenide glass can be structurally altered by the application of heat. This heat is generated by the controlled input of electricity. The chalcogenide can form a crystalline state or an amorphous state. The crystalline and amorphous states each have dramatically different electrical resistivity and this forms the basis by which data is stored. The amorphous, high resistance state is used to represent a binary 1, and the crystalline, low resistance state represents a 0.
Chalcogenide is the same material utilized in re-writable optical media (such as CD-RW and DVD-RW). In those instances, the material's optical properties are manipulated, rather than its electrical resistivity, as chalcogenide's refractive index also changes with the state of the material.
PRAM is both solid-state and XY-addressable, and is an attractive candidate for commercial use as non-volatile computer memory. Although PRAM has yet not reached the commercialization stage, nearly all prototype devices make use of a mixture of germanium, antimony and tellurium called GST. Under high temperature (over 600°C), the chalcogenide becomes liquid. Once cooled, 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 point, but below the melting point, it will transform into a crystalline state with a much lower resistance. This phase transition process can be completed in as quickly as 5 nanoseconds, according to a January 2006 Samsung Electronics patent application concerning the technology.
The properties of chalcogenide glasses were first explored as a potential memory technology by Stanford Ovshinsky of Energy Conversion Devices in the 1960s. In the September 1970 issue of Electronics Magazine, Gordon Moore - co-founder of Intel - published an article on the technology. However, material quality and power consumption issues prevented commercialization of the technology. More recently, interest and research have resumed as flash and DRAM memory technologies are expected to encounter scaling difficulties as chip lithography shrinks.
PRAM can be constructed in a number of different ways but there are two notable methods. In one method, diodes are used as selection elements instead of transistors. This cuts down on cost since a diode is smaller and cheaper than a transistor. Taking this one degree further, Macronix pioneered cross-point PRAM, which is composed simply of a self-aligned chalcogenide cell sandwiched between the address lines (that is, with no transistor or diode selection element). In this manner, the chalcogenide serves as the rectifying element so the low-resistance crystalline state is never used. Instead, the cell is manipulated between distinct amorphous states. This type of cell is very low cost since it only requires two masking steps.
In August of 2004, Nanochip licensed PRAM technology for use in MEMS (micro-electric-mechanical-systems) probe storage devices. These devices are not solid state. Instead, a very small platter coated in chalcogenide is dragged beneath many (thousands or even millions) of electrical probes which can read and write the chalcogenide. Hewlett-Packard's micro-mover technology can accurately position the platter to 3 nanometers so densities of more than 1 terabit per square inch will be possible if the technology can be perfected.
Timeline
September 1966 - Stanford Ovshinsky files first patent on phase change technology
September 1970 - Gordon Moore publishes research in Electronics Magazine
June 1999 - Ovonyx joint venture is formed to commercialize PRAM technology
November 1999 - Lockheed Martin works with Ovonyx on PRAM for space applications
February 2000 - Intel invests in Ovonyx, licenses technology
December 2000 - ST Microelectronics licenses PRAM technology from Ovonyx
March 2002 - Macronix files a patent application for transistor-less PRAM
July 2003 - Samsung begins work on PRAM technology
2003 through 2005 - PRAM-related patent applications filed by Toshiba, Hitachi, Macronix, Renesas, Elpida, Sony, Matsushita, Mitsubishi, Infineon and more
August 2004 - Nanochip licenses PRAM technology from Ovonyx for use in MEMS probe storage
August 2004 - Samsung announces successful 64Mbit PRAM array
February 2005 - Elpida licenses PRAM technology from Ovonyx
September 2005 - Samsung announces successful 256Mbit PRAM array, touts 400µA programming current
October 2005 - Intel increases investment in Ovonyx
December 2005 - Hitachi and Renesas announce 1.5 volt PRAM with 100µA programming current
December 2005 - Samsung licenses PRAM technology from Ovonyx