Phase-change memory
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Phase-change memory (also known as PCM, PRAM, Ovonic Unified Memory and Chalcogenide RAM [C-RAM]) is a type of non-volatile computer memory. PRAM uses the unique behavior of chalcogenide glass, which can be "switched" between two states, crystalline and amorphous, with the application of heat. PRAM is one of a number of new memory technologies that are attempting to compete in the non-volatile role with the almost universal Flash memory, which has a number of practical problems these replacements hope to address.
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[edit] Background
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.
The crystalline and amorphous states of chalcogenide glass have dramatically different electrical resistivity values, and this forms the basis by which data are stored. The amorphic, high resistance state is used to represent a binary 0, and the crystalline, low resistance state represents a 1. 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.
Although PRAM has not yet reached the commercialization stage for consumer electronic devices, nearly all prototype devices make use of a chalcogenide alloy of germanium, antimony and tellurium (GeSbTe) called GST. It is heated to a high temperature (over 600°C), at which point the chalcogenide becomes a liquid. Once cooled, it is frozen into an amorphic 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 five nanoseconds, according to a January 2006 Samsung Electronics patent application concerning the technology. This is comparable to conventional memory devices, for instance, modern DRAM cells have a switching time on the order of two ns.
It is the switching time that makes PRAM, and other flash memory replacements, most interesting. Flash memory works by trapping electrons within the gate of a MOS transistor, which has been constructed with a special gate "stack" designed to trap charges (either on a floating gate or in insulator "traps"). The presence of a charge within the gate shifts the transistor's threshold voltage, 
and represents a logic 1. In order to change the state of the cell, from 1 to 0 for instance, this charge has to be removed. This is accomplished by applying a relatively large voltage across the cell, which effectively "sucks" the electrons out of the insulating layer. This burst of voltage is provided by a charge pump which takes some time to build up power. General write times for common Flash devices are on the order of one ms (for a block of data), about 100000 times the typical 10 ns read time (for a byte). Thus PRAM can offer much higher performance in applications where writing quickly is important. Another difference, and potentially more interesting, is that each burst of voltage across the Flash cell degrades it slightly, so most Flash devices are only rated for something on the order of 10,000 to 100,000 writes. This makes them unsuitable for hard drive replacements in desktop computers. PRAM appears to have no such limit. Combined with their high speed, thousands of times quicker than conventional hard drives, this makes them particularly interesting in roles that are currently performance-limited by memory access speed.
Because they are not based on maintaining a floating charge in the cells, PRAM may be better for archival storage than Flash.
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 itself serves as the rectifying element so the low-resistance crystalline state is never used. Instead, the cell is manipulated between distinct amorphic states. This type of cell is very low cost since it only requires two masking steps.
[edit] 2000 and later
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. The basic idea is to reduce the amount of wiring needed on-chip; instead of wiring every cell, the cells are placed closer together and read by current passing through the MEMS probes, acting like wires.
In September 2006, Samsung announced a prototype 512 Mb device using diode switches[1]. The announcement was something of a surprise, and it was especially notable for its fairly high density. The prototype features a cell size of only 46.7 nm, which is even better than commercial Flash devices currently available. Although Flash devices of even higher capacities were already available (8 GB was just coming to market for instance) other Flash competitors are generally much lower density. The only production MRAM and FeRAM devices are only 4 Mb, for example. The high density of this prototype PRAM device suggests that it could be a real Flash competitor, and not limited to niche roles as these other devices have been. This is especially true in the case of NOR Flash, which allows per-bit addressing (the more common NAND flash can only be accessed in "banks" of many bytes at a time), which has generally lagged NAND densities and appears to be about the same density as this PRAM device.
Samsung's announcement was followed by one from Intel and STMicroelectronics, who demonstrated their own PCM devices at the 2006 Intel Developer Forum in October[2]. They showed a 128 Mb part that had very recently started fabbing at STMicroelectronics's line in Italy. Intel stated that the devices were strictly proof-of-concept, but they expect to start sampling within months, and have widespread commercial production within a few years. Intel is already the leading NOR Flash producer, and appears to be aiming their PCM at the same market as Samsung.
PCM is also a promising technology in the military and aerospace industries where radiation effects make the use of standard non-volatile memories such as Flash impractical. PCM memory devices have been introduced by BAE Systems, referred to as C-RAM, claiming excellent radiation tolerance (rad-hard) and latchup immunity. Additionally, BAE claims a write cycle endurance of 108, which will allow it to be a contender for replacing PROMs and EEPROMs in space systems.
[edit] Timeline
- September 1966: Stanford Ovshinsky files first patent on phase change technology
- June 1969: US Patent 3,448,302 licensed to Ovshinsky claims first reliable operation of phase change memory
- 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 1.5volt PRAM with 100µA programming current
- December 2005: Samsung licenses PRAM technology from Ovonyx
- July 2006: BAE Systems (formerly Lockheed Martin) introduces a Radiation Hardened C-RAM 512Kx8 chip
- September 2006: Samsung announces 512Mbit PRAM device
- October 2006: Intel and STMicroelectronics show a 128Mbit PRAM chip
- December 2006: IBM Research Labs demonstrate a prototype 3 by 20 nanometers[3]
- January 2007: Qimonda licenses PRAM technology from Ovonyx
[edit] References
- ^ SAMSUNG Introduces the Next Generation of Nonvolatile Memory - PRAM
- ^ Intel Previews Potential Replacement for Flash
- ^ Phase Change to Replace Flash?
