July12: Phase-change memory (also known as PCM, PRAM, PCRAM, Ovonic Unified Memory, Chalcogenide RAM and 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.
Phase Change Memory is a promising memory technology that has recently experienced a resurgence of interest. PCM employs a reversible phase change phenomenon to store information through a resistance change in different phases of a material. Advances in memory technology and pioneering work conducted by Numonyx have moved the technology to the forefront of the memory industry R&D activity. PCM offers high performance and low power consumption, combining the best attributes of NOR, NAND and RAM within a single chip. those attributes are: bit-alterable; non-volatile; fast read speed; fast write/erase speed; and good scalability.
Bit alterable: Similar to RAM or EEPROM, PCM is bit-alterable — meaning that stored information can be switched from one to zero, or zero to one, without a separate erase step. Flash memory technology requires a separate erase step in order to change information.
Non-volatile: PCM is non-volatile, as are NOR flash and NAND flash. PCM does not require a constant power supply to retain information, while RAM does.
Read performance: Similar to RAM and NOR flash memory, PCM features fast random access times. This enables the execution of code directly from the memory, without an intermediate copy to RAM. The read latency of PCM is comparable to single bit per cell NOR flash, while the read bandwidth can match DRAM.
Write/erase performance: PCM will achieve write throughput speeds faster than NAND and with lower latency. These features, when combined with a no separate erase step (bit-alterable), will deliver significant write performance improvement over NOR and NAND flash.
Scalability: Scaling is another area where PCM offers a difference. Both NOR and NAND rely on floating gate memory structures, which are difficult to shrink. As the memory cell shrinks on flash, the number of electrons stored on the floating gate shrinks. Because PCM does not store charge (electrons), it is immune to the charge storage scaling issue.
The greatest challenge for phase-change memory has been the requirement of high programming current density (>107 A/cm², compared to 105-106 A/cm² for a typical transistor or diode) in the active volume. This has led to active areas which are much smaller than the driving transistor area. The discrepancy has forced phase-change memory structures to package the heater and sometimes the phase-change material itself in sublithographic dimensions. This is a process cost disadvantage compared to Flash.
The contact between the hot phase-change region and the adjacent dielectric is another fundamental concern. The dielectric may begin to leak current at higher temperature, or may lose adhesion when expanding at a different rate from the phase-change material.
Phase-change memory is susceptible to a fundamental tradeoff of unintended vs. intended phase-change. This stems primarily from the fact that phase-change is a thermally driven process rather than an electronic process. Thermal conditions which allow for fast crystallization should not be too similar to standby conditions, e.g. room temperature. Otherwise data retention cannot be sustained. With the proper activation energy for crystallization it is possible to have fast crystallization at programming conditions while having very slow crystallization at normal conditions.
