For the first time, scientists at IBM Research have demonstrated reliably storing three bits of data per cell, instead of one, in a technology known as "phase-change memory." The breakthrough lowers the cost of the memory, and has a wide array of uses across mobile and high-performance computing.
PCM materials exhibit two stable states: amorphous without a clearly-defined structure; and crystalline phases, of low and high electrical conductivity, respectively. To store individual bits on a PCM cell, a high or medium electrical current is applied to the material. One bit can be programmed to be written in the amorphous phase, with the opposing bit crystalline phase. Then to read the bit back, a low voltage is applied. This is also how re-writable Blu-Ray discs read and write data.
Using IBM's new method, the new cell-state measurement and data decoding from the cell measure a physical property of the PCM cell that remains stable over time, and are insensitive to drift and random data changes. To provide additional stability of the stored data in a cell over ambient temperature fluctuations, a coding and detection scheme is employed. This scheme modifies the thresholds that are used to detect the cell's stored data so that they follow variations that occur in the cell due to temperature change. As a result, the cell state can be read reliably over long time periods after the memory is programmed.
IBM scientists envision standalone Phase Change Memory, as well as hybrid applications, which combine the technology with flash storage, using the PCM as an extremely fast cache. IBM postulates that a mobile phone's operating system could be stored in PCM, enabling the phone to launch in a few seconds. In the enterprise space, entire databases could be stored in PCM for fast query processing for time-critical applications, such as financial transactions.
"Phase change memory is the first instantiation of a universal memory with properties of both DRAM and flash, thus answering one of the grand challenges of our industry," said Dr. Haris Pozidis, manager of non-volatile memory research
at IBM Research. "Reaching three bits per cell is a significant milestone, because at this density the cost of PCM will be significantly less than DRAM, and closer to flash."
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