Current technology is single-level cell (SLC) PCM, which only stores one bit per cell with limited capacity. For example, Samsung produces a 512Mb PCM chip for its GT-E2550 GSM mobile phone. Micron's Numonyx division makes a 128Mb PCM chip and is shipping product to several customers who use it in networking equipment, medical monitoring devices, and security cameras. .
PCM uses electrical charges to change areas on a glassy material from crystalline to random or amorphous. The technique uses far less power than NAND flash to store data and it has data write rates up to 100 times faster because it does not first require existing data to be marked for deletion.
PCM leverages the resistance change that occurs in the material -- an alloy of various elements -- when it changes its phase from crystalline - featuring low resistance - to amorphous - featuring high resistance - to store data bits. In a PCM cell, where a phase-change material is deposited between a top and a bottom electrode, phase change can controllably be induced by applying voltage or current pulses of different strengths. These heat up the material, and when distinct temperature thresholds are reached cause the material to change from crystalline to amorphous or vice versa.
IBM scientists said they were able to address the bit error problem associated with MLC PCM memory by using an advanced modulation coding technique, which addresses the problem of short-term drift. Short-term drift is analogous to a problem in NAND flash memory where electrons leak through the thin walls of cells and create data read errors.
In NAND flash, the problem is addressed through the use of error correction code (ECC) in controller chips. But in PCM, data errors are not corrected, but avoided through the use of specialized code.
"With modulation codes you try to avoid the most probable errors. Modulation codes appear today in hard disk drives as well as optical drives such as Blu-ray discs," Pozidis said. "We apply a voltage pulse based on the deviation from the desired level and then measure the resistance. If the desired level of resistance is not achieved, we apply another voltage pulse and measure again - until we achieve the exact level."
IBM scientists achieved a worst-case write latency of about 10 microseconds, which represents a 100x performance increase over even the most advanced flash memory on the market today.
Pozidis said IBM is currently using PCM circuitry that is 90 nanometers in size, or about twice the width of today's densest SLC PCM products. But that too will shrink over time.
IBM is not planning to produce consumer grade products out of PCM, Pozidis said. The main target for the technology is to license it to memory manufacturers, such as Toshiba and Samsung, and help them accelerate the production of the memory chips for enterprise applications.
Other researchers have been combining carbon nanotube technology with PCM to create chips that sip electricity and could extend the battery life of mobile devices to weeks.
Lucas Mearian covers storage, disaster recovery and business continuity, financial services infrastructure and health care IT for Computerworld. Follow Lucas on Twitter at @lucasmearian, or subscribe to Lucas's RSS feed. His email address is firstname.lastname@example.org.
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