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Enterprise-class phase-change memory on the way, but roadblocks remain

Phase-change memory (PCM) is an emerging class of NVRAM with promising characteristics. Enterprise-class chips are expected within the next few years.

Currently, there are no enterprise-grade storage products available that make use of phase-change memory -- a type of non-volatile random access memory. However, PCM chips are in development for data storage and there are currently chips in production for mobile phones.

Phase-change memory (PCM) chips use a glass-like material called chalcogenide as a storage medium. Chalcogenide is made up of germanium, antimony and tellurium, and is also used in CD-RW and DVD-RWs. While those media rely on the material's optical properties, PCM makes use of the material's electrical resistivity. Each cell in the chip consists of chalcogenide material between two electrodes. Electrical currents are used to heat the material to switch it between two states (or phases) -- crystalline and amorphous. Cells in an amorphous or liquid state represent a "0", and cells in a crystalline or solid state represent a "1." Systems are able to read each bit by recognizing the lower electrical resistance of cells in a crystalline state.

Engineers from the University of Cambridge recently found a way to increase PCM write speed, which previously was slower than that of NAND devices. Their method uses a weak electrical current to "prime" the memory, which allows for much faster crystallization of the phase-change material. This technique allows phase-change memory to switch between states as quickly as 5 nanoseconds, according to a report published in Science magazine. "NAND is very fast if nothing has been written to it, but when you have to erase a block in order to write to it again, that's when it slows down," said Marc Staimer, CDS with Beaverton, Ore.-based Dragon Slayer Consulting. PCM chips do not require blocks to be completely erased before writing to them.

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Q&A: IBM's Haris Pozidis on PCM

Phase-change memory will also offer higher density than NAND flash. "You'll get higher scalability," Staimer said. "Flash is starting to run into die shrink limitation. There are some technical challenges ahead for flash." As NAND cells get smaller, the number of electrons held in the cell decreases. The result is a drop in write throughput and endurance. NAND cells currently measure 20 nanometers across. PCM cells have been demonstrated to work at 10 nanometers.

Another potential benefit of phase-change memory is related to lifespan. Flash storage devices, of course, degrade over time -- a typical SLC SSD is rated to sustain 100,000 writes and MLC SSD can sustain 10,000 writes. PCM degrades as well, but at a much slower rate. According to Staimer, PCM will likely have the ability to sustain up to 10 million writes. However, that remains to be seen.

In June, IBM demonstrated that PCM can reliably store multiple data bits per cell over extended periods of time. The company claims that this development will lead to lower-costs, faster and more durable PCM chips for mobile phones, as well as high-performance applications like enterprise data storage. To achieve this result, IBM used modulation coding techniques to prevent the voltage drift in multi-bit PCM, which previously led to read errors.

In a recent Q&A with, Haris Pozidis, a phase-change memory researcher at IBM's Zurich laboratory, said PCM "will bridge the latency and endurance gap between fast/volatile/expensive main memory -- mostly DRAM today -- and slow/non-volatile/cheap storage -- in the form of NAND flash today."

Still, enterprise-grade PCM products won't be available for a bit. "I wouldn't expect products until 2014 or 2015," Staimer said. "IBM, Micron, Samsung and SK Hynix have all proven that they can make it work, but they are all working through production issues."

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