June 30, 2011

IBM making more reliable phase change memory

Long-term memory: Each cell in this 200,000-cell phase-change memory chip can store multiple bits of data reliably over a period of several months. Credit: IBM

MIT Technology Review - IBM encoding phase-change memory for greater reliability

IBM researchers have developed a programming trick that makes it possible to more reliably store large amounts of data using a promising new technology called phase-change memory. The company hopes to start integrating this storage technology into commercial products, such as servers that process data for the cloud, in about five years.

Like flash memory, commonly found in cell phones, phase-change memory is nonvolatile. That means it doesn't require any power to store the data. And it can be accessed rapidly for fast boot-ups in computers and more efficient operation in general. Phase-change memory has a speed advantage over flash, and Micron and Samsung are about to bring out products that will compete with flash in some mobile applications.

To make multibit memory cells, the IBM group picked four different levels of electrical resistance. The trouble is that over time, the electrons in the phase-change cells tend to drift around, and the resistance changes, corrupting the data. The IBM group has shown that they can encode the data in such a way that when it's read out, they can correct for drift-based errors and get the right data.

The IBM group has shown that error-correcting code can be used to reliably read out data from a 200,000-cell phase-change memory array after a period of six months. "That's not gigabits, like flash, but it's impressive," says Eric Pop, professor of electrical engineering and computer sciences at the University of Illinois at Urbana-Champaign. "They're using a clever encoding scheme that seems to prolong the life and reliability of phase-change memory."

For commercial products, that reliability timescale needs to come up to 10 years, says Victor Zhirnov, director of special projects at the Semiconductor Research Corporation. IBM says it can get there.

The new nonvolatile memory, developed by Eric Pop, professor of electrical engineering and computer sciences at the University of Illinois at Urbana-Champaign, and colleagues, can hold more data than flash while requiring considerably less power (100 times less power).

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