Wednesday, July 06, 2011

#CHIPS: "Smarter Phase Change Memory Beats Flash"

Using a variety of solid-state memory technologies, IBM Research recently demonstrated a flash substitute that is 100 times faster and will last 3,000 times longer than the flash memory, which has replaced hard disks in everything from smartphones and touch-screen tablets to high-end enterprise servers and cloud-based data centers. Solid-state flash memory has largely replaced mechanical hard disks in mobile devices and even in high-performance server farms such as those at, but now IBM has demonstrated a new, faster alternative to flash that lasts longer and can store four times as much information per bit-cell.

Flash memory has almost driven the hard drive makers out of business, with only two major manufacturers left, Seagate and Western Digital. However, hard drives have been hanging on because of the longevity of their spinning metal disks, which experience no wear since their read-write heads never touch the disk's surface. Flash memory, on the other hand, is faster to access, more compact and much lower power. Unfortunately, flash memory bit-cells wear out in as few as 3,000 cycles, making them suitable only for relatively short-lived mobile consumer devices (although server farms use them by monitoring their wear and implementing complex algorithms to map around bad cells until a point of diminishing returns when they have to be replaced.)

IBM's phase change memory test chip was fabricated in its 90-nanometer complementary metal oxide semiconductor (CMOS) process.

Now IBM scientists have demonstrated a solid-state alternative to flash memory that solves most of its problems, plus offers a clear migration path to higher performance, higher density and greatly expanded longevity. The key to this breakthrough memory technology--called phase-change memory--is its ability to store up to four-bits per memory cell. Flash memories have likewise, of late, started storing multiple bits per cell, albeit without solving its short lifetime problem.

"As organizations and consumers increasingly embrace cloud-computing models and services, whereby most of the data is stored and processed in the cloud, ever more powerful and efficient, yet affordable storage technologies are needed," states Dr. Haris Pozidis, Manager of Memory and Probe Technologies at IBM Research – Zurich. "By demonstrating a multibit phase-change memory technology which achieves for the first time reliability levels akin to those required for enterprise applications, we made a big step towards enabling practical memory devices based on multi-bit PCM."

IBM's innovation involves several architectural breakthroughs which together solve all the outstanding problems with flash while boosting its performance by 100-times. Phase change memories work in a manner similar to rewritable optical media, that is by heating and cooling a polymer so that it can take on two states--either crystalline (transparent) or amorphous (opaque). The difference with solid-state phase change memories is that these changes are performed electrically rather than with a laser as with optical disks.

By enlisting not only the phase change from crystalline to amorphous, but also the analog change in resistance of the electrodes between which the memory cells are sandwiched, IBM was able to encode four-bits per cell.

"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,” explains Pozidis.

Through this iterative process, the IBM scientists were able to demonstrate the longevity of their technique while simultaneously showing that even in the worst case access time will be no worse than 10 microseconds, which is still 100-times faster than flash.
Further Reading