Recently results from two academic researchers in Japan will be significant to the NAND Flash market: http://www.electronicsweekly.com/Articles/Article.aspx?liArticleID=44028&PrinterFriendly=true. Clearly the trip from laboratory to volume production is often longer than the early estimates but these results look important.
Back in 2006, Jim Gray argued in Tape is Dead, Disk is Tape, Flash is Disk, & Ram Locality is King that we need a new layer in the storage hierarchy between memory and disk and NAND Flash was an excellent candidate. Early NAND Flash-based SSDs could sustain read rates well beyond 10x of disk random IO rates but the write rates were terrible. Some were as bad as 1/5 the rate of magnetic disk. Second generation devices are solving the random write problem as expected. Costs continue to plunge, overall performance continue to improve, and many very high scale server workloads have been deploying flash devices over the past year. A success by most measures but two issues remain. The first issue is that we have one important metric heading in the wrong direction: endurance. NAND flash can only support a limited number of erase cycles before failing. The second issue is that many don’t expect the feature size to be reduced below 32 nm which, were that to happen, would slow the improvement rate dramatically.
When I first got interested in single level cell (SLC) NAND Flash most published endurance numbers were typically in the 10^6 cycle range. Most current devices are in the 10^5 range and many see as low as 10^4 cycles on the horizon. A million cycles is fine and will not restrict the life of the device. 100,000 cycles is closer to the line but my back of envelope numbers suggest 100k will (barely) be acceptable. 10k cycles is a problem and will restrict longevity of the device.
In this research work Shigeki Sakai and Ken Takeuchi show how Feroelectric gate Field Effect Transistors can dramatically improve the durability, reduce required programming voltage, improve performance, and support further generational reductions in feature size. The device prototype they demonstrated uses 6v to program rather than 20v which may reduce the cost or increase the speed of devices slightly. What’s most important in the demonstrated results is estimated endurance in the 10^8 cycle range which is at least three orders of magnitude better than most current generation NAND parts. That would take endurance completely off the NAND Flash worry list.
Potential feature size reduction is the other improvement of interest in this result. Feature size reduction is the engine of Moore’s law and drives the semi-conductor economics we’ve all become used to. Many experts don’t expect to be able to reduce NAND flash features size below roughly 30nm. The Fe-NAND result shows potential for two more generational feature size reductions down to the 10nm range. This is important in that it drives costs reductions and we all want them to continue.
Fe-NAND looks extremely interesting and, if the research can be confirmed and is manufacturability, we have a very significant technology that can address the two major concerns with current generation NAND flash: 1) rapidly falling endurance, and 2) expected inability to drop down below 32nm feature size. Flash continues to build industry momentum.
Thanks to Jack Creasey for sending this my way.
James Hamilton, Data Center Futures
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