An article has been published in the December 2012 edition of IEEE Spectrum that highlights an interesting and potentially useful discovery by ROM manufacturer Macronix. Researchers there have discovered that applying heat to NAND flash cells can drastically extend their life, thus overcoming one of the biggest problems with the solid state storage technology.
NAND flash is used everywhere, from smartphones to SSDs to thumb drives, and we've written extensively before on how it works. The technology's biggest failing is that NAND flash only lives so long. Every time the flash cells are erased, they retain some residual charge; eventually, they get to where it takes so long to make them change their charge level that they stop being useful as a storage medium.
As NAND flash grows denser, it gets more delicate; in our discussion on the future of flash, we discuss the decreasing lifetimes of NAND flash and the potential alternatives. SSDs rely on complex mathematical gymnastics at the controller level to reduce writes and hence lengthen the life of their flash cells, but the need for those kinds of workarounds could be substantially lessened by the Macronix discovery.
It's long been known that annealing NAND flash—that is, subjecting it to high heat—can force the long-trapped electrons out of the NAND floating gate, reducing its retained charge and returning it to usefulness. But it's been thought all along that such annealing was too energy-intensive and too difficult to do precisely—essentially, an entire NAND chip had to be baked for hours.
However, using techniques borrowed from phase-changing RAM, where heat is applied to a material to change its state from conductive to insulating, the Macronix boffins constructed a redesigned NAND flash package with its existing electrical pathways modified to carry heat to the floating gate, the portion of the NAND transistor that is filled and drained to denote a 0 or a 1.
The modification is a complex one and required substantial engineering, but the results are impressive—a brief and restricted jolt at 800C appears to "heal" the flash cell, removing its retained charge. Macronix estimates that this can be done repeatedly as needed, leading to a flash cell that could potentially last for 100,000,000 cycles, instead of the roughly 1,000 cycles that current 21nm TLC flash cells are rated to last.
Since flash cell life cycle decreases as process size shrinks, this method of heating cells back to life is good news for the future of SSDs. Moore's law charges on; the International Technology Roadmap for Semiconductors projects an eventual arrival at 8nm features, and the useful life of NAND flash at that size is very, very short. If Macronix's method can be commercialized it will have profound implications on the future of the medium.
So far, there is no word on when the discovery will actually be transformed into a usable product, and there's also no word on what impact it might have to the packaging and design of the rest of the device in which it's used. The IEEE article notes that the 800C hot spot is "restricted to the area near the [floating] gate," and that the heating cycle doesn't have to be run very often, so overall phone or laptop battery life won't be affected by the technology's addition. We'll have to wait and see what the tech actually looks like and if it comes with added heat sinks or other SSD changes.
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