Scientists at Philips have developed a material for non-volatile memory chips that's not only better than Flash in almost every respect, but keeps on getting better the smaller you shrink the memory cells.
Philips' material operates on the same principle as rewriteable DVDs - its physical state can be changed back and forth between two forms, known as 'phases', by pumping it with energy. Each phase can be used to represent the 1s and 0s of binary data.
Where DVD RWs simply use the different phases to reflect laser light on and off from a detector, Philips' memory cell contains a thin film of the 'phase-change' material surrounded by silicon-dioxide and changes it from one phase to another again using pulses of electric current. The two phases have different electrical resistance, which represents the binary information.
Philips admitted that the technique isn't new, but it reckons the material it uses - a doped Antimony Tellurium (SbTe) compound - and the structure of the cell are novel, and, unlike other attempts at phase-change memory, well-suited to the standard CMOS process used to make most computer chips.
The SbTe phase-change material flips between amorphous and crystalline states at a low field strength of around 14V per micron, Philips said. The upshot is that the voltage required to drive a current through the material in its high-resistance phase in order to get it hot enough to change to its low-resistance phase is lower than that required by other phase-change materials investigated for memory use to date. The higher voltages required in the past were too large for the kind of products made using CMOS processes.
That problem is compounded when chip makers move to smaller manufacturing processes, such as the shift from 130nm to 90nm. But not with Philips' material, the researchers claimed. Smaller strips of the SbTe material require a smaller phase-transition voltage, keeping it within the voltage limits of the fabrication process. Making the cell requires the addition of just two extra lithography stages, Philips said, so it should be relatively inexpensive to manufacture.
The material's phase change is fast, taking place in under 30ns, in the prototype cell. That, the company said, is 100 to 200 times faster than today's Flash memory cells, and getting awfully close to DRAM speeds.
Philips isn't the only company exploring phase-change memory. Last month, memory maker Elpida licensed phase-change technology developer Ovonyx' material and cell structure, which operate along broadly the same lines as Philips'. Intel and STMicro have also invested in Ovonyx.
Philips' work will be detailed in the next (April 2005) issue of Nature Materials. ®
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