VLSI 2007 – Phase Change Memory Technology

This is my second review of papers to be presented at the VLSI Technology Symposium. Today, I will take a look at the most promising papers on phase change memory technology (PCM or PRAM). Today at its spring development forum in Beijing, Intel announced that it will produce a 128M 90nm PRAM later this year. This is bound to create some extra buzz around the many good PRAM papers to look forward to at VLSI.

Samsung dominates the PRAM submissions as you might expect from some of their announcements in the past year. They will present a phase change cell based on GeSbTe for sub-50nm generations that appears to keep reset current below 260µA.  PRAM designs typically suffer from high currents required to heat and change the phase of the active layers. In another paper, Samsung memory engineers plan to present a novel heat dissipating cell scheme to improve the reset characteristics across a large (512M) array.

Naturally, Intel does have a paper in the NVM category. Oddly, though, it is a traditional floating gate NOR. Intel’s flash team will present “A Scalable Self-Aligned Contact NOR Flash Technology.” Sounds a bit bland, but it promises technology for 40nm and beyond. At 65nm, the cell area is only 0.036 square microns which is a cell area factor improvement from 10.6 on their 65nm production technology down to 8.5. This team’s ability to push the floating gate NOR cell is somewhat at odds to the PRAM announcement that was billed – in the EETimes article anyway - as the replacement for the floating gate.

In a way, it makes sense for Intel to get PRAM into the market first. But you need to look back a few years before STMicroelectronics began to make big strides and invest a lot of energy in PRAM development. Intel was the first major player to invest in phase change memory by funding a spin-off from the creators of ovonics. Energy Conversion Devices was the corporate entity that grew out of the pioneering work of Stanford R. Ovshinsky in 1960. A separate company – Ovonyx – was formed in 1999 to commercialize the “Ovonic Unified Memory” or OUM. Today, OUM has a sexier name. PRAM is “perfect RAM” in the Samsung vernacular.

That’s enough of a history lesson though. It does sound as though we will have a chance for the Intel PRAM and NOR flash to go “head-to-head” soon albeit with PRAM making its mark using older generation litho tools.

Despite the talk of Intel mass production with Samsung and STMicro doubtless charging hard, an IBM paper may have the most interesting new technology to present. Details are sketchy to non-existent at this point regarding, “Novel Lithography-Independent Pore Phase change Memory.” It certainly sounds like the first NVM session will be worth attending to hear about this work along with Samsung’s.

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