The previous blog entry discussed work on PCM (phase-change memory) taking place in an attempt to dethrone NAND Flash memory as the king of nonvolatile semiconductor memory. If PCM technology were a slam dunk, then NAND Flash would never have been born because PCM was invented more than ten years before Flash. However, technologies do not advance at equal paces. Thomas Edison developed a practical incandescent light bulb in 1879 and it was in mass production within a very few years. Nikola Tesla experimented with fluorescent light bulbs during the 1890s but GE put them into mass production only in 1939 and incandescent bulbs, with their original Edison screw-in bases, are only now being phased out. It can take decades for a new technology to become production-ready.

So there must have been barriers to PCM becoming a commercial reality. The first such barrier is write current. PCM cells write bits by melting glass at 600° C. It doesn’t take much imagination to understand that there’s some appreciable amount of power required to do this, particularly at 1970 lithographic sizes. Today, 90nm and 45nm PCM cells require much less write current than 40 years ago, but the amount of current is still not negligible.

Next, there are mechanical issues associated with repeatedly melting a material inside of an integrated circuit. Eventually, voids can form in the melt zone resulting in cell destruction. Fortunately, the failure related to this mechanism always occurs at write time, so the cells can be read after a write to verify that a failure has not occurred.

There are also issues associated with operating temperature. High-temperature PCM operation tends to anneal PCM bits set to the amorphous state. Numonyx says that the retention time for its PCM cells is on the order of 10 years at 85° C. However, it’s 10 hours at 125° C, 10 seconds at 165° C, and 10 microseconds at 225° C. This problem isn’t insurmountable, but it must be understood and addressed by system designers.

Note that all memory technologies have similar problems. NAND Flash memory has well-understood wearout mechanisms. Because they’re well understood, system designers working with NAND Flash memory have little trouble incorporating them into their designs. Novice designers—well that’s a different story.