Numonyx could add refresh to PCM to beat the heat

Such a move might seem counter-intuitive as it would move the memory technology closer to that of DRAM but could be worthwhile, according to Paolo Cappelletti, vice president of technology development at Numonyx.

For nonvolatile memory the typical germanium, antimony, tellurium element ratio is 2:2:5, but this has the disadvantage of a relatively low melting point. "There is no issue that we can guarantee 10 years endurance at 85 degrees C," said Cappelletti, before acknowledging that the conventional phase-change memory has problems with bits resetting when a memory is held at higher temperatures for extended periods. As the temperature goes above 85 degrees C the data retention drops off significantly. Retention is approximately 10 hours at 125 degrees C, 10 seconds at 165 degrees C and 10 microseconds at 225 degrees C.

"When you get to higher temperature reliability in the parts per billion region, there is no [memory] technology that can intrinsically offer the reliability," said Cappelletti. "And in active mode, there is a lot you can do."

When challenged that adding a refresh cycle to phase-change memory was a retrograde step, Cappelletti countered that it would only be necessary to refresh the memory occasionally and that when the system was off and at low temperature the memory would have its nonvolatile quality. The high temperatures were likely to be experienced when the system was powered up and when a refresh cycle would be available, he said. "I would not take for granted that you cannot make use of it."

Cappelletti pointed out that nonvolatile memories based on charge storage have their own problems with soft errors. However, the temperature sensitivity does mean that phase-change memories are not suitable for pre-programming and use with solder reflow ovens.

The alternative approach is to find a different mix of of germanium, antimony and tellurium with an elevated melting point, Cappelletti said. "We are working on different compositions to find a material with 50 degree C higher melting temperature to increase the temperature range. We have seen materials with 50 degree higher melting temperature in research but there are many considerations for production," said Cappelletti.

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