Unlike NAND flash memory, which requires a entire block of memory cells to be rewritten each time new data is stored on a device like a SSD drive, PCM allows for single bits to be changed, greatly increasing the efficiency and performance of a device.
"It has features of a low-latency memory and high bandwidth so we can combine many of the good attributes ... of NAND flash, DRAM and NOR flash," Fazio said.
The breakthrough in stacking PCMS came with the use of a thin film selector substance that is in the same class of materials as chalcogenide and is built above the silicon substrate. The companies are calling the thin film selector an Ovonic Threshold Switch (OTS), which acts like a resister between the stacked layers of PCM chips.
"This switch demonstrates a diode-like behavior. It has a low current in the off state and a high current in the on state," Atwood said. "Combining the OTS with the thin film storage material, a similar material used in phase-change memories today, enables a cell that can be stacked multiple layers high." Additionally, since the silicon substrate that isn't being used as a selector, as it is in today's PCM product, can now be used for building the support circuitry used that's required to decode, read and write to the cells.
"So the combination of these two advantages results in a much smaller die size that's a lower potential cost structure for the memory," Atwood said.
On Dec. 9, the two companies plan to present a paper on their discovery at the 2009 International Electron Devices Meeting, titled "A Stackable Cross Point Phase Change Memory."