New Memory Cell Technology Realizes Large-capacity PRAM

Dec 13, 2012
Jyunichi Oshita, Nikkei Electronics

Low-power Electronics Association & Project (LEAP) developed a memory cell technology that contributes to realizing a PRAM whose capacity is as large as that of NAND flash memory.

LEAP aims to replace NAND flash memory used as SSDs in data centers, etc with PRAM to increase the speed and lower the power consumption of the SSDs.

Many PRAM developers are trying to realize a cross-point memory that rewrites data with two-terminal devices (diodes) by exploiting the fact that data stored in PRAM can be rewritten just by flowing a unidirectional current in its memory cells. A cross-point structure makes it easy to increase the density of memory cells and realize a capacity equivalent to that of NAND flash memory.

Though ReRAM (resistive random-access memory), which is a new-type nonvolatile memory, has a similar potential, Norikatsu Takaura, leader of the Phase-Change Device Group of LEAP, said, "From the viewpoint of operation reliability required for SSDs used in data centers, PRAM (which uses materials that have been mass-produced for optical discs, etc) is superior to ReRAM."

To realize a PRAM having a cross-point structure, it is necessary to reduce the rewriting current of memory cell and enable to rewrite data with small-size diodes. However, the memory cell of traditional PRAM has a large rewriting current.

This time, LEAP found in a simulation of rewriting operation that rewriting current can be reduced by lowering the heat conductivity of memory cell's phase-change material and increasing its resistance value. And it quantitatively calculated their effects. As a result, LEAP succeeded in developing a phase-change material that realizes optimal heat conductivity and resistance values.

Specifically, LEAP added a dielectric additive to Ge2Sb2Te5, which is a common phase-change material. And it checked the property change of the phase-change material by using the amount of the additive as a parameter to find the optimal amount of the additive. As a result, it became possible to reduce rewriting current by 68% and rewriting power by 90%, compared with a traditional memory cell using Ge2Sb2Te5.

Furthermore, LEAP increased operating temperature by more than 100°C, enabling to ensure operations at a temperature as high as 150°C. The nano-sized crystal of the newly-developed phase-change material contributed to lowering rewriting current and improving high-temperature properties, LEAP said. It confirmed that PRAM using the new technologies can be rewritten 107 times.