CD material makes rad-hard memory

Electronics Times, Nov 15, 1999

Materials originally developed for rewriteable CDs are to form the basis of a radiation-hard memory being developed by Lockheed-Martin for use in space. The non-volatile memory technology itself could replace flash and DRAM.

The defence company is licensing technology from Ovonyx, a subsidiary of Michigan-based battery company Energy Conversion Devices, formed earlier in the year by former Micron Technology chief technology officer Tyler Lowrey.

The phase change memory uses resistance measurements in place of the stored charges used by conventional memories, making it far less susceptible to corruption by stray alpha particles, at least in the memory bits themselves.

Ovonyx believes that it is simpler to make than flash memory, needing four extra mask steps on top of a standard logic process.

The extra mask steps are needed to deposit a layer of chalcogenide, the material that is used to form the memory bit. Chalcogenides are specialised materials formed from alloys of group VI elements, such as selenium or tellurium, with group IV or V elements.

Supplied with enough energy, they can switch from an amorphous form through a series of intermediate states into a polycrystalline form that has a much lower resistance. They can also be switched back.

In a rewriteable CD, a laser is used to heat the material to switch it from one form to another. In the proposed semiconductor devices, current pulses are used.

The programming sequence is similar to that needed for a flash memory but Ovonyx claims the materials can withstand 1013 cycles.

In today's processes, a series of short 1mA pulses at a voltage approximately double that of the normal read voltage will set the bit. Raising the current to just under 2mA will reset it.

In common with flash memories, the intermediate states can be used to support multilevel storage. With a resistance difference of about 1000 times, the company says the memory could support 16-state cells, or four bits per cell, in commercial-grade devices.