Phase Change Memory Offers Best Of Both Worlds

Intel and STMicroelectronics are still working to spin off their flash memory joint venture, to be dubbed Numonyx, but the two aren't waiting until the deal settles to begin working on new technologies. The first technology from the venture is based on an idea that has been around since the 1970s, but only now is technologically possible to deliver.

The two companies have begun shipping samples of Phase Change Memory (PCM) to OEM partners for evaluation. The memory, developed under the codename "Alverstone," provides the storage capabilities of flash memory with the speed of DRAM.

The test devices are 256MB multi-level cell devices manufactured in a 90nm process. The sample is actually 128MB but by storing two bits per cell they will be able to double capacity.

PCM memory uses a chalcogenide gas that is kept in one of two states, liquid or crystalline. The two states are used to represent the 0s and 1s of bits, which is how data is stored on a storage device. To change the data, an electrical pulse is fired through the memory causing a phase change; the crystalline melts to liquid, or the liquid solidifies into a crystalline.

All of this is done on a microscopic level, converting materials just a few nanometers in length, according to Giulio Casagrande, who will be vice president of advanced research at Neumonyx once the company is set up. By sending a pulse of electricity through the memory, it's possible to convert many megabytes of memory.

Intel feels it combines the best features of all existing memory technologies. "One of the benefits of flash, NOR (define) or NAND (define) is it's non-volatile compared to DRAM (define)," said Cliff Smith, technology initiatives manager for Numonyx. "But flash is not bit alterable. It's organized in blocks and you have to do pre-conditioning to write to it, making it slower. With DRAM, you just write to it. PCM has that DRAM-like quality of being bit alterable."

He added the caveat that most flash applications, like cameras and MP3 players have been structured to live with that block orientation of flash, and it will be a while before PCM is cheap enough to compete with DRAM. The primary benefit to PCM when it finally hits the market will be that it uses less power and performs faster than flash, said Brookwood.

Even there, he said, expect flash to put up a fight. "You've got a whole bunch of really smart people at places like Samsung and Toshiba and Sandisk working real hard to maintain and enhance the products that have made them successful. Those mature technologies don't just lie down and die gracefully," he said. Sandisk recently showed off the ability to store three bits per cell, thus one-upping PCM's two bits per cell. Traditional flash stores one data bit per cell.

PCM has one advantage going for it: as it shrinks and gets to a smaller manufacturing process, its performance will improve, since the electrical pulse to read and write data literally has less area to cover. So increasing the density and shrinking it from its 90nm size now will actually make it faster, said Casagrande.

Intel has no set release date planned for the memory. Smith figured it's at least a year or two away from coming to market. Setting up Numonyx will be completed by the end of this quarter, he said.


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