Memory chips improve in a flash

Researchers at the California Institute of Technology and Agere Systems have developed a technique that could result in a new generation of reliable nanoscale memory chips.

The research development, announced on December 13, 2000, at the International Electron Devices Meeting, applies to ‘flash’ memory, which continues to store information even when a device is turned off.

This information could include personal phone directories in a cellular phone or the pictures captured by a digital camera. There are between16 to 32 million bits of data stored on a silicon flash memory chip in a typical cellular phone, and each bit of data is stored in a part of the flash memory chip called a ‘cell’. As the size of silicon memory chips decreases, the chips are more and more difficult to make leakproof, resulting in the loss of stored date.

Using an aerosol technique developed at Caltech, the researchers formed memory cells by spraying silicon nanocrystals through a bath of high-temperature oxygen gas. The end result was memory cells comprised of silicon on the inside with a silicon dioxide outer shell. The silicon nanocrystals store the electrical charge, whereas the insulating silicon dioxide shell makes the nanocrystal memory cells more leakproof.

‘As compared to conventional flash memories, these silicon nanocrystal memories offer higher performance, simpler fabrication processes, and greater promise for carrying memory miniaturisation to its ultimate limit,’ said Harry Atwater, professor of applied physics and materials science at Caltech and project director.

To overcome the potential leakage problem, Atwater and Richard Flagan, McCollum Professor of Chemical Engineering, and their students at Caltech, and colleagues Jan de Blauwe and Martin Green at Agere Systems developed a method to break up each memory cell into 20,000 to 40,000 smaller cells. Therefore, even if several of the smaller cells spring a leak, the vast majority of the charge will not be lost and the bit of data stored in the whole memory cell will be retained.

The aerosol approach is said to have several advantages over the conventional lithographic techniques currently used to make flash memory cells. Because it requires fewer steps, it is less expensive and takes less time to produce. In addition, the aerosol approach will allow researchers to continue making smaller and smaller devices.

So far, the researchers are said to have created extremely robust flash memory cells. They have charged and dissipated a single cell one million cycles without significant degradation, whereas with traditional silicon chips, 10,000 cycles is considered satisfactory.

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