THE miniaturisation that has led to a seemingly endless rise in the capacity of flash memory cards – which many cellphones, cameras and music players rely on to store data – may soon hit a wall. Fortunately, a way through that wall has been lurking in your CDs and DVDs all along.
Flash memory cards store data in the form of electrical charge, with the presence of charge signifying a 1 and its absence a 0. The charge is stored in a combination of a capacitor and a transistor, etched into silicon chips. So far, chip makers have relied on making these elements ever smaller: today’s flash memory transistors are just 65 nanometres wide and the next generation will be a mere 45 nanometers.
They cannot be made smaller forever, says Greg Atwood of Numonyx, a manufacturer of flash memory chips in Santa Clara, California. “Serious issues will begin to arise when dimensions get below 20 nanometres,” he says. One issue is that “parasitic” charge builds up on the transistor every time new data is written to it. This limits the cards’ use to around 10,000 write-and-erase cycles, with the problem becoming more serious as transistors get ever smaller (91av, 28 July 2007, p 26).
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Now Atwood and his colleagues have shown that an alloy used to store data on CDs and DVDs could be an ideal replacement for silicon. In a CD or DVD, a laser beam heats a thin layer of an alloy of tellurium, antimony and germanium, called GST, causing it to switch between disordered and crystalline states. The two states reflect light differently, and can thus be used to store laser-readable 0s and 1s.
Each state also has a unique electrical resistance, and Numonyx wants to exploit this property to store data. It has built a form of memory in which a blob of GST sits on top of a resistor. Current applied to the resistor heats the blob and changes the GST’s resistance. A second, much lower current can “read” the blob’s resistance, allowing it to store either a 0 or a 1 (Science, ).
Numonyx engineers have used this technique to build a chip with 128 megabits of what they call “phase change memory” (PCM). Researchers at IBM have already demonstrated that a PCM memory cell can be shrunk down to about 5 nanometres wide. As PCM does not accumulate parasitic charge, it can be rewritten at least a million times.
But Malcolm Penn, of UK based market research company Future Horizons, says PCM’s economic viability is doubtful, at least for now. “We don’t see the classic transistor running out of steam for at least 10 years,” he says.
