Scientists from IBM Research – Zurich are claiming a world-first, for their recent demonstration of “reliable multi-bit phase-change memory [PCM] technology.”
PCM involves the use of materials that change between crystalline and amorphous states, the two states having different levels of electrical resistance – data is stored in a binary fashion, using one level to represent a 0, and the other to represent a 1. By applying new techniques to existing PCM technology, the researchers were reportedly able to write and retrieve data 100 times faster than is possible with Flash.
The IBM team used a 200,000-cell PCM test chip, fabricated in 90-nanometer CMOS technology. The phase-change material, an alloy consisting of various elements, was deposited between electrodes in the chip. By applying voltage to those electrodes, the alloy was heated, causing it to change back and forth between its low-resistance crystalline state, and its high-resistance amorphous state. What’s more, by varying that voltage, the amount of material between electrodes that changed state could also be controlled – this allowed the scientists to store multiple bits of data on each cell.
One of the problems meeting previous PCM efforts has been one of resistance drift, or the tendency of the amorphous material’s resistance to gradually increase after the initial phase-change. This has resulted in read errors. In order to get around this problem, the Swiss researchers utilized an “advanced modulation coding technique,” that compensated for resistance drift. As a result, the chip has been able to reliably retain data for up to five months, and counting. Previously, only single bit-per-cell PCM chips had proven to be as reliable.