Better Phase-Change Memory
Improved technology could compete with flash
Source: “A Multi-Level-Cell Bipolar-Selected Phase-Change Memory”
Ferdinando Bedeschi et al.
International Solid-State Circuits Conference, February 3-7, 2008, San Francisco
Results: Researchers at Intel and STMicroelectronics have developed an algorithm that doubles the amount of data that can be stored in a single phase-change memory cell, which represents bits as distinct arrangements of a material’s atoms.
Why it matters: For the past decade, flash memory has provided compact, sturdy storage for small devices such as iPods and cell phones. But flash chips, which store data as electric charge, may soon reach the limits of their capacity. Phase-change memory is among the alternatives that engineers have been pursuing. Previously, a phase-change memory cell could represent only one bit of data at a time. But the researchers found a way to store two bits in each cell, doubling the capacity of phase-change memory and making it competitive with flash.
Methods: A typical phase-change memory cell uses a type of glass that can switch back and forth between amorphous and crystalline states; the crystalline state represents a 1, the amorphous state a 0. The researchers created two-bit memory cells by giving the glass two more states in between amorphous and crystalline. To write data to a memory cell, an electrode heats it until a crystalline filament forms in the amorphous material. The bigger the filament grows, the more current passes through the memory cell. In the amorphous state, the memory cell represents two 0s. In the semiamorphous state, it represents a 0 and a 1. In the semicrystalline state, it represents a 1 and a 0. In the crystalline state, with memory cell resistance at its lowest point, it represents two 1s.
Next steps: The phase-change chips were made using existing fabrication processes that yield memory cells larger than those of a flash memory chip. Future versions of phase-change memory should use newer processes that produce smaller cells, but researchers need to make sure that the technique holds up as the cell sizes shrink.