SanDisk tackled these problems with new algorithms that run on a flash-memory chip controller. In order to write and read data to and from cells, engineers employ some of the transistors on a flash chip to control the other transistors used to store data. These algorithms are significant factors in reliably cramming in four bits per cell.
“We’ve introduced a number of key concepts that allow us to manage the memory side of it,” says Quader. “The complexity of this distribution is so much different than what you’re doing with two bits per cell.”
Usually, a single applied voltage is used to write data to a memory cell, but this approach won’t work with four-bit cells because they are so small and close together. Writing to one cell can easily erase a neighboring cell due to electrical coupling effects. Using an approach called three-step programming gets around this problem. A small voltage is applied to one cell, effectively programming only 3 of its 16 states. Next, the neighboring cells are programmed to 15 and 3 levels respectively, using different voltages. Finally, the original cell is programmed a second time. Writing data in this stepwise manner produces electrical characteristics within the cell that ensure reliable storage of bits.
Because the programming scheme takes slightly longer than do traditional approaches, SanDisk developed a feature that senses the voltages stored within cells by effectively remembering the values sensed previously. The end result is a chip that can write data at a rate of 7.8 megabytes per second–close to the speed at which existing chips can be accessed. SanDisk’s Quader says that the 64-gigabit chips will be in production before the second half of this year, using 43-nanometer lithography technology.
Mark Bauer, a research fellow at memory company Numonyx and chair of the conference session, says that the real innovation behind SanDisk’s work is the controller technology. “You won’t see four-bit flash without that controller,” he says.
Bauer adds that, while some experts have predicted that flash memory is reaching its storage limits, clever engineering keeps breathing new life into the technology. “Four years ago, people were saying that flash was hitting a roadblock, but the improvements keep coming,” he says. “We can’t tell what solutions being explored today will solve the problems tomorrow.”