Nonvolatile memory, which stores data even when power is removed, is familiar as the basis for flash memory in thumb drives. But flash technology has essentially reached its size and performance limits, according to some physicists and researchers. For several years, the computer industry has been searching for a replacement. Research being done at the National Institute of Technology (NIST) suggests resistive random access memory (RRAM) could form the basis for the next-generation of nonvolatile computer memory.
RRAM could surpass flash in many key respects: It is potentially faster and less energy-intensive. It could also pack far more memory into a given space—its switches are so small that a terabyte could be stored in a space the size of a postage stamp. But RRAM has yet to be broadly commercialized because of technical hurdles that need addressing.
One hurdle is its variability. A practical memory switch needs two distinct states representing either a one or a zero, and designers need a predictable way to make the switch flip. Conventional memory switches flip reliably when they receive a pulse of electricity, but we’re not there yet with RRAM switches, which are still flighty.
“You can tell them to flip and they won’t,” says NIST guest researcher David Nminibapiel. “The amount needed to flip one this time may not be enough the next time around, but if you use too much energy and overshoot it, you can make the variability problem even worse. And even if you flip it successfully, the two memory states can overlap, making it unclear whether the switch is storing a one or a zero.”
This randomness cuts into the technology’s advantages, but the NIST research team has found a potential solution. The key lies in controlling the energy delivered to the switch by using several short pulses instead of one long pulse.
Typically, chip designers use relatively strong pulses of about a nanosecond in duration. The NIST team, however, decided to try less energetic pulses of 100 picoseconds, about a tenth as long. They found that sending a few of these gentler signals was useful for exploring the behavior of RRAM switches as well as for flipping them.
“Shorter pulses reduce the variability,” Nminibapiel says. “The issue still exists, but if you tap the switch a few times with a lighter ‘hammer,’ you can move it gradually, while simultaneously letting you check it each time to see if it flipped successfully.”
Because the lighter touch does not push the switch significantly from its two target states, the overlapping issue can be significantly reduced, meaning one and zero can be clearly distinguished. The use of shorter pulses also proved instrumental to uncovering the next serious challenge for RRAM switches—their instability.
“We achieved high endurance, good stability, and uniformity comparable to using longer pulse widths,” notes Nminibapiel. “Instability affects our ability to maintain the memory state, though. Eliminating this instability is a problem for another day, but at least we’ve clarified the problem for the next round of research.”