Replacement for Flash Memory

Today flash memories or thumb drives are commonly used as devices that store information even without power—nonvolatile memory. However, physicists and researchers are of the opinion that flash memory is nearing the end of its size and performance limits. Therefore, the computer industry is in search of a replacement for flash memory. For instance, the National Institute of Technology (NIST) conducted research is suggesting resistive random access memory (RRAM) as a worthy successor for the next generation of nonvolatile computer memory.

RRAM has several advantages over flash. Potentially faster and less energy hungry than flash, it is also able to pack in far more information within a given space. This is because its switches are tiny enough to store a terabyte within a space the size of a postage stamp. So far, technical hurdles have been preventing RRAM from being broadly commercialized.

One such hurdle physicists and researchers are facing is the RRAM variability. To be a practical memory, a switch needs to have two distinct states—representing a digital one or zero, and a predictable way of flipping from one state to the other. Conventional memory switches behave reliably when they receive an electrical pulse and switch states predictably. However, RRAM switches are still not so reliable, and their behavior is unpredictable.

Inside a RRAM switch, an electrical pulse flips it on or off by moving oxygen atoms around, thereby creating or breaking a conductive path through an insulating oxide. When the pulses are short and energetic, they are more effective in moving ions by the right amount for creating distinct on/off states. This potentially minimizes the longstanding problem of overlapping states largely keeping the RRAM in the R&D stage.

According to a guest researcher at NIST, David Nminibapiel, RRAMs are as yet highly unpredictable. The amount of energy required to flip a switch may not be adequate to do the same the next time around. Applying too much energy may cause it to overshoot, and may worsen the variability problem. In addition, even with a successful flip, the two states could overlap, and that makes it unclear whether the switch is actually storing a zero or a one.

Although this randomness takes away from the advantages of the technology, the researcher team at NIST has discovered a potential solution. They have found the energy delivered to the switch may be controlled with several short pulses rather than using one long pulse.

Typically, conventional memory chips work with relatively strong pulses lasting about a nanosecond. However, the NIST team found less energetic pulses of about 100 picoseconds, which were only a tenth of the conventional pulses, worked better with RRAM.  Sending a few of these gentler signals, the team noticed, was more useful not only for flipping the RRAM switches predictably, but also for exploring the behavior of the switches.

That led the team to conclude these shorter signals reduce the variability. Although the issue does not go away totally, but tapping the switch several times with the lighter pulses makes the switch flip gradually, while allowing checking to verify whether the switch did flip successfully.