NJIT Engineers Reduce Power Consumption in Future Computer Memory

Most people haven’t heard of resistive RAM — one of several evolving types of computer memory that could become mainstream someday — but its chances at commercial success improved recently, because of insightful new research from NJIT and commercial partner Tokyo Electron.

Resistive memory is in a category of what’s called non-volatile memory, which means it remembers data even when switched off. Ordinary memory goes blank when it loses power. But resistive memory, although it’s a nanoscale device with just two terminals, still has some engineering challenges such as high power consumption.

An electrical connection called the conductive filament connects the electrodes. Prof. Durga Misra, chair of NJIT’s electrical and computer engineering department, explained that his team built filaments which are reliable and require much less power. It works by adding hydrogen plasma to a zirconium dioxide layer, ultimately serving as a switching layer, in their prototype memory device. They knew from their own previous research that this would generally help, but now they know precisely where and how much hydrogen to install.

“Initially many researchers had trouble with high-quality switching layers that are energy efficient. Then during the deposition process they were trying to reduce the oxygen flow, to create oxygen vacancies that help in conducting filament formation and switching operation. It was non-controllable,” he continued. “We observed a better performance when we used a bi-layer because of the way these oxygen vacancies are distributed — a graded distribution with higher concentration near the top electrode. We tried the plasma treatment at the midpoint during the deposition of a single layer and observed much lower switching power. We observed that if the bottom layer is thinner, and after plasma treatment if we have a slightly thick top layer, the switching power reduces further.”

“In a single device we reduced the power consumption by 500 times, by using plasma treatment for the optimized device as compared to devices without any plasma treatment. For a single user it will be in the same range, whereas in a data center it could reduce power consumption by 100 times,” Misra noted. “In the initial experiment we did the plasma treatment at the midpoint of the switching layer, but by increasing the top layer the power consumption reduced further. We are continuing the research further to understand it better.”

With the hydrogen placement hurdle behind them, there remain further considerations to keep conductive filaments stable. “If the devices are set to a particular resistance level, then after thousands of operations it should [remain] in that level. But it changes values after 1,000 cycles. We are trying to improve that as well. We are trying to address this. This variability can also be addressed by peripheral circuit design and system architecture design,” he observed. Moreover, “We are trying to reduce the switching power from 240 picowatts to 40-50 picowatts in a single device. That will reduce the data center power requirement further.”

The team’s research, led by graduate student Aseel Zenati, is published as Engineering of ZrO2-based RRAM devices for low power in-memory computing from a recent issue of the Journal of Vacuum Science and Technology - B.