Stanford engineers find that a new memory technology may be more energy efficient than previously thought
Scientists often discover interesting things without completely understanding cause and effect, but such unknowns are anathema to engineers. Their job is to translate basic knowledge into useful technologies and that means getting down to the nitty gritty.
That is the spirit behind a paper that a Stanford team will present when the IEEE International Electron Devices Meeting (IEDM) brings leading researchers to San Francisco on December 5.
Led by electrical engineer H.-S. Philip Wong, the team developed a deeper understanding of a new type of data storage technology that would be ideal for smart phones and other mobile devices where energy-efficiency is vital feature.
This new technology is called resistive random-access memory, or RRAM for short.RRAM is based on a new type of semiconductor material that forms digital zeroes and ones by resisting or permitting the flow of electrons. RRAM has the potential to do things that aren’t possible with silicon: for instance, being layered on top ofcomputer transistors in new three-dimensional,high-rise chips that wouldbe faster and more energy efficient than current electronics.
But while engineers can observe that RRAM does store data, they don’t know exactly how these new materials work. “We need much more precise information about the fundamental behavior of RRAM before we can hope to produce reliable devices,” Wong said.
Soto help engineers understand some of the unknowns, Wong’s team built a tool to measure the basic forces that make RRAM chips work.
Graduate student Zizhen Jiang of the Stanford team explained the basics.
RRAM materials are insulators, which normally do not allow electricity to flow, she said. But under certain circumstances insulators can be induced to let electrons flow. Past research hadshown how: jolting RRAM materials with an electric field causesa pathway to form that permitted electron flows. This pathway is called a filament. To break the filament researchers apply another jolt and the material becomes an insulator again. So each jolt switchedthe RRAM from zero to one or back, which is what makes the material useful for data storage.
But electricity is not the only force at play in RRAM switching. Pumping electrons into any material raises its temperature. That’s the principle behind electric stoves. In the case of RRAM, introducing voltage tothe materials raisedtheir temperature in ways that caused filaments to form or break. But at what voltage/temperature state? No one knew. Before the new Stanford study researchers thought short bursts of voltage, sufficient to generate temperatures of about 1,160 degrees Fahrenheit – hot enough tomeltaluminum – was the switching point. But those were estimates because there was no way to measure the heat generated by an electric jolt.
“In order to begin to answer ourquestions we had to decouple the effects of voltage and temperature on filament formation,” said Ziwen Wang, another graduate student on the team.
Essentially, the Stanford researchers had to heat the RRAM material without using an electric field. So theyput an RRAM chip ona micro thermal stage (MTS) device -- a sophisticated hot plate capable of generating a wide range of temperatures inside the material. Of course the objective was not merely to heat the material, but also measurehowfilaments formed. Here they took advantage of the fact that RRAM materialsareinsulatorsin theirnatural state. That makes them digital zeroes. As soon as a filament formedelectrons wouldflow. The digital zero would become a digital one, which the researchers could detect.
Using this experimental model theteamput RRAM chipsonthe burner and cranked up the heat, starting at about 80 degrees Fahrenheit – roughly the temperature of a warm room -- all the wayup to 1,520 degrees Fahrenheit, hot enough to melt a silver coin. Heating the RRAM to various temperatures in between these extremes, the researchers measuredprecisely if andhowRRAM switched from its native zero to a digital one.
To their pleasant surprise the researchers observed that filaments couldform more efficiently at ambient temperatures between 80 degrees Fahrenheit and 260 degrees Fahrenheit, which is hotter than boiling water – contrary to prior expectation that hotter was better. If confirmed by subsequent research, this would be good news because in a working chip the switching temperature would be created by the voltage and duration of the electric jolt. Efficient switching at lower temperatures would require less electricity and make RRAM more energy efficient and extend battery lifewhen they wereused as the memory in mobile devices.
Much work remains to be done to make RRAM memory practical but this research provides the test bed to vary conditions systematically instead of relying on hit and miss hunches.
“Now we can use voltage and temperature as design inputs in a predictive manner and that is going to enable us to design a better memory device,” Wang said.
Henry Chen, a Stanford alumnus who earned his PhD in Wong’s lab gave this research a big assist and was a co-author on the paper. Chen, now with the Chinese memory chip-manufacturing firm GigaDevices Semiconductor Inc., helped develop the concepts and instruments that enabled theresearchers to make the measurements being reported at IEDM.