Phase-change memory: A thousand times faster than silicon

Story

September 06, 2016

While silicon chips can store data in billionths of a second, phase-change memory may be 1,000 times faster – and require less energy and space. A 19-member group of researchers led by Stanford University reports that phase-change memory, which is based on a new class of semiconductor materials, could both store data permanently and allow certain operations to run as much a thousand times faster than today’s memory devices.

The group’s work is “fundamental but promising,” says Aaron Lindenberg, an associate professor of materials science and engineering at Stanford, and of photon science at the SLAC National Accelerator Laboratory (operated by Stanford University for the U.S. Department of Energy Office of Science). “A thousandfold increase in speed coupled with lower energy use suggests a path toward future memory technologies that could far outperform anything previously demonstrated.” Their findings provide new insights into the experimental technology of phase-change memory.

Phase-change materials

Memory chips are typically based on silicon technologies that are capable of efficiently switching electron flows on and off, representing the ones and zeros of digital software. But silicon has its limits, so researchers around the world are on a quest to find new materials and processes requiring less energy and space.

Why phase-change memory? It shows potential as a next-generation technology, thanks to certain materials that boast flexible atomic structures with appealing electronic attributes. Phase-change materials are capable of existing in two different atomic structures, with different electronic states. A crystalline or “ordered atomic structure” enables the flow of electrons, while an amorphous or “disordered structure” prevents their flow.

Researchers have figured out how to flip the structural and electronic states of these materials – changing their phase from “1” to “0” and back again – via short bursts of heat (applied electrically or optically). In terms of being used as a memory technology, phase-change materials are appealing because they retain whichever electronic state conforms to their structure. Once atoms flip to form a “0” or a “1,” the data gets stored until another jolt of energy changes it, according to Stanford University. This means that phase-change memory is nonvolatile, like the silicon-based flash memory in smartphones today.

The catch? Any next-generation memory technology must also be capable of performing certain operations much faster than today’s chips. To analyze this, the researchers tapped extremely precise measurements and instrumentation to demonstrate the speed and energy potential of phase-change technology.

What they discovered is extremely promising. “Nobody had ever been able to investigate these processes on such fast time scales,” Lindenberg says.

Looking at speed

To take on the speed end of things, the researchers focused on the brief interval when an amorphous structure begins to switch to crystalline – a digital “0” flips to a “1.” This phase, during which the charge flows through the amorphous material like a crystal, is known as “amorphous on.”

By jolting a small sample of amorphous material with an electrical field comparable in strength to a lightning strike, the Stanford researchers’ instrumentation detected that the amorphous-on state occurred less than a picosecond after the jolt. This interval means that phase-change materials can be transformed from “0” to “1” by a picosecond excitation, and implies that emerging technology could store data many times faster than silicon random access memory (RAM) for tasks that require memory and processors to work together to perform computations.

Although the researchers weren’t able to establish exactly how much time would be required to completely flip an atomic arrangement from amorphous to crystalline or back, their results suggest that phase-change materials could perform superfast memory chores and permanent storage – depending on how long the thermal excitation is engineered to stay inside the material.

While work still remains to turn the group’s discovery into functioning memory systems, attaining such speed via a low-energy switching technique on a material that can store more information in less space means that phase-change technology may one day revolutionize data storage. Any new technology that “demonstrates a thousandfold advantage over incumbent technologies is compelling,” Lindenberg notes. “We’ve shown that phase change deserves further attention.”

Storage/endurance

Clearly, phase-change memory has attracted researchers’ attention. Earlier this year, in another phase-change memory research effort, IBM was able to demonstrate reliably storing three bits of data per cell in a 64k cell array at elevated temperatures after one million endurance cycles via the technology.

Phase-change memory’s combination of read/write speed, endurance, nonvolatility, and density prevent data loss when powered off, unlike DRAM, and can endure at least 10 million write cycles, compared to an average USB stick, which maxes out at 3,000 write cycles. IBM expects its phase-change memory storage breakthrough to help speed up machine learning and access to the Internet of Things, mobile phone apps, and cloud storage.