Electronic Design
The phasechange memory chip on a circuit board Image courtesy of IBM Research

The phase-change memory chip on a circuit board. (Image courtesy of IBM Research).

IBM Scientists Mark New Advances in Phase-Change Memory

To hear scientists describe memory that can access data with incredible speed, retain information when powered down, and endure huge workloads, you might believe they were talking about the human brain. But these are the same features that IBM scientists have built into a relatively new kind of memory chip.

It is called phase-change memory and the scientists said that they had tripled the amount of data that it can hold. The breakthrough could lead to new advances in everything from mobile phones to the servers behind Watson, IBM’s artificial intelligence platform.

The material inside phase-change memory changes state when an electrical current is applied. The memory becomes either amorphous or crystalline, with each state having different electrical conductivity. Each state can be programmed to represent bits—0 or 1—with low voltages applied to read back the data.

The scientists at IBM’s Zurich laboratory have been tinkering with phase-change memory for years, trying to get certain qualities of  dynamic rapid-access memory (DRAM) and flash, which is widely used in mobile devices and personal computers. Though flash memory is much slower than DRAM, it also has the ability to save information after losing power, unlike DRAM.

That sweet spot between DRAM and flash is one of the latest priorities for memory chipmakers. Intel and Micron last year revealed 3D XPoint a memory chip both fast and nonvolatile, meaning that it can serve as either system memory or digital storage. Other types of memory that could inhabit that space are resistive RAM, magnetoresistive RAM, and memristors.

Haris Pozidis, a member of the research team and manager of non-volatile memory research at IBM. (Image courtesy of IBM Research).

PCM also falls within that range. It retains data when powered down and can endure at least 10 million write cycles, compared to an average flash USB stick, which tops out at 3,000 write cycles.  In separate experiments, IBM scientists have shown that PCM can write and retrieve data 100 times faster than flash memory.

The amount of data that can be stored within phase-change memory, however, has always posed a problem. The scientists had been limited to storing 1 bit per cell, before making new advance that enabled 3 bits per cell at elevated temperatures and after 1 million endurance cycles. Storing that amount of data could make PCM chips closer in cost to flash memory, which is extremely cheap and plentiful.

“Phase-change memory is the first instantiation of a universal memory with properties of both DRAM and flash, thus answering one of the grand challenges of our industry,” Haris Pozidis, a member of the research team and manager of non-volatile memory research at IBM, said in a statement.

The research team, which shared its findings at the International Memory Workshop in Paris, envisions PCM as general-purpose data storage. It could be paired with flash storage, with PCM serving as an extremely fast cache. For instance, a mobile phone’s operating system could be stored in PCM, enabling the phone to turn on within seconds.

It could also be plugged into data servers to lower the amount of time it takes for processors to access memory. That could help machine-learning algorithms scan through large datasets, like medical records, network security checks, or financial transactions. IBM has tested the memory with its OpenPower server architecture.

Two advances from the IBM scientists helped to ensure that PCM cells could reliably store additional data. First, the memory was designed to read a physical property of the semiconductor material that remains stable over time, so that the memory can reliably recall data. This makes the memory cells insensitive to drift, or the tendency of the memory to degrade over time.

The scientists also found a new way to account for slight changes in temperature, which can slowly change the cell’s electrical conductivity, corrupting the data. The scientists built a “coding and detection scheme” that tracks temperature variations within the cell, ensuring that the bits survive long periods of time. That, the research team says, also makes the memory non-volatile.

“This is actually the first time that someone was able to store three bits per cell in phased-change memory arrays—so, not a single cell—and also address all the major reliability challenges with this technology,” Pozidis said in a video about the research.

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