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IBM researchers announce major breakthrough in phase change memory

For years, scientists and researchers have looked for the so-called Holy Grail of memory technology — a non-volatile memory standard that’s faster than NAND flash while offering superior longevity, higher densities, and ideally, better power characteristics. One of the more promising technologies that’s been in development is phase-change memory, or PCM. IBM researchers announced a major breakthrough in PCM this week, declaring that they’ve found a way to store up to three bits of data per “cell” of memory. That’s a significant achievement, given that previous work in the field was limited to a single bit of data per memory cell.

Phase change memory exploits the properties of a metal alloy known as chalcogenide. Applying heat to the alloy changes it from an amorphous mass into a crystal lattice with significantly different properties, as shown below:

Theseus1

Scientists have long known that chalcogenide could exist in states between crystal lattice or amorphous, but building a solution that could exploit these in-between states to store more memory has been extremely difficult. While phase-change memory works on very different principles than NAND flash, some of the problems with scaling NAND density are conceptually similar to those faced by PCM. Storing multiple bits of data in NAND flash is difficult because the gap between the voltage levels required to read each specific bit is smaller the more bits you store. This is also why TLC NAND flash, which stores three bits of data per cell, is slower and less durable than MLC (2-bit) or SLC (single bit) NAND.

IBM researchers have discovered how to store three bits of data in a 64K array at elevated temperatures and for one million endurance cycles.

“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,” said Dr. Haris Pozidis, an author of the paper and the manager of non-volatile memory research at IBM Research – Zurich. “Reaching 3 bits per cell is a significant milestone because at this density the cost of PCM will be significantly less than DRAM and closer to flash.”

Here’s how the PR blast describes the breakthrough:

To achieve multi-bit storage IBM scientists have developed two innovative enabling technologies: a set of drift-immune cell-state metrics and drift-tolerant coding and detection schemes.

More specifically, the new cell-state metrics measure a physical property of the PCM cell that remains stable over time, and are thus insensitive to drift, which affects the stability of the cell’s electrical conductivity with time. To provide additional robustness of the stored data in a cell over ambient temperature fluctuations a novel coding and detection scheme is employed. This scheme adaptively modifies the level thresholds that are used to detect the cell’s stored data so that they follow variations due to temperature change. As a result, the cell state can be read reliably over long time periods after the memory is programmed, thus offering non-volatility.

“Combined these advancements address the key challenges of multi-bit PCM, including drift, variability, temperature sensitivity and endurance cycling,” said Dr. Evangelos Eleftheriou, IBM Fellow.

There’s still a great deal of work to do before phase-change memory can be considered as a candidate to replace NAND flash or DRAM in certain situations. The performance and power impact of these new structures has not been characterized and the switching time hasn’t been revealed.

Universal-PCM

The graphic above is from an IBM video explaining how PCM memory works and some general information on this latest breakthrough. Note that PCM, like NAND flash, takes a performance hit when it shifts to a multi-bit architecture. While single-bit PCM is nearly as fast as DRAM (according to IBM), multi-bit PCM is significantly slower. Data retention (how long data remains in the cell) was also worse than NAND flash, which has lower endurance (how many read/write cycles the cells can withstand) but higher data retention.

Phase-change memory is theoretically capable of replacing DRAM in at least some instances, but if these density gains come at the cost of programming speed, the net gain may be minimal. Phase-change memory also requires large amounts of power to program and generates a great deal of heat as a result.

This video from IBM walks through the history of phase-change memory, explains the basics of its function, and covers the most-recent breakthrough. We think IBM’s discovery here could help pave the way for a long-term replacement to NAND flash, but we’re still years away from that. Intel’s Optane 3D XPoint memory may make its own play for the server and data center space, and Micron, which used to manufacture PCM for the mass market doesn’t build it anymore.

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