Nikkei Electronics Asia -- February 2007
Tech Feature
Phase-Change Memory Switch 500x Faster than Flash

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Jan 26, 2007 16:14 Nikkei Electronics Asia
A phase-change memory (PCM) that potentially could be 500 times faster than today's Flash memory chips was among the many new devices and technologies described at the recent International Electron Devices Meeting in San Francisco, California.

IBM Corp of the US and partners Macronix International Co Ltd of Taiwan, and Qimonda AG of Germany, have created a new PCM switch with an alloy composed of germanium and antimony. PCM is a type of non-volatile memory that can be switched between crystalline and amorphous states by applying heat.  

The prototype that was demonstrated switches more than 500 times faster than Flash memory and uses less than half the power to write data into a cell. The cross-section of the switch measures 3 x 20nm, much smaller than today's Flash memories.
The new alloy, composed of germanium and antimony, was designed using mathematical simulations at IBM's Research Lab in San Jose, California. Small amounts of other elements were added, or doped, to the material to enhance its properties.

Scaling, Moore's Law
The researchers said their result shows that unlike Flash memory, PCM technology can improve as it gets smaller with Moore's Law advancements. Some current semiconductor technologies are expected to hit a performance barrier over the next couple generations of size decreases.

"Many expect Flash memory to encounter significant scaling limitations in the near future," said T C Chen, vice president of Science & Technology at IBM Research. This "...new PCM material has high performance in an extremely small volume. This should ultimately lead to PCMs that will be very attractive for many applications."

Most current memories are based on the presence or absence of an electrical charge in a confined region in the cell. Static RAM and dynamic RAM use inherently leaky memory cells, so they need to be powered constantly. DRAM also must be refreshed often. Most Flash memories use a floating gate, charge-storing cell that is designed not to leak, so data is retained. Power is needed only to read, write or erase information, making this non-volatile memory popular in portable devices with batteries.

But the benefit of having data stored safely comes at a price: writing data onto Flash memory is about 1,000 times slower than doing so onto DRAM or SRAM. And Flash memory cells tend to degrade and become unreliable after being rewritten about 100,000 times, the researchers said. While that may not be too much of a problem in consumer electronics uses, it is not practical in applications that must be rewritten often, such as the main memories of computers or the buffer memories in networks or storage systems.

Another concern is that it might be difficult to retain the current cell design as Moore's Law shrinks the feature sizes smaller than 45nm. The joint research project in PCM demonstrated a non-volatile phase-change material that can switch 500-plus times faster than Flash memory, with less than half the power consumption, and at a scale of 22nm.

Materials Advances
The core of the PCM is a tiny piece of semiconductor alloy of germanium and antimony that can be changed quickly between a crystalline phase with lower electrical resistance to an amorphous phase that is disordered but has a much higher electrical resistance. No electrical power is needed to maintain either phase of the material, so PCM is non-volatile memory.

The phase that the material is in is set by the amplitude and duration of the electrical pulse that heats the alloy. When it is heated to just above melting point, the alloy's energized atoms move in random arrangements. By stopping the electrical pulse suddenly, the atoms freeze into a random, amorphous phase. Turning off the electrical pulse more gradually - for example, over 10 nanoseconds - allows enough time for the atoms to rearrange themselves into the well-ordered crystalline phase that they prefer.

by Lori Valigra