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Mobile devices to undergo a phase memory change

By Bernard Cole
iApplianceWeb
(12/28/06, 1:54 AM GMT)

Researchers from IBM, Macronix and Qimond have come up with a new “phase memory” approach that could change the way memory is allocated and used in digital camera, mobile devices and portable music players.

Working together at IBM Research labs on both U.S. coasts, the scientists designed, built and demonstrated a prototype phase-change memory device  that switched more than 500 times faster than flash while using less than one-half the power to write data into a cell.

The device's cross-section is a minuscule 3 by 20 nanometers in size, far smaller than flash can be built today and equivalent to the industry's chip-making capabilities targeted for 2015.

Their research indicates that unlike flash, phase-change memory technology can improve as it gets smaller with Moore's Law advancements.

"These results dramatically demonstrate that phase-change memory has a very bright future," said Dr. T. C. Chen, Vice President, Science & Technology, IBM Research. "Many expect flash memory to encounter significant scaling limitations in the near future.

"This should ultimately lead to phase-change memories that will be very attractive for many applications."

The new material is a complex semiconductor alloy created in an exhaustive search conducted at IBM's Almaden Research Center in San Jose, Calif. It was designed with the help of mathematical simulations specifically for use in phase-change memory cells.

“Emerging memory technologies, like phase-change memory, are important elements of Qimonda’s advanced memory development," said Dr. Wilhelm Beinvogl, Senior Vice President, Technical Innovation, Qimonda AG. “Phase-change memories have the clear potential to play an important role in future memory systems.”

A computer memory cell stores information -- a digital "zero" or "one" -- in a structure that can be rapidly switched between two readily discernible states. Most memories today are based on the presence or absence of electrical charge contained in a tiny confined region of the cell.

The fastest and most economical memory designs – SRAM and DRAM, respectively – use inherently leaky memory cells, so they must be powered continuously and, in case of DRAM, refreshed frequently as well. These "volatile" memories lose their stored information whenever their power supply is interrupted.

Most flash memory used today has a "floating gate" charge-storing cell that is designed not to leak. As a result, flash retains its stored data and requires power only to read, write or erase information.

This "non-volatile" characteristic makes flash memory popular in battery-powered portable electronics. Non-volatile data retention would also be a big advantage in general computer apps, but writing data onto flash memory is thousands of times slower than DRAM or SRAM.

Also, flash memory cells degrade and become unreliable after being rewritten about 100,000 times. This is not a problem in many consumer uses, but is another show-stopper for using flash in applications that must be frequently rewritten, such as computer main memories or the buffer memories in networks or storage systems.

A third concern for flash's future is that it may become extremely difficult to keep its current cell design non-volatile as Moore's Law shrinks its minimum feature sizes below 45 nanometers.

The IBM/Macronix/Qimonda joint project's phase-change memory achievement is important because it demonstrates a new non-volatile phase-change material that can switch more than 500 times faster than flash memory, with less than one-half the power consumption, and, most significantly, achieves these desirable properties when scaled down to at least the 22-nanometer node, two chip-processing generations beyond floating-gate flash's predicted brick wall.

At the heart of phase-change memory is a tiny chunk of a semiconductor alloy that can be changed rapidly between an ordered, crystalline phase having lower electrical resistance to a disordered, amorphous phase with much higher electrical resistance. Because no electrical power is required to maintain either phase of the material, phase-change memory is non-volatile.

The material's phase is set by the amplitude and duration of an electrical pulse that heats the material. When heated to a temperature just above melting, the alloy's energized atoms move around into random arrangements.

Suddenly stopping the electrical pulse freezes the atoms into a random, amorphous phase. Turning the pulse off more gradually – over about 10 nanoseconds – allows enough time for the atoms to rearrange themselves back into the well-ordered crystalline phase they prefer.

The new memory material is a germanium-antimony alloy (GeSb) to which small amounts of other elements have been added (doped) to enhance its properties. Simulation studies enabled the researchers to fine-tune and optimize the material's properties and to study the details of its crystallization behavior.

To learn more, go to www..ibm.com  or www.mxic.com.

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