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Closeup cross-section of phase change bridge
A new memory technology using semiconductor alloy is faster, smaller and more resilient than flash

Scientists from IBM, Macronix and Qimonda today disclosed joint research results that detail a new type of computer memory with the potential to be the successor to flash memory chips. The advancement is "phase-change" non-volatile memory, which appears to be much faster and can be scaled to dimensions smaller than flash. Non-volatile memories do not require electrical power to retain their information. By combining non-volatility with good performance and reliability, this phase-change technology may one day be a universal memory for mobile applications.

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. This new result shows that unlike flash, phase-change memory technology can improve as it gets smaller with Moore’s Law advancements.

Flash memory cells, while also non-volatile, degrade and become unreliable after being rewritten about 100,000 times. This is not a problem for many consumer uses, but is a showstopper in applications that must be frequently rewritten, such as computer main memories or the buffer memories in network 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 designs shrink 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. A patent has been filed covering the composition of the new material.

The view a couple animations of how phase-change memory works, click here. The technical details of this research will be presented this week at the IEEE’s 2006 International Electron Devices Meeting in San Francisco.





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