Scientists from IBM, Macronix and Qimonda announced joint research results that give a major boost to a new type of computer memory with the potential to be the successor to flash memory chips, which are widely used in computers and consumer electronics like digital cameras and portable music players.
The advancement heralds future success for "phase-change" memory, which appears to be much faster and can be scaled to dimensions smaller than flash – enabling future generations of high-density "non-volatile" memory devices as well as more powerful electronics. 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 also enable a path toward 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.
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.
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 applications, 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. A patent has been filed covering the composition of the new material.
The technical details of this research will be presented this week at the Institute of Electronics and Electrical Engineer’s (IEEE’s) 2006 International Electron Devices Meeting (IEDM) in San Francisco (Paper 30.3: "Ultra-Thin Phase-Change Bridge Memory Device Using GeSb" by Y.C. Chen et al. Wednesday morning, December 13.) This paper was also one of only five to be chosen for the "Highlights of 2006 IEDM" session at the IEEE’s International Solid-State Circuits Conference, which will be held in San Francisco in February 2007.
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