Morphing memory: superfast atom shuffling inspires data-storage alternatives

Science News, June 4, 2005 by Peter Weiss

Anyone who purchases an electronic camera, cell phone, voice recorder, travel disk, or PDA, typically brings home a stick, card, or some other medium containing a chip ready to store information via a technology known as flash memory. Last year, consumers worldwide bought almost $12 billion worth of flash products, which depend on electrons to store data. The semiconductor industry expects global demand to surpass $18 billion by 2007.

Nonvolatile memory systems, in which data remain intact even when the power is off, are widespread as the magnetic-disk drives of computers. More recently, portable consumer products have taken advantage of nonvolatile memory provided by fast, high-capacity microchips. In these products, flash rules.

Although camera buffs, for instance, can today store hundreds of images on a stamp-size chip costing less than $100, they're demanding more data-dense, cheaper storage components. Engineers working to create the next generation of data-storage devices consider flash to be "the technology to beat," says Matthias Wuttig of RWTH Aachen University in Germany.

Several technologies have potential to dominate the future of microchip nonvolatile memory. The newest contender relies on a principle already at work in any computer that can burn a rewritable CD. Alaser heats spots on an inner layer of the CD to between 300[degrees]C and 600[degrees]C for a few nanoseconds. That's all it takes to rearrange the atoms in that layer in a way that imprints one bit of digital data--the proverbial 1 or 0. Over the past decade, phase-change material, a class of silvery semiconductors about as soft as lead, has emerged as a star ingredient of write-your-own optical disks.

Now, researchers are striving to recast electronic memory chips by taking advantage of this material. Rather than accumulating electrons to store data, these upcoming chips instantly toggle patches of atoms between order and disorder.

What makes phase-change material particularly suitable for fast-memory devices is that it "can go from amorphous to crystalline [or back] with minimum motion of the atoms," notes Gary A. Gibson of Hewlett-Packard Laboratories in Palo Alto, Calif. Consequently, it can switch with lightning speed between arrangements that have dramatically different optical properties or electrical resistances. "This is really magic," Wuttig says.

Phase-change memory developers are resurrecting a decades-old invention that was eclipsed by the success of such materials in optical disks. In the 1960s, Stanford R. Ovshinsky of Energy Conversion Devices in Rochester Hills, Mich., made the seminal discoveries that revealed the potential for those materials to be a medium for electronic--as well as optical--data storage.

Scientific and commercial interest in the electronic version of the technology has exploded in the past few years, Ovshinsky says. That version is known as phase-change random access memory, or ovonic memory, in reference to Ovshinsky. Those who are most bullish about it, forecast that the technology could end up stealing not only flash-memory markets but also those now dominated by volatile-memory technologies, such as the dynamic random access memory (DRAM) and static random access memory (SRAM) used by computers.

IT'S ELEMENTARY Although in the kitchen, making ice cubes and softening butter aren't the speediest operations, freezing and melting serve as the basis for the new form of fast computer memory. On microscopic scales, materials can freeze and melt at blinding speeds. Associating 1 or 0 with each of these states of matter provide the makings of memory.

Compounds known as chalcogenides have opened new vistas of data storage because of the changes they undergo when suddenly heated. At the heart of each of those compounds is one or more of such elements as sulfur, selenium, and tellurium--which appear in oxygen's column of the periodic table--combined with other semiconducting or metal-like elements such as germanium, indium, and antimony.

In optical disks, a laser's heat switches a ehalcogenide patch between an orderly crystalline form and a more disordered, amorphous one. Because the mirrorlike crystalline patches bounce light in a given direction than the somewhat translucent, amorphous patches do, a detector in a CD or DVD player can almost flawlessly discern which bits are Is and which are Os.

In the new applications, jolts of electric current, rather than bursts of light, trigger the reversible crystal-to-amorphous structural change. Going from crystal to amorphous is straightforward. The electric jolt instantly melts the patch of chalcogenide, and when the nanoseconds-long zap ends, the patch's temperature plummets so quickly that the jumbled atoms freeze in place before they can snap back into crystalline order.

Going the other way, from an amorphous to a crystal state, the scientists apply a slightly longer, less-intense dose of current that warms, but doesn't melt, the amorphous chalcogenide patch. The technique takes advantage of the greater stability- and lower energy of the crystalline state. The warmth mobilizes the atoms of the amorphous state just enough so that they can rearrange themselves into an orderly lattice.