A new type of nonvolatile memory based on carbon nanotubes has dramatically lower power requirements than current technology. It uses the nanotubes to read and write data to small islands of phase-change materials, which store information. With further development, the new technology could extend battery life in mobile devices and also make desktop computers more efficient.
Itty bitty bits: Three low-power phase-change memory bits are positioned between carbon-nanotube electrodes that have been colorized. The middle bit is “off” and the other two are “on.” The bits are arrayed on a silicon substrate that has been colored light blue.
Nonvolatile memory stores information even when the power is switched off. The standard technology for it, flash memory, is used in smart phones, cameras, USB sticks, and fast-booting netbook computers. But the storage density of flash memory is reaching its limit because the transistors used to make flash memory arrays cannot be miniaturized any further. The power needed to write to flash is also a speed limitation, and it drains the batteries in portable devices.
The new nonvolatile memory, developed by Eric Pop, professor of electrical engineering and computer sciences at the University of Illinois at Urbana-Champaign, and colleagues, can hold more data than flash while requiring considerably less power.
A few replacements for flash are in development. The one that's closest to commercialization is phase-change memory. The "bits" in phase-change memory are small islands of materials called chalcogenides that switch between glassy and crystalline states when rapidly heated. The two phases have different electrical resistances that can represent "1" or "0"—the bits are read by passing a small current through an electrode to read the resistance. Samsung and Numonyx, a memory company owned by Micron, have both promised to release phase-change memory products soon.
While phase-change memory promises to be denser than flash, for the most part it still has relatively high power requirements. "The traditional drawback of phase-change memory is that you need significant heat to change the phase," says Victor Zhirnov, director of special projects at the Semiconductor Research Corporation.
The device designed by Pop and colleagues at the University of Illinois tackles the power consumption problem by allowing the phase-change memory bits to be further miniaturized. The smaller the chalcogenide bit, the less the energy required to heat it up and change its phase.
The limit in today's technologies isn't the phase-change material itself but the size of the electrical contacts needed to connect to the islands where information is stored. "The smallest contacts they have been able to make are about 50 nanometers," says Pop. His devices use narrow, highly conductive carbon nanotubes as the electrical contacts. The nanotubes range from one to five nanometers in diameter, and they connect to pieces of phase-change material that are about 10 nanometers in size.
Pop's group uses established methods to grow arrays of flat, parallel carbon nanotubes on silicon chips. Then they add metal electrical contacts to the nanotubes and zap the nanotubes with a large pulse of electricity until they snap in two, leaving a tiny gap between the two halves. The device is then coated with a phase-change material called GST (a compound of germanium, antimony, and terbium). Each bit consists of an island of GST in the gap in a nanotube. When a small amount of current is passed through the nanotubes, it heats the material, changing its phase. The nanotubes can also be used to read the bit.
It takes about 0.1 milliamps to switch a conventional phase-change memory bit. This week in the journal Science, Pop's group reports writing to their phase-change memory using 100th as much current. They've shown that they can write and rewrite each bit hundreds of times and have made arrays of about 100 bits. Pop says the next step is to demonstrate millions of read-write cycles and larger arrays.
If the device can be produced in high volume, it could benefit not just portable electronics but also desktops and mainframes, says Zhirnov. In today's computers, moving information between processors and memory, and reading and writing to memory, takes a lot of energy and generates waste heat. Bringing the logic and memory closer together, onto the same chip if possible, is a major goal in computing, says Zhirnov.
Putting the two on one chip isn't possible with flash, because writing to flash memory requires voltages about 20 times higher than the voltages needed to operate digital logic. The nanotube devices, on the other hand, operate in the same voltage range as the transistors used for digital logic. In principle, this means they could be used to make chips that integrate memory and logic and operate very efficiently.
The Illinois researchers must also demonstrate that the memory is reliable. There's a lot of variation in the size and conductive properties of carbon nanotubes. Pop says that so far, these variations don't seem to matter in the memory devices. But that remains to be proved on a larger scale.
By Katherine Bourzac
From Technology Review