IBM makes tiny circuit from carbon

Breakthrough could replace use of silicon

By KENNETH CHANG New York Times

Last Updated: Aug. 26, 2001

In another step toward post-silicon computers, IBM scientists have built a computer circuit out of a single strand of carbon.

The IBM circuit performs only a single simple operation - flipping a "true" to "false" and vice versa - but it marks the first time that a device made of carbon strands known as nanotubes has been able to carry out any sort of logic. It is also the first logic circuit made of a single molecule.

At least another year or two of research is needed before IBM can even evaluate whether a practical computer chip can be manufactured from nanotubes, said Phaedon Avouris, manager of nanoscale science at IBM Research and the lead scientist on the project.

But the fact that the researchers were able to build the circuit raises hopes that nanotubes could eventually be used for computer processors that pack up to 10,000 times more transistors in the same amount of space.

Avouris declined to speculate on when a chip with nanotube circuitry might appear commercially, but he described nanotubes as "the best candidate from what we've seen" in the emerging field of molecular electronics.

"This is yet another test that nanotubes have passed," Avouris said. "The physics works."

The processing power of computer chips has consistently doubled every year or two as the size of transistors continues to shrink. But current chip-making technology, which etches circuits into silicon, is expected to run up against fundamental physics limits in 10 to 15 years.

A nanotube, which resembles a rolled-up tube of chicken wire, is about one hundred-thousandth the thickness of a human hair. Its thinness, only about 10 atoms wide, makes it a promising candidate for circuits in future faster and smaller computer chips. It takes its name from nanometer, a unit of measurement one-billionth of a meter long, which is a convenient length for specifying molecular dimensions.

Charles M. Lieber, a professor of chemistry at Harvard University and an expert in the field of nanotechnology, called the IBM achievement "quite significant." The effort to incorporate nanotubes in computer chips is a "great strategy and one that could be implemented relatively quickly," he said.

The IBM researchers presented their findings on Sunday at a meeting of the American Chemical Society in Chicago. An article describing the results will appear in the September issue of the journal Nano Letters.

Nanotubes are not the only approach to building ultratiny circuits. Other researchers, like those at Hewlett-Packard, have designed custom molecules that act as on-off switches. However, unlike transistors, the switches do not amplify electric signals, and a computer made of molecular switches would have to employ a different method for performing calculations, one that scientists still are working to devise.

"They don't even have a transistor," Avouris said. "In that sense, we're way ahead in the game. You don't have to worry about finding a different architecture. You use what exists now."

In April, the same researchers at IBM's T.J. Watson Research Center in Yorktown Heights, N.Y., reported that they had constructed vast arrays of transistors made out of carbon nanotubes, but the arrays were not wired to perform any calculations.

In the latest research, the scientists succeeded in hooking up two of these transistors to perform the true-false flipping operation. Even more remarkably, the two transistors exist along sections of the same nanotube.

-- Anonymous, August 27, 2001