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Beyond Silicon Valley

Scientists are developing molecular-sized computer chips that would be as fast as a conventional supercomputer and would assemble themselves. They're on a road that leads . . .


The computer chips that physicist Marvin Cohen and his colleagues at the Lawrence Berkeley Laboratory have in mind are so small and powerful that four wine bottles could contain enough to store all the information in all the human brains in the world. A thread of the same circuits less than one-hundredth of an inch long could easily hold all the information in all the books ever written.

These theoretical computer circuits would be constructed from complex carbon molecules called nanotubes that have the same electrical properties as the silicon semiconductors used in most computers today. They would be a hundred times stronger than steel, as fast as a conventional supercomputer and, best of all, would assemble themselves. These are chips measured in nanometers: one billionth of a meter--the size of some viruses. They promise circuits 100 times smaller than the most miniature devices available today--computers that can be woven into clothing, painted onto walls, injected into the bloodstream or sprinkled like fairy dust in the air.

It is one vision of what lies beyond Silicon Valley, when the technology of conventional semiconductors has exhausted its possibilities and the cost of producing increasingly complex silicon chips becomes more than anyone can pay.

Researchers at Caltech, Stanford and MIT also are exploring the quirky quantum properties of subatomic particles for special purpose calculators. At USC and other centers, groups have harnessed the genetic code to program molecules of DNA so that the natural biological machinery of life will solve scientific equations.

"Just as the transistor was mind-boggling 50 years ago, and then the silicon chip, the idea that you are getting down to devices that are just a few atoms thick truly is mind-boggling," said Cohen of UC Berkeley.

But scientists have yet to discover a way to assemble these molecules for nanocomputers into the flawless circuits that today's computers demand.

It is not for want of trying. Already, scientists wielding electron beams like arc welders have built experimental structures thousands of times smaller than a human hair--gears that turn, pumps that operate, electric turbines only 60 microns in diameter that run on static electricity, transistors only 10 atoms in diameter. IBM researchers recently built a working abacus in which carbon molecules slide along microscopic copper grooves.

Not to be outdone, two Cornell University scientists crafted a guitar just 10 microns long, about the size of a single cell. They pluck its six silicon strings--each about 100 atoms wide--with an atomic force microscope.

But even this skill in molecular machining falls short of the manufacturing perfection required for conventional circuits, said computer designer Philip J. Keukes at Hewlett-Packard Laboratories in Palo Alto.

Instead, scientists like UCLA chemist James R. Heath hope this next generation of molecular machines will build themselves. In theory, they could be grown as the complex lattices of crystals, which arise spontaneously from the right combination of chemicals. "They would be transistors in a beaker," Heath said.

The problem is that some percentage of these self-assembling molecules always would be defective, due to the inherent nature of the chemistry used to assemble them.

At first glance, the nanotube, a molecule discovered in 1991, seems the perfect candidate for a self-assembling computer circuit.

So tiny that 10,000 will fit in the thickness of a human hair, nanotubes form naturally from a mist of heated carbon vapor in sheets exactly one atom thick. Without any prompting, the sheets neatly roll up into tubes as part of their natural chemistry. They can conduct electricity as well as any copper wire and they also can form the semiconductors used to make computer circuits.

At the same time, however, the tubes also tangle themselves into snarls of conducting and semiconducting tubes that contain countless flaws, any one of which would crash a conventional computer.

Now, Heath at UCLA and Keukes at Hewlett-Packard have found a way around that stumbling block. In the process they turn conventional computing on its head.

There is no need to eliminate the errors in complex chips, they determined. Instead, let the computer diagnose and heal itself. "It is possible to make detours around defects," Keukes said.

They proved their idea on an experimental computer built at Hewlett-Packard called Teramac, which contains 220,000 hardware defects, any one of which would be fatal to a conventional computer.

The key to the Teramac design are the thousands of extra connections between its components--11,000 wires on each chip, compared to several hundred on a Pentium chip--that let the computer steer around any defect. It can still work even if only a fraction of its components are functioning properly.

Despite the defects, the refrigerator-sized Teramac computer not only works; it operates 100 times faster than a normal high-performance computer workstation.

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