Cascading molecules drive IBM's smallest computer

By R. Colin Johnson
EE Times
October 25, 2002 (7:15 a.m. EST)

  SAN JOSE, Calif. — IBM researchers have created a simple computation engine that's more than 250,000 times smaller than the most advanced silicon circuitry. Called the world's smallest computer, the system relies on a "molecular cascade" that pushes a handful of carbon monoxide molecules across a copper surface to perform digital logic functions.

"Our molecular cascades are still research, but their small size is literally generations smaller than today's silicon circuitry," said Andreas Heinrich, a physicist at IBM's Almaden Research Center here. "Our 3-input sorter implemented in next-generation CMOS technology requires an area of over 50 square microns, but our molecular cascade implementation uses just 200 square nanometers. Even if CMOS density follows Moore's Law for 40 more years, molecular cascades are still going to be smaller."
Heinrich was lead researcher on the project that also included Christopher Lutz, Jay Gupta and Donald Eigler. Though years away from real-world applications, IBM's molecular cascades demonstrate how all circuitry will eventually be shrunk to nanoscale levels, Heinrich said.
The molecular cascades rely on the natural attraction that the carbon ends of CO molecules have towards copper. On the lattice of a single-crystal copper substrate, CO molecules are positioned in a way that's somewhat like trying to cram tennis balls into an egg carton. IBM has lined up these molecules in a staggered 0.25-nanometer grid. The CO spontaneously hops to adjacent grid sites, nudging one another in a chain reaction that performs a preset calculation.

Domino calculations

"Imagine two lines of dominoes that curve toward each other, and at their end there is a single domino that can be toppled by either line — that's an OR gate," Heinrich said. "Your input is either a nudge to topple the first one in a given line of dominoes, or a zero is no nudge."

By placing CO molecules on a 0.25-nm grid of crystalline copper, IBM set up logical calculations in domino code. A scanning tunneling microscope (STM) was used to push a naturally occurring grid of CO molecules atop copper into a preset pattern needed to perform a given calculation. Then the STM supplied the "input" to the domino-coded circuit by manually nudging the first molecule in the cascade.
So far, the molecular cascades have a perfect operational record, Heinrich said.

"We have seen over 10,000 of these hops and we have never seen an incorrect one," he said.

In fact, it was this incredible reliability of the cascade that first attracted Heinrich to invent the domino code. After researcher Lutz mentioned it to him, Heinrich immediately went out and bought 600 dominoes. The domino code for an OR gate was easy, he said, the AND gate was "exceedingly hard," and he hasn't yet created a NOT gate.
The domino theory has had his laboratory abuzz for months, and the accounting department as well after they saw Heinrich's domino purchase on his expense report.
"I got in trouble with accounting for the dominoes, but it was worth it," he said. "The whole project has been fun for everyone involved. As soon as people grasp the concept, they want to see it work — you really have to see all the animation we have put up on our Web site."

The slow operation of the gates — some required seconds to settle — underscores the fact that the work was part of a research project. "We have made extraordinarily small, albeit exceedingly slow, logic circuits," Heinrich said.