January - February 2003 The International Reference for Chip-Scale Electronics, Flip-Chip Technology, Optoelectronic Interconnection and Wafer-Level Packaging Email the editor

Terrence E. Thompson Senior Editor

In this economic downturn, it's fortunate for the chip community's future that not all R&D has stopped. For example, IBM researchers at San Jose's Almaden Research Center recently built and operated the world's smallest working computer circuits using an innovative new approach in which individual molecules move across an atomic surface like toppling dominoes.

The new "molecule cascade" technique enabled the scientists to make working digital-logic elements some 260,000x smaller than the ones used in today's most advanced ICs.

The circuits are made by creating precise patterns of carbon monoxide molecules on a copper surface. Moving a single molecule initiates a cascade of molecular motion, just as toppling a single domino can cause a large pattern to fall in sequence. Tiny structures demonstrated the fundamental digital-logic OR and AND functions, data storage and retrieval and the wiring necessary to connect them into functioning computing circuitry.

The most complex circuit built by IBM-a 12nm x 17nm three-input sorter-is so small that 190 billion of them could fit atop a standard pencil eraser 7mm in diameter!

IBM scientists have designed methods to link pairs of molecules to form "a cascade of molecular motion." (Source: IBM)

Andreas Heinrich, IBM Almaden physicist, said "the molecule cascade is not only a novel way to do computation, it is also the first time all of the components necessary for nanoscale computation have been constructed, connected and then made to compute. It is much smaller than any operating circuits made to date."

"Molecule cascades show how we are learning to harness the properties of very small structures," added IBM Fellow Don Eigler. "I was amazed at how rapidly we progressed from initial discovery to design and operation of functional circuitry," he said.

Nanoscale Science

This molecule cascade and the quantum mirage that the IBM researchers discovered are intriguing examples of novel nanoscale science and information-processing approaches that also yield new insights into the properties and interactions of atoms, molecules and surfaces.

The molecule cascade works because CO molecules can be arranged on a copper surface in an energetically metastable configuration that can be triggered to cascade into a lower energy configuration, just as with falling dominoes.

The metastability is due to the weak repulsion between carbon monoxide molecules placed only one lattice spacing apart.

This situation is analogous to placing tennis balls next to each other in an egg carton. Since the tennis balls are slightly larger than the lattice spacing of the carton, they push against each other and can't nestle down into the hollows of the carton as deeply as they could if they were more widely separated.

A Triad of CO Molecules

Just as placing three tennis balls in a row of an egg carton is unstable, Heinrich and his colleagues learned that a triad of carbon monoxide molecules arranged in a chevron-shaped pattern on the copper surface would be spontaneously rearranged by the outward motion of the central molecule.

The scientists then designed ways to link pairs of molecules so the rearrangement of an initial chevron formed a new chevron, and so on, in what they describe as "a cascade of molecular motion."