Pressure produces smaller circuits

By Eric Smalley, Technology Research News

Figuring out how to etch ever tinier lines in silicon chips is only one of the challenges to keeping computer speeds doubling every 18 months or so. This doubling, described by Moore's Law, is the main reason computers keep getting more powerful.

Being able to make smaller lines allows manufacturers to etch tinier transistors, the electrical switches that form the building blocks of computer processors and memory.

In order to pack more circuits into the same area, the metal wires that connect the transistors into useful circuits also have to shrink. The trick is figuring out how to efficiently make metal wires 1,000 times narrower than a human hair.

Taking advantage of the same principle that pressure cookers use to speed dinner preparation, researchers at the University of Massachusetts at Amherst have made copper and nickel wires narrower than 100 nanometers. A nanometer is one millionth of a millimeter. In addition to producing smaller interconnects for computer chips, the technique promises to reduce the amount of pollution the chipmaking process generates.

The researchers' chemical fluid deposition technique is a hybrid of plating, which spreads liquid metal over a surface, and chemical vapor deposition, which coats a surface with a mist of metal atoms, said James J. Watkins, an associate professor of chemical engineering at the University of Massachusetts.

"We developed a new approach to metal deposition that involves the use of supercritical fluids as the reaction medium," he said. Like the extra-hot water in a pressure cooker, supercritical fluids exist under pressure and at temperatures hotter than those that would turn them into gases under ordinary pressure.

Supercritical fluids go beyond what is possible in a pressure cooker, however; they are heated and compressed to the point where they behave like a cross between a liquid and a gas. "The supercritical fluid has a density that approaches those of liquids but [has] transport properties of a gas," said Watkins.

The researchers suspend tiny metal particles in supercritical carbon dioxide. This makes it possible to apply more metal to a surface than is possible using metal vapor and apply it faster and more consistently than using liquid metal, according to Watkins.

The researchers made electrically conductive wires by filling tiny trenches carved into silicon chips with particles of copper or nickel suspended in the supercritical carbon dioxide. The trenches can be narrower than 100 nanometers, which is too small to fill using ordinary liquid or vapor techniques, said Watkins.

Chemical fluid deposition also sidesteps both the hazardous emissions produced by chemical vapor deposition and the water baths used in metal plating processes that result in contaminated wastewater, according to Watkins.

The researchers are doing "outstanding" work in supercritical fluid chemical deposition, said Gregory L. Griffin, a chemical engineering professor at Louisiana State University. They will probably have to demonstrate higher growth rate or a similar advantage before the technique will be adopted, however, he said. "Previous groups, including one commercial vendor, have shown similar trench-filling ability using conventional chemical vapor deposition."

Chemical fluid deposition could be used in practical applications in two to three years, said Watkins. The main challenge is adapting the other materials and equipment used to manufacture the chips so they can handle the high pressure needed to make supercritical fluids. "The technique can be adapted [for manufacturing] in a straightforward manner. The problem of handling wafers in a high-pressure environment is presently being addressed by a number of manufacturers," he said.

Watkins' research colleagues were Jason M. Blackburn, David P. Long and Albertina Cabanas of the University of Massachusetts. They published the research in the October 5, 2001 issue of the journal Science. The research was funded by the National Science Foundation (NSF), the David and Lucile Packard Foundation and Novellus Systems, Inc.

Timeline:   2-3 years
Funding:   Corporate; Government; Private
TRN Categories:   Materials Science and Engineering; Integrated Circuits
Story Type:   News
Related Elements:  Technical paper, "Deposition of Conformal Copper and Nickel Films from Supercritical Carbon Dioxide," Science (online), September 13, 2001


October 17, 2001

Page One

Atom laser fits on a chip

Email takes brainpower

Teamed computers drive big display

Holograms control data beams

Pressure produces smaller circuits


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