Making silver film fall flat.

A cross section of a computer chip reveals multiple layers of metals, insulators, and semiconductors-a materials sandwich that would make Dagwood Bumstead proud. Now, researchers have added an improved sandwich- building skill to their repertoire-depositing an especially flat metal film on top of a semiconductor.

In films, flatness denotes flawless crystal structure-a characteristic critical to a chip's performance. Usually, the spacing of the metal and semiconductor atoms must be very well matched, says Chih-Kang Shih of the University of Texas at Austin, a requirement that limits the possible combinations available to manufacturers. However, Shih and his colleagues have found a way to deposit a flat silver film on gallium arsenide, a metal and a semiconductor with very different atomic characteristics.

In addition to the work's potential practical applications, it suggests an "ideal interface" for studying theoretical models of surface interactions, Shih says. The group reports its results in the July 12 Science.

Silver atoms tend to form islands when deposited onto gallium arsenide at room temperature. To get around this problem, the researchers chilled the gallium arsenide to -138#161#C, taking away the energy the atoms need to move around.

On this cold substrate, the silver atoms collected in nanometer-scale clusters only one-hundredth the size typical of islands at room temperature. Then, after the film warmed back up, the nanoclusters rearranged into a flat film with a critical thickness of 1.5 nanometers.

This particular combination of materials probably won't make "a great impact on any emerging devices," says Lloyd J. Whitman, a physicist at the Naval Research Laboratory in Washington, D.C., who studies the structure of material interfaces. But, he adds, the work may eventually lead to useful systems like aluminum on silicon.