Microscope writes beneath a metal surface.

Microscope writes beneath a metal surface

Physicists have discovered a method for etching microscopic features inside thin sandwiches of metal and semiconductor. The technique may yield new insights into electronic properties of the boundary between metals and semiconductors--a region crucial to the performance of many microelectronic devices. It may also lead to new structures for high-density data storage.

Previously, other scietists have found ways to record microscopic images on the surfaces of metals using a scanning tunneling microscope (STM) (SN: 11/17/90 p.310). But the STM couldn't rearrange atoms below the surface -- a property essential for etching images there. Nor could it probe internal boundaries between structural materials -- zones that can strongly influence the properties of electronic devices.

In 1988, researchers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., developed ballistic electron emission microscopy (BEEM), a new method for investigating microscopic details of the interface between metals and semiconductors. They injected electrons from the tiny tip of an STM into a thin metal layer coating a semiconductor. At high enough energies, some of the electrons not only crossed the metal-semiconductor interface but also passed completely out the other side. How much current passed through both layers depended on the atomic structure of the metal-semiconductor interface at that particular location. Scientists went on to harness this relationship to chart atomic details of the interface.

Using a silicon semiconductor buried under a thin layer of gold, a group of researchers at Cornell University in Ithaca, N.Y., has now applied the BEEM technique at a higher voltage than ever before.

Unexpectedly, the high-voltage electrons rearranged some of the atoms near the interface without altering the gold surface, the team reports in the Dec. 24, 1990 APPLIED PHYSICS LETTERS. This does not happen at lower voltages.

"It was a surprise. We discovered it by a student [Hans D. Hallen] saying, 'What happens if I go to a higher voltage?'" group leader Robert A. Buhrman told SCIENCE NEWS.

By slowly moving the STM tip across the gold surface, the investigators found they could create lines at the subsurface interface just 8.5 nanometers wide -- about one-ten-thousandth the width of a human hair. This represents the first time anyone has produced characters this small at a boundary between two materials, the researchers believe.

Although they haven't nailed down the mechanism behind the "writing" effect, they suspect that the powerful current moves loosely held gold atoms to form the lines.

"With this technique we can better understand the properties of such interfaces and how they change and deteriorate under electrical stress," Buhrman says. For instance, he says, researchers might induce defects in computer chips and watch them, to better understand how they form. Scientists may also be able to engineer better data storage by "writing" information onto the interfaces, then reading the information back electronically, he says.

The Cornell scientists plan to investigate the cause of the high-voltage effect more carefully and to apply their technique to metals other than gold. Although they're not sure where their studies will lead, "these things have a habit of growing," says John Silcox, who participated in the work.