Protein masks for etching tiny holes.

The outer wall of the bacterium Sulfolobus acidocaldarius acts as a sieve. Its two-dimensional array of regularly spaced, microscopic holes permits the organism to regulate the passage of substances into and out of its body.

When deposited on a smooth graphite surface, this holey layer of protein molecules also provides scientists with a template for rapidly fabricating patterns of tiny holes in a thin, metal-oxide film (SN:3/24/90, p.191). Now physicists have discovered that the resulting metal-oxide screen can act as a mask in transferring the same pattern of holes to the underlying graphite substrate.

This novel method may in certain cases represent an inexpensive, efficient, "bench-top" alternative to the use of electron beams or such techniques as X-ray lithography for etching arrays of holes on graphite, silicon, and other surfaces, says Kenneth Douglas of the University of Colorado at Boulder. Douglas, Genevieve Devaud, and Noel A. Clark describe their work in the July 31 SCIENCE.

The researchers begin by depositing a layer of bacterium-derived protein crystals onto a graphite surface. They coat these crystals with a thin film of titanium, which subsequently reacts with oxygen in air to form a metal-oxide layer about 3.5 nanometers thick.

Using a technique known as "fast-atom beam" milling, the researchers train a stream of accelerated argon atoms ions onto the metal-oxide coating to gradually wear it away. But the erosion occurs unevenly. More metal-oxide disappears from the hollows representing holes in the protein layer than from the peaks marking the presence of protein. This imbalance produces holes, each only 10 nanometers in diameter, that puncture the metal-oxide layer and penetrate into the underlying graphite substrate, while leaving a network of protein, still coated with titanium dioxide, surrounding the holes.

"There seems to be a kind of self-focusing effect that's created by armoring or reinforcing the biological material, which by itself would normally be too fragile to withstand the ion beam," Douglas says. The researchers suggest that the hole-formation process involves both the removal and the redistribution of metal oxide.