Unlikely Partners in Patterning

Success in matchmaking depends on choosing partners with just the right character traits. If the parties are incompatible, the union won't succeed. The same is true of materials. Scientists who combine materials to make complex structures are mindful of materials compatibility issues and often are limited by them. But those limitations have now become less restrictive, thanks to a new study.

Researchers at Hebrew University of Jerusalem have devised a versatile method for forming nanometer-scale patterns using pairs of materials that don't lend themselves to patterning via other methods. The technique may form the basis for future nanofabrication strategies and provides new opportunities for fundamental investigations.

When one material is deposited on top of another--for example, through vapor deposition methods, which are common in semiconductor processing--the behavior of the top layer depends on the strength of its interactions with the bottom layer. If the two materials interact only weakly, then the top material will form bonds to other atoms or molecules in the top layer, causing the material to ball up. But if the top material prefers to associate with the bottom material, the top layer will wet the layer below and remain firmly attached.

The upshot is that weakly interacting materials are easily manipulated by a variety of methods and can be coaxed into forming complex patterns. But materials that interact strongly, such as pairs of metals, or metals and semiconductors, bond tightly to one another and resist patterning.

What the Israeli team has demonstrated is a technique for forming complex patterns of one metal on top of another. By combining the use of a gaseous buffer layer to sidestep strong metal-metal interactions and a laser-patterning technique, chemistry professor Micha Asscher and graduate student Gabriel Kerner prepared periodic designs (parallel stripes) of monolayer-thick potassium on ruthenium [Surf. Sci., 557, 5 (2004)].

And in recent follow-up experiments, the team patterned ruthenium surfaces with long parallel lines of gold with widths and periodicities as narrow as a few hundred nanometers. Ultimately, the technique should be capable of patterning surfaces with closely spaced lines of metal less than 50 nm wide and 5 mm long, the group says.

To form the patterns, the researchers cool the support metal and expose it first to xenon and then to the second metal, which results in a sandwich of weakly interacting layers. Then they use a pair of low-intensity laser beams to create a bright-and-dark diffraction pattern of parallel lines on the surface. In the bright areas, just enough energy is pumped into the system to remove the xenon layer and the metal above it without damaging the surface. Then, by warming the surface slightly, the remainder of the gas desorbs, allowing the metal to be deposited gently, thereby maintaining the pattern.

In a commentary in the same issue of Surface Science, John H. Weaver and Vassil N. Antonov, materials scientists at the University of Illinois, Urbana-Champaign, describe the work as "novel and exciting." Weaver, who developed the buffer-layer procedure to prepare metal clusters on semiconductors, remarks that the laser-plus-buffer approach is a general technique useful for "synthesis and patterning of nanostructures of almost anything on anything."