ANOTHER use for synchrotron rays was highlighted last month when a group at the University of Wisconsin announced that they had made the world's smallest metal gears -- only slightly wider than the thickness of a human hair.
The project employed the same kind of lithography used to etch the super-thin circuit lines on computer chips: Electromagnetic rays are projected through a patterned mask onto a surface covered with a "photoresist." Where the beams strike the surface, they alter the chemistry of the resist, making it easy to wash away with a solvent. The shadowed sections remain, leaving the mask pattern etched into the surface.
Conventional microchip lithography employs comparatively weak forms of ultraviolet light (and more recently, X-rays) that penetrate only a very short distance into the resist. But the Wisconsin team used a synchrotron to generate the high-energy X-rays needed to etch the resist layer much deeper than is common. They then filled the grooves with relatively hefty layers of nickel -- about one-fourth the thickness of a human hair -- with center holes about 50 millionths of a meter wide. Similarly tiny gears have been made before of silicon. But the strength of metal is needed for many expected applications, including tiny "roto-rooters" for blood-vessel surgery, miniature pressure sensors and accelerometers, "microknives" designed to cut a single fertilized egg in half and ultra-precise gear clusters for satellites.
"If you drive something that small," says Edward Lovell of the UW team, "it requires only trivial amounts of power and might run for years in a spacecraft." Moreover, "these gears can rotate very fast -- hundreds of thousands of revolutions per minute -- with little or no lubricant" because their mass is so slight.