Feeling the Tiniest Stresses

An extremely sensitive microscope can measure the strain in a material over just tens of atoms. The achievement, described in the 3 August issue of Physical Review Letters, will allow researchers to probe the stability and durability of their smallest designs and perhaps lead to better computer chips and other miniature devices.

Devices called scanning tunneling microscopes and atomic force microscopes have been used for more than 15 years to scrutinize the surfaces of materials. These instruments can reveal bumps and dents just a few individual atoms across. Many materials also vary in stiffness and flexibility over such minute lengths, but until now this information could only be obtained for centimeter-sized areas. A few years ago, however, Oleg Kolosov, a materials scientist at Oxford University, U.K., designed the Ultrasonic Force Microscope (UFM), which measures stiffness over atomic lengths with a tiny cantilever that taps rapidly against a material and measures how readily the material bounces back.

To test the UFM's capabilities, Kolosov's collaborators at the Hewlett-Packard laboratories in Palo Alto, California, grew nanometer-sized islands of the semiconductor germanium on a silicon surface. The UFM's cantilever then scanned the surface, tapping away about three million times each second. The UFM data told the team that the germanium islands were less rigid than the stiff silicon surrounding them, which they expected. However, the UFM also revealed something new--the center of each germanium dot was considerably stiffer and also under more strain than the outer edges. The reason, says team member Stanley Williams of Hewlett-Packard, is because atoms squashed near the center don't have as much space to vibrate as those on the edges.

Phaedon Avouris, a physicist at the IBM research center in Yorktown Heights, New York, believes that the technique will be useful to map out the elastic properties of tiny structures that are grown literally atom by atom. "Strain is a very important parameter," he says, and often determines a structure's stability and why it grows the way it does. Thus the UFM, he says, could help materials scientists design even more stable microscopic devices such as transistors and other micromachines.