MEMS Could Pave the Way for Atomic-Scale Assembly
Microelectromechanical systems (MEMS) could provide the key to manufacturing atomic-scale, self-assembling devices of the sort presently observed only in nature's complex living organisms. Often built with the same materials and lithographic techniques used in making integrated circuits, micron-sized MEMS motors, pumps, resonators, actuators, and sensors have been developed in the lab but not yet much deployed in marketable devices.
However, this may be changing soon. Michael Roukes of Caltech leads an effort focused on creating and exploring the properties of simple nano-electromechanical devices (NEMS), which have already proven useful in improving MRI sensitivity and wireless communications. The latter will most likely be crucial to future space exploration. At the APS March Meeting in Los Angeles, he specifically described the effort to coax small suspended columns of silicon into vibrations at GHz frequencies, making them into possible radio wave transmitters.
Using lithography and etching techniques, he has fabricated a 10x10x100-nm suspended beam of silicon which oscillates at an estimated frequency of 7 GHz (although no detector can yet "hear" the vibrations). Such a resonator will eventually be used in microwave signal processing (for modulating or filtering signals). The speed and stability of nanoscopic silicon arms might even facilitate the advent of some new kind of Babbage-type computer in which mechanical levers once again serve as processing or memory elements. Silicon structures in this size regime will also be used as cantilever probes in magnetic resonance force microscopy - the goal being atomic-resolution NMR imaging - and as calorimeters for the study of quantized heat pulses.
Roukes' colleague, Andrew Cleland (University of California, Santa Barbara), described a paddle-shaped silicon structure (whose smallest lateral feature was 200 nm) for detecting very small amounts of electrical charge, with a potential application in high-sensitivity photodetection. At the same session, Rex Beck of Harvard reported a NEMS force sensor which integrates a field effect transistor into a scanned probe microscope. The present sensitivities are about 10 angstroms for displacement and 5 pico-Newtons for force, but Beck expects improvements as the size of the device shrinks. The smallest transistor-probe structure Beck reported had dimensions of 3x2 microns x 140 nm. Stanford's Thomas Kenny reported on the use of slender cantilevers in atomic force microscopes to measure forces at the attonewton (10-18 newton) level.
MEMs may be a key enabling technology for NASA to explore the solar system with ultra-miniaturized, robust, softball-sized spacecraft, one of the most promising application areas. JPL scientist William Tang described the integration of microgyroscopes, microseismometers, quadrulpole mass spectrometers, micropropulsion engines and other sub-pint-sized gadgets on future space missions. When successful, these devices will serve as models for developing components and systems for spacecraft in the new millenium. "Space exploration in the coming century will emphasize cost-effectiveness and highly focused mission objectives," said Tang, who believes this new approach should result in frequent multiple missions, which will be faster, better, smaller and cheaper. "The aim is to broaden the scope of space science and to validate new technologies on a timely basis."
Dennis Polla (University of Minnesota) has deposited ferroelectric thin films on silicon-based MEMs to make sensors and actuators with a broad range of applications. These include acoustic emission sensing in integrated diagnostics; measurement of acceleration; biochemical sensing using molecular recognition microcantilevers; miniature stepper motors for precision positioning; and surgical and scientific micro-instruments. Chih-Ming Ho (University of California, Los Angeles), reported on recent studies of microscale fluid flows, in which the surface force dominates the force field, although, said Ho, other experiments indicate that the viscous force alone cannot account for the observed results.
Ultimately, MEMs research and the ensuing devices are expected to provide possible foundations of nanotechnology and the so called "Holy Grail" of atomic-scale assembly. "When we get there, nanotechnology will provide techniques for the mass production of tiny functional machines assembled, atom-by-atom, with perfect precision," said Rourkes. "But right now, Mother Nature is really the only true nanotechnologist.
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