Microcircuits, fabricated on silicon chips, have produced a revolution in electronics. Now researchers are using the same chip-fabrication techniques to build micromachines, complete with motors, gears and other moving parts--all smaller than a dust speck.
And the combination of electronic circuits and micromachines, all on the same chip, offers the prospect of microscopic robots.
"Clearly this will be a new industry," said George Hazelrigg of the National Science Foundation, which is funding some of the basic research. "I think it's going to be a big one."
This field is so new that it lacks even an agreed-upon name. At AT&T Bell Laboratories, the term is "microdynamics." Japanese researchers speak of "micromachines." Rodney Brooks of the Massachusetts Institute of Technology talks of "microbots" and "microbotics." Others use terms such as "micro-electromechanical devices," a lack of settled terminology that illustrates the unsettled nature of the field.
Indeed, its beginnings trace only to the late 1970s. By then, techniques were well in hand for etching silicon to make microcircuits. Various small groups were using these same methods to make specialized instruments. A tiny pit etched in silicon, roofed over with a thin and flexible layer, could offer a microscopic pressure sensor, with the thin layer flexing under pressure.
In 1982 Kurt Petersen, then an IBM researcher, wrote a pathfinding paper that established a firm foundation for this field. "There were little sporadic efforts, people working on things all over the country," he recalls. "They didn't know each other. They hadn't really grasped the idea that they were making mechanical devices out of silicon, which is an electronic material."
Since then, the development of silicon micro-instruments has flourished. In late-model cars, a jewel-like pressure sensor, no more than two or three millimeters in size, rides beneath the hood. Auto makers buy them by the millions as part of the system that controls the air-fuel ratio in engines.
Also, the Pentagon's Strategic Defense Initiative is developing miniaturized guidance systems featuring accelerometers the size of a match head. They feature a roofed-over pit etched in silicon, but the thin roof is cut away at the edges, allowing it to flex like a diving board when its rocket accelerates.
"We couldn't form them by machining a block of metal," said Brig. Gen. Malcolm O'Neill of the SDI. "Small, finely machined parts are hard to make." But silicon etching methods can be controlled to far greater accuracy.
Hazelrigg expects that within five years, the next generation of microdevices will be ready. These will feature cutting wheels and other tools with moving parts, driven by motors the width of a human hair. These would not be robots, since they would not move or make decisions on their own. Instead they would be particularly precise surgical tools manipulated by a doctor, or highly complex and compact mechanical systems that would be under the control of an outside computer.
The necessary motors and wheels are under vigorous research. At Bell Laboratories, William Trimmer and Kaigham Gabriel have fabricated sets of three meshing gears, with diameters as small as 0.005 inches--about a hair and a half. A similar wheel, powered by air from a hypodermic needle, spins at 24,000 revolutions per minute. "As with microchips, we get hundreds of thousands of devices off a single wafer of silicon," Gabriel said. "If one turbine breaks, we just reach in and get another one."
To build a micro-electric motor, the wheel must be made to spin using electricity rather than air. At UC Berkeley in 1988, a group of graduate students led by Richard S. Muller built a micro-motor whose rotor, or turning wheel, was only two-thirds of a hair in diameter.
The researchers arranged a microcircuit that would apply charges of static electricity around the rotor's periphery, shifting the charges' locations rapidly. The rotor turned by following the electrical attraction of the charges. Muller declared that "the important thing is that now we know a rotor this small can work. We had no proof before."
A more recent advance, at the University of Utah, addresses a major problem with such motors: They are of little use when spinning at very rapid speeds. Instead, they must turn a shaft at much slower speeds but without wasting power. The shaft then has torque, or turning force, and can do useful work.
The Utah invention is the "wobble motor." Its shaft rolls around the inside a cylinder, leaving only a small gap between rod and cylinder. Hence the rod must make 40 or more "wobbles," or rolls, for each rotation it makes, a design that paradoxically minimizes friction.
Steven Jacobsen, the project leader, declares that the longest-running microscopic motor, prior to his wobble motor, ran for only three minutes before stopping. His motor, by contrast, has operated continuously for a month.
