Timeless machine detects electric charge

Science News, March 14, 1998 by Corinna Wu

A 200-year-old scientific instrument is new again. Unlike its classic counterpart, however, the modern version can only be seen under a microscope.

In 1784, French physicist Charles-Augustin de Coulomb developed the torsion-balance electrometer, a sensitive device that measures electric forces. Now, researchers have scaled Coulomb's invention down to just a few micrometers in size. Andrew L. Cleland of the University of California, Santa Barbara and Michael L. Roukes of the California Institute of Technology in Pasadena fashioned the miniature electrometer out of silicon. They describe it in the March 12 Nature.

The new device, which moves in response to tiny amounts of electric charge, is "quite similar in principle" to Coulomb's original, says Roukes. When electric charge accumulates in a pair of electrodes--one that is fixed and one that rotates--the electrodes attract and draw closer together. In the silicon electrometer, the movable electrode rests on a paddle attached to a thin, flexible beam that twists and vibrates in response to electric attraction. By applying a magnetic field, the researchers can detect that motion. The vibrating beam cuts through the magnetic field, generating a voltage that is sensed by another electrode in the device.

"This is just the beginning," says Roukes, who sees the electrometer as a demonstration of what an integrated microelectromechanical system can do (SN: 7/26/97, p. 62). Other small-scale charge detection devices that use superconducting materials are much more sensitive, he says, but they operate best at a few thousandths of a degree above absolute zero. The mechanical electrometer can operate at slightly above 4.2 kelvins, the temperature of liquid helium. That's still very cold, but doing experiments above that temperature is "about a factor of 2 easier," says Ted Fulton, a physicist at Lucent Technologies in Murray Hill, N.J. Scientists would eventually like to have probes that work at room temperature.

Tiny electrometers could be used to "pick up electrical field signatures on the surface of a semiconductor," adds Fulton. With such probes, scientists could scan a semiconductor's surface, mapping out the distribution of charges on the material to gain a better understanding of it. With this in mind, he and his colleagues fabricated transistors that can sense individual electrons.

Before it can be used in any kind of scanning instrument, the new electrometer will need to get much smaller, Fulton notes. "Most of the interesting fine detail is very close together" on a semiconductor surface. Roukes expects that the electrometer can be scaled down further to make it more useful and to explore the physical limits of such devices.