System Senses Individual Electrons

Kathleen G. Tatterson

MURRAY HILL, N.J. -- A system that can monitor and image individual electrical charges on and below the surface of semiconductor devices could be the next step in the never-ending quest for smaller, more powerful integrated circuits.
Scientists at Bell Laboratories have combined technologies from their single-electron transistor and the scanning probe microscope to develop the single-electron transistor scanning electrometer. According to the Bell Labs team, the technology will help bring forth the day when only a handful of electrons will control a device's performance. The instrument could aid in locating charged dopant atoms in semiconductor material. Typically, a single-electron microscope senses the combined electric field of up to 300 electrons at one time, explained Harald F. Hess, a researcher at Bell Labs.
"The location and the amount of dopants and how they release electrons is critical to device performance," he said. With the electrometer, scientists can image such charges with improved sensitivity.

The single-electron transistor scanning electrometer can produce images of single electrons and even small fractions of electrons.

The single-electron transistor is a cryogenic device of microscopic size with an electrical resistance that is very sensitive to the electric fields coming from nearby electric charges. Its sensitivity allows it to detect single electrons or as little as 1 percent of an electron's electromagnetic field.
The probe is made of a sharpened glass fiber that looks like a sewing needle, onto which scientists have planted the device's circuitry. Its fine tip tapers to a small, almost flat area at the end measuring 100-nm wide. When near a semiconductor or other surface, the probe picks up small signals and measures the total charge to a small fraction of an electron. An "electric" image of the charge distribution emerges as the probe scans back and forth over the area.
Hess considers the device a research tool and said that the technology will not be ready for commercialization for another 10 years. "By pushing the frontiers of technology this far, we gain expertise and insight for closely related, more applied measurement problems," he said. The team is working to shrink the device, make it operate in a more realistic environment (it operates at only cryogenic temperatures of 2458 °F) and produce images with finer details.