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Microcantilevers: ORNL's Sensors with Sensitivity

Thanks to a problem with a high-tech microscope, researchers at the Department of Energy's (DOE) Oak Ridge National Laboratory (ORNL) have developed microscopic sensors. These hairlike, silicon-based devices are at least 1,000 times more sensitive and 1,000 times smaller than currently used sensors. Each microsensor spans the width of a human hair.

They can detect and measure relative humidity, temperature, pressure, flow, viscosity, sound, natural gas, mercury vapor, and ultraviolet and infrared radiation. They show potential as biosensors-devices that can detect DNA sequences and proteins.

In 1991 Thomas Thundat of ORNL's Health Sciences Research Division (HSRD) was examining the effect of humidity on DNA using an atomic force microscope, which employs natural repulsion between atoms' electron clouds to image surface variations. The humidity affected the performance of the microscope's cantilever, which like a phonograph stylus tracing record grooves, is used to map the atomic mountains and valleys of surfaces. It occurred to him that the cantilever is a potential sensor, and from this observation came a new series of physical and chemical sensors developed at ORNL. Fortunately, newly available micromachining techniques make it possible to fabricate microcantilever sensors that are rugged and extremely sensitive yet cost little and consume little power.

Microcantilevers of silicon or silicon nitride have been made that are smaller than this period. These "microscopic diving boards" project from miniature chips about the size of a grain of rice. For this innovation of microminiature sensors, ORNL developers Thundat, Eric Wachter, Mitch Doktycz, Rick Oden, and Bruce Warmack, all in HSRD, were elected to the International Hall of Fame of the Inventors Clubs of America. ORNL's microcantilever sensor technology received an R&D 100 Award from R&D Magazine. The awards recognize the year's most significant technological innovations.

"We showed that a microcantilever would bend in a measurable way if its tip is coated with a material that attracts another material from the air," Warmack says, "For example, a gold-coated cantilever absorbs mercury vapor, which stiffens the cantilever, causing it to bend and changing the way it vibrates. A gelatin tip absorbs water, measuring humidity."

"These sensors can also respond sensitively to heat," Thundat says. "A silicon microcantilever coated with aluminum bends more with rising temperature because aluminum expands more than silicon. Such a device can measure temperature and even detect infrared radiation and heat-generating chemical reactions."

When set in motion, microcantilevers have a natural vibration that changes in the presence of sound waves or a fluid (enabling measurements of viscosity and pressure).

Changes in cantilever position or vibration rate can be detected by measuring wobble in reflected laser beams. "Future silicon devices," Warmack says, "will probably be based on piezoresistance-changes in electrical resistance induced by increased bending or reduced vibration."

One of ORNL's patented microminiature sensor technologies has been licensed to Consultec, Inc., which has fabricated a prototype mercury vapor sensor and infrared thermometer. Sensor development has been funded by DOE's Office of Health and Environmental Research, Measurement Sciences Division.

ORNL, one of the Department of Energy's multiprogram national research and development facilities, is managed by Lockheed Martin Energy Research.

(Photo of Thomas Thundat with microminiature sensor available on request) .