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Si MEMS Record Set
Mar 2008
ITHACA, N.Y., March 18, 2008 -- A device that generates a 4.5-GHz signal, the highest achieved in silicon, has been developed at Cornell University. With a quality factor, or Q, close to 10,000, it pushes the upper limits of microresonator frequency, Cornell said in a statement.

MQ, which is a measure of how sharply tuned an oscillator is and, therefore, of the amount of energy stored in the resonator, commonly decreases with increasing frequency.

"You can trade off quality factor for frequency, so you want to maximize their product [frequency multiplied by Q]," said Sunil Bhave, an assistant professor of electrical and computer engineering at Cornell. "If both are high, designers have more leeway."
Scanning electron micrograph of a dielectrically transduced silicon bar resonator. The inset shows the thin (15 nms) dielectric layer. (Photo courtesy Cornell University OxideMEMS Lab)

Bhave and graduate student Dana Weinstein described the device at the 2007 IEEE International Electron Devices Meeting, held in Washington, D.C., in December (their paper appears in the conference proceedings).

The device uses of a silicon bar set into vibration by a process called 'dielectric transduction.' An alternating voltage is applied to an electrode separated from the bar by a dielectric, or insulator. Attractive forces between electric charges in the electrode and the bar create mechanical vibrations that travel back and forth along the bar like sound waves in a flute or organ pipe.

Previously, researchers have used an air gap as the dielectric. "Substituting a solid dielectric makes it easier to read out oscillations at higher frequencies, but the solid dielectric damps vibrations and reduces the efficiency of transmission of energy to the oscillator," the university said.

Weinstein found by mathematical analysis that efficiency could be increased by moving the dielectric layers from the ends of the bar partway toward the middle. The ideal positions are two-thirds of the way from the middle to the end of the bar, which are the points of maximum strain when it is vibrating.

The resulting device is a silicon bar 8.5 microns (millionths of a meter) long, 40 µms wide and 2.5 µms thick, divided by two dielectric layers of silicon nitride just 15 nms (billionths of a meter) thick. The 4.5-GHz signal is the ninth harmonic of the resonator's fundamental vibration along its length. The method could produce resonators with frequencies exceeding 10 GHz, the researchers said.

Weinstein's research is supported by a National Defense Science and Engineering Graduate Fellowship awarded by the US Department of Defense. The devices were manufactured with use of the facilities of the Cornell NanoScale Science and Technology Facility and the Michigan Nanofabrication Facility, both part of the National Nanofabrication Infrastructure Network funded by the National Science Foundation.

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