For decades, quartz-crystal oscillators have served as clocks in all sorts of electronic gadgets. Placing a voltage across the crystal causes it to resonate at a predictable frequency, allowing all parts of a circuit to operate in synchrony. But these quartz clocks are relatively bulky, their size a significant barrier to shrinking circuits. Recently, researchers have developed silicon versions that offer smaller, lower-power, tunable alternatives to quartz.
Now, researchers at Cornell University have created a silicon microresonator that vibrates at 4.51 gigahertz, the highest frequency ever recorded in such a silicon device. Other researchers have demonstrated silicon microresonators that vibrate up to 1.5 gigahertz, say the Cornell researchers.
The Cornell microresonator, which was fabricated by Sunil Bhave, an assistant professor of electrical and computer engineering at Cornell, and graduate student Dana Weinstein, reaches the high frequency without compromising signal strength and purity–how sharply tuned the signal is to a particular frequency. Usually, as frequency increases, the Q factor, which is a measure of an oscillator’s stability, drops. Essentially, the Q factor is a measure of quality: it indicates how long an oscillator can maintain a vibration at a certain frequency. A high Q factor means that the oscillations die out more slowly. The higher the number, the better. The Q factor for the Cornell device at 4.51 gigahertz is close to 10,000, which compares well with quartz resonators. “The main idea of the design of the resonator is that it’s actually projected to work even better at higher frequency,” Weinstein says. “We’re trying to push the limit and get to higher frequency for a variety of applications.”
Exactly what those applications would be are yet to be known, but the high-frequency resonators could find uses as timekeepers for telecommunications and microprocessing. One of the advantages of silicon microresonators is that they can be integrated directly into microchips using conventional manufacturing techniques, making them cheaper to produce and easier to fabricate small. Also, multiple resonators of different frequencies could be put on the same chip, says Ville Kaajakari, an assistant professor of electrical engineering at Louisiana Tech University. In a cell phone, for example, high-frequency resonators could filter out interference from other sources of radio signals. The Cornell device is 8.5 micrometers long and 40 micrometers wide, compared with a width of about a millimeter for a quartz resonator.
The novel design of the microresonator allows it to reach its high frequency, Weinstein says. Other silicon resonators have a dielectric material, or insulator, on the ends to enhance their ability to transmit energy to the resonator. By mathematical analysis, Weinstein found that positioning the dielectric material–in this case, a thin film of silicon nitride–in the body of the microresonator makes this transmission more efficient. She is now working on increasing the frequency even more. “Now, it’s a fabrication challenge,” she says.