MEMS Sources Offer Alternative To Quartz

Micromachining techniques have delivered resonators and reference oscillators that are a fraction of the size of conventional ceramic and quartz-crystal clock oscillators.

Frequency generation has long relied on the quartz-crystal resonator as a timekeeper. Because of the required wavelengths (for typical frequencies of 10 and 20 MHz), however, quartz resonators cannot be miniaturized beyond about 1 or 2 mm on a side. In contrast, microresonators based on microelectromechanical-systems (MEMS) technology promise the spectral performance of quartz crystal in a fraction of the size. Starting with a resonator beam size of 30 × 8 µm, the 19.2-MHz MRO-100 micro-oscillator from startup Discera (Campbell, CA) can be fabricated as part of a miniature monolithic multiband wireless transceiver solution. For evaluation purposes, the miniature sources are supplied in standard 3 × 3-mm and 4 × 4-mm chip-scale packages.

Discera, still relatively unknown outside of their customer base, made waves at the recent Microwave Theory & Techniques Symposium (Philadelphia, PA, June 8-13, 2003) with prototype results for their MRO-100. The company, founded in 2001 by Dr. Clark T.-C. Nguyen, a Professor of Electrical Engineering from the University of Michigan, and Rick Snyder, CEO of Ardesta LLC, is financed by means of seed funding from Ardesta.

The company is currently sampling the MEMS-based micro-oscillators (Fig. 1) to a number of companies involved in wireless communications. Discera's resonators and oscillators are fabricated as micromachined mechanical structures, with resonant beams that actually exhibit microvibrations at precise frequencies. And since these sources can be produced at a fraction of the size of traditional quartz-crystal or surface-acoustic-wave (SAW) oscillators, wireless handset manufacturers currently employing multiple-stage receiver architectures are interested in the potential of a tiny, tri-band MEMS oscillator.

The one "fly in the ointment" for this technology, however, is that the vibrating elements within the MEMS microresonators and micro-oscillators have such low mass. As a result, they must be isolated at certain frequencies from air molecules, thus requiring a package capable of maintaining a fairly low-vacuum environment. Similarly, the package must be hermetic to isolate any contamination, such as water molecules, from the low-mass resonant structures. In spite of the low mass of these structures, however, they are rugged enough to withstand even the shock and vibration endured by most cellular telephones.

Resonators, of course, are building blocks for RF architectures, and can be used for a number of different components, including filters, oscillators, and switches. At present, the company is achieving resonators with quality factors (Q's) of 10,000 at 20 MHz, based on a prototype fabrication process. According to Discera's CEO Didier Lacroix, the Q's should easily surpass 20,000 at 20 MHz once the firm makes the transition to a higher-volume production MEMS process. Compare these values to Q's of about 20 for an integrated-circuit (IC) filter and about 2000 for a SAW resonator.

Because they are so small, a bank of the miniature resonators can be fabricated onto a single die to produce a chip-scale switch/filter bank. Discera's micro-oscillators are based on polysilicon resonators fabricated with a standard silicon MEMS/semiconductor process, allowing integration with monolithic active and passive components.

The company's current batch of MRO-100 micro-oscillators are designed for +2.5-VDC operation—a considerably lower operating voltage than commercial MEMS components previously announced from other suppliers. During measurements, a 3 × 3-mm surface-mount-packaged 19.2-MHz oscillator with +2.45-V internal regulated supply was mounted on an oscillator test board and powered by a +3-VDC 20-mm coin-cell battery (with 220 mAh rating). Current draw was just 2.7 mA in this discrete implementation, although it is expected that a more integrated oscillator would operate with considerably less current. Output signals were AC coupled from the test board at an amplitude of 650 mV peak to peak. The measured output power at 50 Ω was −10.5 dBm. The measured phase noise was −106 dBc/Hz offset 1 kHz from the carrier, reaching a noise floor of −110 dBc/Hz (Fig. 2).

According to Lacroix, the current prototype process achieves critical dimensions of 1 µm. With improvements in lithography expected with the shift to a larger-scale production process, stability and phase noise should improve dramatically. The resonator/oscillator design included mechanical temperature compensation—allowing for structural expansion and contraction with increases and decreases in temperature, respectively. The mechanical temperature compensation, which support stable frequency performance over a wide temperature range of −40 to +150°C, eliminates the need for power-inefficient thermistor- or oven-based temperature compensation.

In addition to their benefits of small size and low power, the MEMS-based MRO-100 micro-oscillators feature versatile modulation capability, tuning by means of applied voltage. Capable of tuning from the center frequency by as much as 1000 PPM/V, a tuning voltage can be used to set the final operating frequency, make adjustments to a reference frequency, or introduce modulation on the reference source (with setting time of less than 1 ms). The MEMS-based micro-resonators can also be modulated on and off, allowing them to double as RF switch elements within more complex switched filters.

The company is currently exploring packaging options for its RF MEMS devices. Discera offers its technology as packaged, discrete devices, but will also license its patented microresonator technology to other companies, such as IC developers, interested in incorporating miniature reference oscillators in their designs. Discera, Inc., 51 East Campbell Ave., Suite 102, Campbell, CA 95008; (408) 376-4150, FAX: (408) 376-4151, e-mail: [email protected], Internet:

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