Electrostatic Transduction for MEMS Resonators

The vast majority of RF systems currently in production use some form of the heterodyning architecture developed by Edwin Armstrong 75 years ago. Today, this architecture relies on discrete components such as quartz crystals and ceramic filters to provide frequency selection and stable references. However, quartz and ceramics cannot be easily integrated with on-chip circuitry, which precludes a fully monolithic wireless transceiver. 
In recent years, high quality factor (Q) electrostatically transduced micromechanical resonators have emerged as a possible alternative to quartz and ceramic components. The key benefit of lateral air-gap resonators is the ability to fabricate multiple frequencies in a single lithography step. This dissertation presents experimental data proving that microresonators fabricated from low-temperature poly-SiGe films (205 MHz Bulk Longitudinal Resonator with Q of 3,500 and 84 MHz Shear Ring Resonator with Q of 850) have comparable Q's to resonators fabricated in polysilicon.
Fully differential (two electrodes for differential actuation and two electrodes for differential sensing) electrostatic transduction is introduced as a powerful technique for minimizing the ohmic resistance of the proof-mass and reducing capacitive feedthrough. The technique is experimentally verified by characterizing a 173 MHz poly-SiC Lame-mode resonator with a Q of 9,300 in air. A fully differential 2-D Lame checkerboard filter consisting of five mechanically-coupled Lame-mode resonators and 173 MHz center frequency, < 2 dB pass-band ripple and 12 dB stop-band rejection is demonstrated.
Differential air-gap electrostatic transduction enables characterization of microresonators using two-port transmission measurements. However, their large motional impedance makes integration with CMOS mandatory. Internal electrostatic transduction, in which the air-gap is filled with a high-dielectric-constant (high-K) material, offers orders-of-magnitude higher transduction efficiency for excitation and detection of bulk acoustic modes. Using Si3N4 as the dielectric material and single crystal silicon as the resonator material, a 121 MHz Bulk Longitudinal Resonator demonstrates internal electrostatic transduction, with a measured Q of 2,100 at ambient pressure and a 9.2 kohm motional impedance.
Tsu-Jae King
Oliver M. O’Reilly
Publication date: 
February 28, 2005
Publication type: 
Ph.D. Dissertation
Bhave, S. A. (2004). Electrostatic Transduction for MEMS Resonators. United States: University of California, Berkeley.

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