CMU MEMS Laboratory Publication Abstract


in M.S. Thesis, June 2004, Carnegie Mellon University, Pittsburgh, PA.
Damping in CMOS-MEMS Resonators
J. Brotz
This research examines the energy loss in micromechanical resonators fabricated in a CMOS-MEMS process. Characterization and understanding of energy loss is a first step to optimal design in MEMS mixer and filter applications. The known energy loss mechanisms for micromechanical resonators - air damping, acoustic anchor loss, thermoelastic damping, and internal friction - are discussed. Theory is given to support expected levels of damping for air damping and thermoelastic damping; design methods to reduce energy loss are discussed for acoustic anchor loss. Squeeze-film and Stoke’s damping in electrostatic gap resonators are analyzed over varying pressure. A tuning fork method and a quarter-wavelength method for reducing acoustic anchor loss are given. Cantilever and fixed-fixed resonator topologies are designed in order to test for air damping and acoustic anchor loss. Air damping theory matches the measured data from 10 µTorr to atmospheric pressure with 25%-69% error. The quality factor becomes fixed in value for pressures below about 500 mTorr. The cantilever tuning fork shows a decrease in damping by 67% over a single cantilever and the fixed-fixed tuning fork by 73% over a single fixed-fixed beam. The free-free beam provided a minimal decrease in damping. Thermoelastic damping is theoretically shown to be negligible for beam widths smaller than 5 µm, and internal friction is believed to be negligible as well, although it has not been quantifiable to this point.
© 2004 Carnegie Mellon University, Department of Electrical and Computer Engineering.
Full paper (PDF) (opens in new window).

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