CMU MEMS Laboratory Publication Abstract


in M.S. Thesis, July 2003, Carnegie Mellon University, Pittsburgh, PA.
CMOS MEMS Resonant Mixer-Filters
J. Stillman
This research expands the technology developed for electromechanical filters to CMOS MEMS mixer-filters. Simple mixer-filters composed of coupled CMOS MEMS resonators mix, downconvert and filter electronic inputs. The devices apply the CMOS MEMS advantages of integration of mechanical structures with CMOS circuits and multiple conductive layers on a single released structure to the growing field of MEMS signal processors.

A model is developed for the mixing and filtering functions of electrostatically actuated, laterally moving CMOS MEMS structures, with a brief discussion of their spectral response and distortion products. Mechanisms for coupling multiple resonators to form arbitrary filter shapes are discussed, including mechanical spring coupling, parallel operation, cascading resonators with different center frequencies, and electrostatic coupling. An integrated CMOS circuit and the effects of its input impedance on the frequency response are discussed.

Topologies for very simple CMOS MEMS resonators based on laterally vibrating beams are explored. A design which cuts down on feedthrough from the drive electrode to the sense electrode by separating the electrodes by several microns on a square attached to the end of the cantilever beam is presented. A model for the shape function and frequency response of this resonator matches finite element simulation closely. Tuning mechanisms, including traditional electrostatic spring softening and introducing axial-tension electrostatic spring softening, are compared. Guidelines for resonator design in CMOS include curl matching to accommodate vertical residual stress gradients, inclusion of etch holes and open areas for releasing high aspect ratio structures, and minimizing the effects of mask misalignment. Optimum sizing of a CMOS MEMS resonator is extremely application-dependent, but for the cantilever with an electrode square, larger, stiffer resonators are better because of a higher maximum polarizing voltage and superior robustness to process variations and voltage noise.

Fabricated devices are tested on a custom printed circuit board in vacuum. A bandpass filter composed of two resonators has a stopband rejection of 28 dB, a center frequency of 398.5 kHz, a ripple of 37%, and a Q of 1,533 when operated at 8 μTorr with a 23-V polarizing voltage. Mixing, downconverting resonators demonstrate Qs of more than 3000 at 8 μTorr with input frequencies of 15 MHz and 15.4 MHz and input magnitudes of 1 V.

Fixed-fixed 119 μm x 1 μm x 5 μm beams demonstrated crippling residual compressive stress, which brought the resonance frequency to 5 Hz from the stress-free resonance frequency of 362 kHz. Additional higher-order filters and alternate topologies are fabricated but not yet tested.

© 2003 Carnegie Mellon University, Department of Electrical and Computer Engineering.
Full paper (PDF) (opens in new window).

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