Electrostatic Comb Drive for Resonant Sensor and Actuator Applications


Interdigitated finger (comb) structures are demonstarted to be effective for electrostatically exciting the resonance of polycrystalline-silicon (polysilicon) microstrucutres parallel to the plane of the substrate. Linear plates suspended by a pair of folded-cantilever truss as well as torsional plates suspended by spiral and serpentine springs are fabricated from a 2 um-thick phosphorus-doped low-pressure chemical-vapor deposited (LPCVD) polysilicon film.  Three experimental methods are used to characterize quasi-static and resonant motions: microscopic illumination, observation with a scanning-electron microscope (SEM), and capacitive sensing using a frequency-modulation technique. Resonant frequencies of the laterally driven structures range from 8kHz to 80 kHz and quality factors range from 20 to 130 at atmospheric pressure, to about 50,000 in vacuum (10^-7 torr). For linear structures suspended with compliant springs, a static electromechanical transfer function of 40 nmV^-2 is demonstrated. Resonant vibration amplitudes of up to 20 um peak-to-peak are observed.

First-order mechanical theory is found to be adequate for calculating spring constants and resonant frequencies, using a Young's modulus between 140 and 150 Gpa and neglecting residual strain in the released structures. Finger gap is found to have a more pronounced effect on comb characteristics than finger width or length, as expected from simple theory.  A finite-element program is used to simulate the vertical levitation associated with the comb drive.  This phenomenon is due to electrostatic repulsion by image charges mirrored in the ground plane beneath the suspended structure and is characterized as an electrostatic spring.  As a result, the applied dc bias modulates the vertical resonant frequency. By electrically isolating alternating drive-comb fingers and applying voltages of qual magnitude and opposite sign, levitation can be reduced by an order of magnitude, while reducing the lateral drive force by less than a factor of two.  These results agree well with first-order theory incorporating results from finite-element simulations.

A two-dimensional manipulator based on an orthogonally coupled comb-drive pair is designed and analyzed for use with a resonant micomotor and a microdynamometer.  These devices can be fabricated and tested with the same technology and methods as the basic comb-drive structures.

Publication date: 
November 30, 1990
Publication type: 
Master's Thesis
Tang, W. C. (1990). Electrostatic Comb Drive for Resonant Sensor and Actuator Applications. United States: University of California, Berkeley.

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