Electrostatic comb drive for vertical actuation Page: 3 of 14
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Electrostatic comb drive for vertical actuation
Abraham P. Lee, Chuck F. McConaghy, Peter A. Krulevitch, Eugene W. Campbell, Gary E. Sommargren,
Jimmy C. Trevino*
Lawrence Livermore National Laboratory, MS L-222, Livermore, CA 94550
Microtechnology Center, Electronics Engineering Technologies Division
Keywords: electrostatic comb drive, vertical actuation, vertical position control, interferometry
The electrostatic comb finger drive has become an integral design for microsensor and microactuator applications. This pa-
per reports on utilizing the levitation effect of comb fingers to design vertical-to-the-substrate actuation for interferometric ap-
plications. For typical polysilicon comb drives with 2 pm gaps between the stationary and moving fingers, as well as between
the microstructures and the substrate, the equilibrium position is nominally 1-2 pm above the stationary comb fingers. This dis-
tance is ideal for many phase shifting interferometric applications. Theoretical calculations of the vertical actuation character-
istics are compared with the experimental results, and a general design guideline is derived from these results. The suspension
flexure stiffnesses, gravity forces, squeeze film damping, and comb finger thicknesses are parameters investigated which affect
the displacement curve of the vertical microactuator. By designing a parallel plate capacitor between the suspended mass and
the substrate, in situ position sensing can be used to control the vertical movement, providing a total feedback-controlled sys-
tem. Fundamentals of various capacitive position sensing techniques are discussed. Experimental verification is carried out by
a Zygo distance measurement interferometer.
The laterally driven electrostatic comb drive resonant structure has become an integral component in numerous MEMS de-
vices over the last 8 years 1. Applications are very broad and include accelerometers3-6, gyroscopes , impact actuators, micro-
mechanical filters9, and micromechanical voltmeters0. Most of these devices utilize the lateral motion of engaging and
disengaging the interdigitated comb fingers. Constant electrostatic force is generated with an applied DC bias voltage', inde-
pendent of the position of the suspended mass. This same device generates a levitation force on the suspended structure as a
bias voltage is applied, due to the asymmetric distribution of the electrical fields 11. However, this levitation force is often the
source of unwanted out-of-plane motion causing tilting to many laterally-driven sensors and actuators ". One exception is in
the accelerometer designed and fabricated by Yun and Howe3, where the levitation force was designed to provide the feedback
force for a E-A sensing loop. Comb finger drives have also been applied towards integrated micro optical systems12. By com-
bining comb fingers with microfabricated hinges13, many devices including laser-fiber packaging modules14, micro optical
scanners15, and even raster-scanning displays have been demonstrated6
Parallel motion of micromirrors can be used for interferometry in phase shifting, modulating Fabry-Perot cavities, or the
construction of extemal-cavity semiconductor-laser modules. For vertical out-of-the-plane actuation in optical applications,
most existing devices are used as tilting mirrors for imaging, displays17, or scanning. Parallel motion micromirrors by Comtois
et al.'8 utilize parallel plate capacitive configurations where a suspended micromirror is attracted to a bottom plate via an elec-
trostatic force. The parallel plate configuration has an unstable region where the mirror will collapse against the bottom plate
with infinitesimal amount of electrostatic force, and stand-offs must be designed to avoid shorting out the biased electrodes.
Interferometry is an ideal application for electrostatically-driven microactuators because the required actuation force is low
*. Further author information -
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Lee, A. P., LLNL. Electrostatic comb drive for vertical actuation, article, July 10, 1997; (https://digital.library.unt.edu/ark:/67531/metadc619911/m1/3/: accessed April 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.