Electrostatic comb drive for vertical actuation Page: 11 of 14
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voltage. The maximum displacement approaches 1.2 pm corresponding to the theoretical plot as indicated in Fig. 6. For dynam-
ic responses, a square (step) input voltage illustrates the damping and time constant of the microstructure (Fig.12). Since the
levitation of the suspended mass reduces the optical path distance, it is represented by a drop in amplitude. As observed in Fig.
12, the time constant for the electrostatic levitation is approximately 50 msec, and the time constant for the restoring drop is
approximately 400 msec, matching the capacitive sensing results in Fig.9. The difference in time constants is due to the squeeze
film damping effect39 required to displace the trapped air in the small gap between the suspended mass and the substrate. To
reduce this damping effect, an optimization must be carried out, with the gap and the size of the suspended mass as the optimi-
zation variables.
6 Volt Sqare Wave Drive
1200.Frequency 1 Hz1000
0 800
60
00
C
200
0
-2000 0.2 0.4 0.6 0.8 1
Time in Seconds
Figure 12.dynamic response of square wave input voltage
8. CONCLUSION
This paper explores the application of electrostatic levitation forces in comb finger drives for vertical small-displacement
(1-2 pm) microactuators. The layout of this microactuator is simple and standard polysilicon or SOI surface micromachining
processes can be utilized. On-chip in situ capacitive position sensing provides a feedback loop to control the vertical displace-
ment down to accuracies better than 5 nm resolutions. Interferometry for measuring the displacement with different input volt-
ages was set up. These initial tests of the vertical microactuator demonstrated the match between theoretical predictions and
experimental verifications. For future designs, it is desired to reduce the squeeze film damping effect to increase the speed of
response by reducing the area of the suspended mass. The device presented in this paper can become an integral component in
phase shifting interferometry.
9. ACKNOWLEDGMENT
The authors would like to thank Dr. Michael Houston and the H Plus Technologies for their assistance in the supercritical
release and the SAM coatings for anti-stiction treatments. Mr. Bill Benett is credited for designing the mechanical compartment
for the lens-microactuator system with alignable x-y-z stages. Mr. Ronald Frattaroli built this mechanical compartment. Ms.
Asuncion Lemoff is appreciated for her assistance in testing of the devices. Funding from LLNL's Engineering techbase funds
is appreciated. This work was performed under the auspices of the US Department of Energy by the Lawrence Livermore Na-
tional Laboratory under contract no. W-7405-ENG-48.I
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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/m1/11/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.