Electrostatic comb drive for vertical actuation Page: 8 of 14
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folded beam flexures,.
Figure 7. SEM of poiysiiicon micromirror
Polysilicon surface micromachining and SOI surface micromachining were employed to microfabricate the devices. The
polysilicon process was carried out by MCNC using the MUMPS service. 'Ihe polysilicon microstructures were freed from the
substrate with a CO2 supercritical release and the surfaces were passivated with anti-stiction self-assembled monolayers'6.
Stress was evident from two sources: the intrinsic residual stress in the polysilicon films, and thermal residual stress from metal
coatings on the polysilicon suspended mass. The former caused the suspended plate to bow downwards whereas the latter gen-
erAt an tJW pward. c..rv...... In .ut..- workI.2. ....e ..wo 1. c U-d A---- deige to copnaefr a -o As fo [h rlT -mi---a-
CGIZU i! fJWU tU~lVALUV l ILCr WUM% UIM LWVU LU I UUb~irIvU LU LU11iIIKIRMdlZ GAU A OUL uii. Ht11theI Sul 1loultJIa-
rhining, large anrhnrc werp rlPeignepd en that inAerrnutting of the varifiial hturAA nvirlp uinrlrnnath tho anhnrc AM nnt
completely etch away. The SOI structures have the advantage of a smoother surface for a better optical mirror surface as op-
posed to the polysilicon surfaces. Another advantage of the SOI process is the potential of etching the backside of the wafer to
create a transmissive window or the potential to coat both sides of the suspended mass with metal thin films, avoiding residual
stress induced bimetallic curvatures resulting from only coating a single side. Both the polysilicon vertical actuators and the
SOI vertical actuators were successfully tested.
-'mirror nAms FIILtr riliei
SineWave fi f l I 1 fl
Generator k- ' ITl-] I H I
N*r li I I V -- syn-
Figure 8. Circuit used to measure capacitive sensor
5. IN SITU CAPACITIVE POSITION SENSING
A capacitor was built into the micromirror for the purpose of measuring the displacement of the mirror above the substrate.
The bottom plate of the capacitor is formed by a layer of polysilicon which is the same area as the mirror itself. The top plate
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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/m1/8/: accessed May 27, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.