Gyroscope and Micromirror Design Using Vertical-Axis CMOS-MEMS Actuation and Sensing

摘要

Most CMOS-MEMS processes are based on thin-film microstructures which often require wet etching for release. The Carnegie Mellon thin-film CMOS- MEMS process requires only dry-etch steps and is maskless, but has drawbacks of curling due to residual stress and temperature coefficient of expansion of the multi-layer structures. This thesis introduces a deep reactive-ion-etch (DRIE) CMOS-MEMS process which incorporates bulk Si into microstructures using backside etch. Electrically isolated silicon is obtained using a silicon undercut. A technique for vertical-axis sensing and actuation using comb-finger sidewall capacitance is developed. For vertical-axis sensing, this technique has small parasitic capacitance compared to the counterparts that have an electrode on the substrate. For vertical actuation, this technique has a very large gap to substrate set by the process, so that the actuation range is not limited. A unique curled comb drive design demonstrates a maximum 62 micrometer out-of-plane displacement for a micromirror scanning. 0.5 mm by 0.5 mm microstage moves 0.3 micrometers vertically at 14 V d.c. Thin-film and DRIE z- axis accelerometers (both are about 0.5 mm by 0.6 mm) have noise floors of 6 mG/(square root Hz) and 0.5 mG/(square root Hz), respectively. The noise floor of a lateral-axis gyroscope is improved from 0.8 degrees/s/(square root Hz) for the thin-film CMOS-MEMS process to 0.02 degrees/s/(square root Hz) for the DRIE CMOS-MEMS process. The electrostatic micromirror rotates 5 degrees at 18 V d.c. The thermally actuated micromirror rotates 17 degrees at 12 mA current and has been installed into an endoscopic optical coherence tomography imaging system for in-vivo imaging of biological tissue. Transverse and axial resolutions of roughly 20 micrometers and 10 micrometers, respectively, are achieved. Cross- sectional images of 500 x 1000 pixels covering an area of 2.9 x 2.8 square millimeters are acquired at 5 frames/second.