CMU MEMS Laboratory Publication Abstract


in Ph.D. Thesis, May 2002, Carnegie Mellon University, Pittsburgh, PA.
Gyroscope and Micromirror Design Using Vertical-Axis CMOS-MEMS Actuation and Sensing
H. Xie
CMOS-MEMS processes leverage mainstream CMOS technologies to achieve on-chip interface and signal processing circuitry, scalability, multi-vendor accessibility, short design cycles, more on-chip functions and low cost. However, 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. The resultant microstructures are flat and have no release holes. Electrically isolated silicon is obtained using a silicon undercut. This process is suitable for applications that require large mass and flat surfaces such as inertial sensors and micromirrors.

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 µm out-of-plane displacement for a micromirror scanning.

Several thin-film and DRIE devices are fabricated and characterized. A 0.5 mm by 0.5 mm microstage moves 0.3 µm 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/Hz½ and 0.5 mG/Hz½, respectively. The noise floor of a lateral-axis gyroscope is improved from 0.8 °/s/Hz½ for the thin-film CMOS-MEMS process to 0.02 °/s/Hz½ for the DRIE CMOS-MEMS process.

Solutions for improving the gyroscopes’ performance and yield are proposed, including a new process flow for independently controlling silicon undercut and a vertical-axis electrostatic force cancellation technique for compensating the off-axis motion.

The electrostatic micromirror rotates 5° at 18 V d.c. The thermally actuated micromirror rotates 17° 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 µm and 10 µm, respectively, are achieved. Cross-sectional images of 500 × 1000 pixels covering an area of 2.9 × 2.8 mm² are acquired at 5 frames/s.
© 2002 Carnegie Mellon University, Department of Electrical and Computer Engineering.
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