Date of Award

Spring 1-1-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Victor. M. Bright

Second Advisor

J. Scott Bunch

Third Advisor

Y.C. Lee

Fourth Advisor

Siavash Pourkamali

Fifth Advisor

Robert Fitch

Abstract

Microelectromechanical resonators utilizing active transduction schemes are emerging as a strong alternative to traditional capacitive and piezoelectric devices. Thermally actuated piezoresistive readout devices are one such class that shows unique promise. Their unique ability to self-oscillate under DC bias through internal feedback is especially promising for both on-chip sensing and clocking applications. Previous work theorized through a lumped parameter system model that further miniaturization of designs should both increase the operating frequency and improve resonator performance. This work examined this assertion experimentally with the design, fabrication, and characterization of devices with area footprints as low as 50 μm2 (pad interconnects removed). Devices were fabricated in multiple I-shaped geometries on 340 nm and 2 μm thick n-type single crystal silicon (SCS) using different submicron patterning methods. Device operation in the VHF regime as both resonators and self-sustained oscillators was achieved in both ambient air and low vacuum conditions (50-70 Torr). Resonators were demonstrated up to 206 MHz, over 3x higher frequency than previous work, demonstrating quality factors >20,000 in vacuum and >10,000 in ambient air. Self-sustained oscillation was demonstrated up to 160 MHz (4x higher than prior work) in ambient air with peak-peak signal amplitudes up to 40 mV. Device operation as oscillators was examined using a laser sensing testbed to verify the mechanical frequency of oscillation and explore changes in device response to illumination. Frequency tuning and on/off control of the self-sustained oscillation was demonstrated by adjusting the laser power. Modeling and simulation was performed using COMSOL multiphysics software to examine structural modes and electrothermomechanical response in support of experimental findings.

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