Date on Master's Thesis/Doctoral Dissertation

12-2012

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Mechanical Engineering

Committee Chair

Bradshaw, Roger D.

Author's Keywords

Fabrication; piezoresistive; microcantilever beam; CFD model; gas sensor; damping

Subject

Electric resistors

Abstract

Symmetric piezoresistive microcantilever beams have been demonstrated in previous research to be capable of sensing the presence of surrounding gas. This occurs as the damping effect of the gas changes the beam resonance behavior. Device sensitivity has been increased dramatically after changing the symmetric beam base to an asymmetric beam base. This dissertation seeks to improve on beam fabrication and simplify the fabrication procedure compared to earlier approaches. By changing to a polymer mask and using new equipment, an entire wafer can be fabricated in far less time compared to the previous approach. While this new approach shows great promise, additional research is needed to demonstrate consistent device quality comparable with earlier approaches. This dissertation also focuses on the continued development of such devices with an emphasis on modeling to better understand the resonant behavior in a gas. Past work at the University of Louisville and elsewhere has relied upon simplified fluid mechanics models to relate changes in resonance with gas properties. The current work considers a combination of Stokes' oscillating cylinder model and computational fluid dynamics simulation to better characterize the damping effect including the effect of the rectangular cross-section and the presence of a boundary (the silicon handle layer) located 2 µm below the beam. The beam is induced to vibrate by electrical attractive forces at the end that change with applied driving electrode voltage and beam voltage. The electrostatic force, the displacement of the beam tip, the change of resistance of beam base due to piezoresistive effects, and the resulting signal received by the lock-in amplifier is established by a combination of analytical models and finite element simulation. This simulated output signal provides valuable insights to address issues of proper parameters to use during testing. This new information developed in this dissertation helps to advance the state of the art for microresonating beams for gas detection. This information is expected to play a key role as the systems in this work are transitioned to use in practice.

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