Date of Award

Fall 2005

Document Type

Thesis - Restricted

Degree Name

Master of Science (MS)

Department

Electrical and Computer Engineering

First Advisor

Josse, Fabien

Second Advisor

Heinrich, Stephen M.

Third Advisor

Hossenlopp, Jeanne

Abstract

Chemical sensing applications have seen tremendous advancements in recent years. With the increasing demand of micro sensors in the fields of biowarfare and medicine, there is a need for the investigation of miniaturized, effective and highly sensitive devices for the detection of gaseous and aqueous contaminants/antibodies. Recent advancements in MEMS fabrication techniques have made it possible to design, produce and integrate miniature sensors. One such MEMS device commonly investigated is the microcantilever sensor. In this thesis dynamic hybrid microcantilever chemical sensors are investigated for detection in vacuum. The hybrid geometry consists of a base elastic layer (Silicon) and a sensitive viscoelastic coating (Polyisobutylene). The hybrid geometry presents some interesting problems. Analytical expressions for the stress-strain relations, neutral axis and the moment-curvature relationship are developed for the dynamic hybrid beam. A detailed analysis reveals that the neutral axis of the dynamic hybrid beam is time dependent. Approximations to this parameter are proposed and their accuracy is compared with the time-varying response and an established technique proposed by Oberst. Using the moment-curvature relation, expressions for the natural and resonance frequencies, frequency shift, quality factor, sensitivity and limit of detection are developed. A detailed analysis of viscoelastic behavior in a chemical sensing environment reveals that the response of the sensor is due to a combination of mass loading and viscoelasticity effects. The change in the viscoelastic property of the coating is not negligible and must be taken into account when designing a sensor to detect accurate quantities of analyte molecules. The quality factor of the hybrid configuration in vacuum is significantly influenced by the damping of the coating, with or without analyte sorption. Simulations are presented for each of the parameters described above for the detection of toluene vapor on a PIB coating. The sensitivity of the sensor can be tailored by optimizing the geometry of the micro-beam. However, each parameter has a combination of effects on the sensor system and a blind increase or decrease in their values may not always contribute to a better and more sensitive response. Hence, a detailed analysis is presented in this thesis to investigate the influence of the geometry and material properties on the resonance response parameters.

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