Date of Award

Fall 2003

Document Type

Thesis - Restricted

Degree Name

Master of Science (MS)

Department

Electrical and Computer Engineering

First Advisor

Josse, Fabian

Second Advisor

Hock, Jeffery

Third Advisor

Cernosek, Richard

Abstract

Thin polymer films are commonly used in the implementation of chemical and biochemical sensors, as partially selective and sensitive coatings for the detection of chemical agents, or as waveguiding layers in guided shear-horizontal surface acoustic wave (SH-SAW) sensor platforms. The design and implementation of these sensors rely on the characteristic information about the viscoelastic properties of the polymer films and subsequent changes upon exposure to environmental factors, such as liquid contact and temperature change. In this thesis, the use of the thickness-shear mode (TSM) quartz resonator is proposed for the rapid characterization of an organic resin (commonly known as SU-8) under various conditions. It is convenient to use an equivalent circuit model to describe the electrical behavior of the TSM resonator. With only a few lumped elements, the modified Butterworth-VanDyke (BVD) model simulates the electrical characteristics of the TSM resonator over a range of frequencies near resonance. This model can explicitly relate the lumped elements in the circuit to physical properties of the TSM, as well as the surface loading (liquid load and/or viscoelastic film). Fitting the circuit model to the measured admittance data allows the extraction the physical properties of the surface perturbation. Both hard-baked and unbaked polymer films are studied at the fundamental and third harmonic frequencies of the TSM resonators. The storage modulus (G') and loss modulus (G' ') are extracted at various thicknesses for the SU-8 films. The most accurate values for the extracted shear moduli G (G=G '+jG'') are obtained at high thickness where the viscoelastic contribution to the TSM response is substantial. Apparently, the polymer film is in a near glassy state due to the high value of G' and high ratio of G' to G'' at the operating frequency 9 MHz of the TSM resonator. Study of the effects of temperature variations and of water uptake also proves to be useful using this technique. This technique also shows to be very useful in the study of the polymer/solvent interaction and the sorption kinetics, which governs the chemical sensor responses in liquid. It is found that the polymer approaches a totally glassy state below -60 °C. It also reveals that the Fickian-diffusion model can be used to describe the sorption behaviors of the SU-8 film for water, with an effective value of 1.52 x 10·9 cm2 s·1 for the diffusion constant at room temperature. This value of diffusion constant for SU-8 film is almost one order lower than that for the PMMA film. The frequency shift of the SU-8 film exposed to water suggests a relatively lower water uptake by the SU-8 film (about 2.1 μg water sorbed by a 1200 μg SU-8 film). These results, combined with the actual extracted values of G' and G", indicate that SU-8 film can be used as an effective, stable waveguiding layer in the implementation of guided SH-SAW sensors in liquid environments.

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