Characterization of epoxy resin SU-8 film using thickness-shear mode (TSM) resonator

Characterization of an organic resin (commonly known as SU-8), being investigated as waveguiding layer in the guided shear-horizontal surface acoustic wave (SH-SAW) sensor platforms, is presented. The impedance-admittance characteristics of the equivalent circuit models of both the unperturbed and coated resonators are analyzed to extract the storage modulus (G') and loss modulus (G"). The accuracy of the extracted shear modulus parameters occurs at high thickness where viscoelastic contribution is substantial. Apparently the polymer film is in a glassy state due to the high value of G' and high ratio of G' to G". Device resistance is less than 20 Omega when the thickness of film is less than 5 mum, beyond which viscoelastic properties significantly contribute to the frequency response. The extraction of the storage and loss moduli as a function of temperature ranging from -75 degreesC to 40 degreesC is also shown in order to study the effects of temperature on the shear modulus. Below -60 degreesC, both the frequency shift and R(2) (the electrical resistance arising from the SU-8 film) are less dependent on temperature, which indicates that the polymer is almost totally elastic below -60 degreesC. Exposure of SU-8 film to water and solutions of organic solvents such as acetone and ethanol also indicates, through frequency shift and resistance change, some degree of change in G' and G". However, stability is rapidly reached with exposure to DI water, indicating relatively lower water absorption.