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 Ω when the thickness of film is less than 5 μm, 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°C to 40°C is also shown in order to study the effects of temperature on the shear modulus. Below -60°C, both the frequency shift and R₂ (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°C. 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.