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

Characterization of an epoxy resin film, commonly known as SU-8, is presented using thickness shear mode (TSM) quartz resonator. The impedance-admittance characteristics of the equivalent circuit models of the unperturbed and coated resonators are analyzed to extract the storage modulus and loss modulus (G' and G). Those parameters are needed to establish SU-8 film as an effective wave-guiding layer in the implementation of guided shear-horizontal surface acoustic wave (SH-SAW) sensor platforms. Both cured and uncured polymer films are studied at the fundamental and third harmonic frequencies of the TSM resonators. The storage modulus (G') and loss modulus (G) of the SU-8 film approach constant values of 1.66 X 10(10) dyne/cm(2) and 6.0 X 10(8) dyne/cm(2), respectively, for relatively thicker films (>20 mum) at a relatively low frequency of 9 MHz. 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. The effect of temperature on the storage and loss moduli is determined for the range of -75 to 40 degreesC. It is found that the polymer approaches a totally glassy state below -60 degreesC. Exposure to water appears to follow Fickian diffusion behavior at short times and this exposure also results in changes to both G' and G. However, stability is rapidly reached with exposure to water, indicating relatively lower water absorption, consistent with the extracted diffusion coefficient. (C) 2004 Wiley Periodicals, Inc.