Theoretical and Experimental Investigation on the Use of a Surface Acoustic Wave Sensor for SU-8 Thin-Film Adhesion Characterization

This paper investigates the application of a surface acoustic wave (SAW) sensor for characterizing the adhesion of SU-8 thin films. The proposed sensor consists of a thin aluminum nitride (AlN) film deposited on a silicon substrate with two sets of interdigital (IDT) electrodes patterned on the AlN film in a delay line configuration. The SU-8 layer is patterned between the input and output IDT electrodes. A theoretical model for calculating the wave dispersion profile for the SU-8/AlN/Si configuration is developed. The effect of changing the adhesion of the SU-8 film on the wave velocity is implemented by assuming the SU-8/AlN interface consists of a layer of distributed massless springs. Different levels of adhesion are accounted for by changing the stiffness of the interfacial springs and the corresponding changes in the wave dispersion profiles are plotted. Four designs for the SAW sensor are fabricated, for each design two sensor configurations are developed to investigate the change in adhesion of the SU-8 film. In the first configuration, a thin gold film is patterned above the AlN film prior to patterning the SU-8 layer. In the second configuration, an Omnicoat layer is patterned above the gold film prior to SU-8 patterning. Omnicoat is an adhesion promoter commonly used to increase the adhesion of SU-8 to gold. The shift in the center frequency values from both configurations is used to characterize the adhesion of SU-8. The theoretical model is then used to find the equivalent spring stiffness values that fit the theoretical to the measured wave velocities.