Mass Loading in Coupled Resonators Consisting of SU-8 Micropillars Fabricated Over SAW Devices

Surface acoustic wave (SAW) sensors extensively employ mass loading effect of a sensing film coated over the surface of a SAW device. We report experimental and simulation results of mass loading by SU-8 micropillars (in place of sensing film) attached normal to the surface of SAW resonators. For certain size of micropillars, we have observed an increase in the resonance frequency of the resonator, in contrast to the decrease in the resonance frequency normally caused by mass loading. The SAW resonator and the pillars constitute a system of coupled resonators, and for high values of stiffness at the interface of the pillar and substrate the shift in the resonance frequency is positive.