Influence of the acoustic energy distribution on the in-liquid sensitivity of thin film electroacoustic resonators

In contrast to quartz crystal microbalances, the sensitivity of thin film bulk acoustic wave resonators (FBARs) is strongly dependent on all layers composing the composite structure. Previous studies of the pure mass sensitivity of suspended FBARs, proved that placing low acoustic impedance materials at the sensing surface of the device can enhance their sensitivity by carefully controlling the energy trapping effects. Here we extend those studies by investigating if the in-liquid sensitivity of shear-mode AlN-based solidly mounted resonators (SMRs), working at 2 GHz, display a similar dependence on the device configuration (top electrode thickness and material). We use the finite element method (FEM) and experimental results to demonstrate that if one is restricted by the readout circuit to a certain resonant frequency, the sensitivity of the devices (particularly in-liquid sensors or biosensors) can be boosted by proper design while preserving the initial frequency. This is possible since the variations in sensitivity are strongly dependent on the energy distribution within the whole resonant structure.