Optimization of shear mode Solidly Mounted Resonators based on fully-dielectric acoustic reflectors for biosensing applications

Electroacoustic biosensors are widely studied nowadays as they can provide a complete and cheap solution for the real time detection of virus, or different diseases, in a very small liquid sample (e.g. blood, urine, saliva). One of these electroacoustic sensors are based on Solidly Mounted Resonators (SMRs). These devices have proven their potential as biosensors but never been optimized from their acoustic reflector point of view. Most of the optimizations were focused on the deposition of tilted piezoelectric materials to excite an acoustic shear mode, the one needed to operate in liquid. The objective of this thesis was to perform a full optimization of the SMR technology for its use as biosensor. The optimization included the design, simulation, and fabrication of the device and its final test in a real biosensing experiment. Unfortunately, the Covid-19 emergency brought some difficulty and the work had to be re-organized in a different way. The work was then focused more on a simulation approach as we could fabricate just few devices. The biosensing verification was finally not performed, however the few devices fabricated could prove the theoretical simulations. The main focus of the thesis was the optimization of the acoustic reflector for the shear mode operation, as this acoustic mode doesn’t couple with liquids and allows the resonator to work in a liquid medium. Two different approaches were used, and the designed acoustic reflector performances were simulated with the 1D Mason’s model and with a 2D FEM analysis. The same reflectors were also fabricated, and besides the emergency, reliable measurements were obtained. The results of the simulations suggest that the optimized designs significantly improve the performances of the resonators in terms of reflectance of the mirrors and resonator Q factor values. Moreover, the experimental results verify the theoretical predictions.