Theoretical investigation of Lamb wave A0 mode in thin SiC/AlN membranes for sensing application in liquid media

The propagation of the fundamental Lamb antisymmetric A0 mode along amorphous-SiC/c-AlN and 3C-SiC/c-AlN thin composite plates is investigated with respect to the AlN and SiC layers thickness, propagation direction and electrical boundary conditions. The A0 mode phase velocity and the electromechanical coupling coefficient (K2) dispersion curves were theoretically studied for four different electroacoustic coupling configurations. For each configuration the highest K2 values (from 1 to 2%) were found corresponding to A0 mode phase velocity lower than the liquid medium compressional velocity (1480 m/s). The gravimetric sensitivity of these configurations was then calculated, specifically addressing the design of enhanced-coupling mass sensors able to work in liquid ambient. The performances of the A0 mode AlN/SiC plates were compared with those of the enhanced coupling (K2 = 4%) ZnO/SiN plates for liquid sensing applications: the former higher operation frequencies and remarkable endurance to high temperature and chemical etching, make it the ideal candidate for the development of elettroacoustic devices able to survive to harsh environment.