Amorphous SiC/c-ZnO-based Lamb mode sensor for liquid environments

The propagation characteristics of the fundamental symmetric Lamb mode S0 along thin composite plates based on amorphous SiC and piezoelectric c-ZnO layers was studied, aimed at the design of a high frequency electroacoustic device suitable to work in liquid environment. The investigation of the acoustic field profile across the plate thickness revealed that, up to a a-SiC thickness-to-wavelength ratio h/?=0.1 the propagating modes have polarization predominantly oriented along the propagation direction, and hence suitable for operation in liquid environment. The presence of the ZnO layer alters the acoustic field profile that is no more symmetric about the mid-plane of the plate but there is a ZnO limited thickness range corresponding to quite unperturbed field profile of the propagating acoustic mode at the bare SiC layer side. The phase velocity and coupling coefficient K2 of the S0 mode were studied with respect to the SiC and ZnO layers thickness and the electrical boundary conditions, and a K2 =7% was achieved. The thin a-SiC/ZnO acoustic waveguide theoretically investigated here can be fabricated by using conventional thin film deposition techniques, such as rf sputtering, and bulk micromachining of the Si substrate. The fabrication procedure of this acoustic device offers the advantage of providing the monolithic integration of the device with the signal processing electronics. The theoretical investigation suggests that a-SiC/ZnO materials combination is a very promising substrate material suitable for developing high-frequency, IC compatible, very simple electroacoustic devices with enhanced K2 and suitable for working in liquid environment