Surface acoustic wave devices and applications in liquid sensing

In this contribution fundamental design considerations for surface transverse wave devices (STW devices) as liquid sensors are presented. Detailed studies have shown that the interference of the surface transverse wave on y-rotated quartz cuts with the surface skimming bulk wave (SSBW), the triple-transit echo (TTE), and the electromagnetic crosstalk (EC) causes perturbations, which can exceed the real sensor signal caused by the STW. With an optimized sensor design, these perturbations can be suppressed and reproducible measurements become possible. The industrial use as sensors for physical liquid properties, e.g. viscosity, in rough environments demands a passivation of the interdigital transducers (IDTs) and the sensing area. Silicon carbide (SiC) has an excellent chemical and mechanical resistance and is deposited on the sensors in a PECVD process. A viscometer for Newtonian liquids, a direct immunosensor system, and a dip-stick sensor for the measurement of mineral oil viscosity are presented in this paper.