Passivation of the interdigital transducers of acoustic sensors based on surface localized HPSWs for measurements in liquids

One of the greatest problems of utilizing the huge potential of surface localized horizontally polarized shear wave (SLHPSW, surface transverse wave (STW)) sensors is to prevent contact between the interdigital transducers (IDTs) and the liquid measurand. This contact leads to a non-negligible perturbation of the measurement due to dielectric interaction between the electric field of the IDTs and the dielectric properties of the liquid. This work presents the passivation of the IDTs with a thin polyimide film. Thicknesses, suitable for screening effectively the electric field of the IDTs from the liquid media, can be attained. A flow cell can now be sealed outside the sensor chip and no mechanical stress of the sensor is exerted.