Influence of step-like portions on the surface of ZnO/glass SAW filters on their frequency characteristics

The frequency amplitudes of surface acoustic wave (SAW) filters mass produced of zinc oxide (ZnO) films on glass were found to be different among the filters even when their center frequencies are the same. We attempted various experiments in order to reduce the deviation of amplitudes. We accidentally found that the frequency characteristics and the amplitude deviation could be improved by mirror-polishing the ZnO film surface. In a SAW filter with a ZnO/interdigital transducer (IDT)/glass substrate structure, periodic step-like portions due to the thickness of the finger electrode of IDT and fine roughness were present on the ZnO film surface. As a result of investigating the effect of surface structure on amplitude deviation, the step-like portions did not affect the electromechanical coupling factors but reduced the SAW phase velocities, experimentally and theoretically. In addition, it was clarified that the step-like portions due to the finger electrodes and fine roughness on the ZnO surface caused deviations in the SAW velocities and their reflections, causing the deviation in the amplitude characteristics.