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 p roduced 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 ampl itudes. We accidentally found that the frequency characteristics and t he amplitude deviation could be improved by mirror-polishing the ZnO f ilm surface. In a SAW filter with a ZnO/interdigital transducer (IDT)/ glass substrate structure, periodic step-like portions due to the thic kness of the finger electrode of IDT and fine roughness were present o n the ZnO film surface. As a result of investigating the effect of sur face structure on amplitude deviation, the step-like portions did not affect the electromechanical coupling factors but reduced the SAW phas e velocities, experimentally and theoretically. In addition, it was cl arified that the step-like portions due to the finger electrodes and f ine roughness on the ZnO surface caused deviations in the SAW velociti es and their reflections, causing the deviation in the amplitude chara cteristics.