REDUCTION OF DEVIATION OF FREQUENCY-CHARACTERISTICS OF SAW FILTERS BY MECHANICAL POLISHING OF ZNO FILM SURFACE

It was found that the surface acoustic wave (SAW) amplitudes in SAW filters made of ZnO films vary from filter to filter even when the center frequencies are the same and the deviation of amplitudes must be reduced. It was found also that the frequency and amplitude characteristics could be improved by mirror polishing of the ZnO film surface. In an SAW filter with a ZnO/interdigital transducer (IDT)/glass substrate structure, periodic convex areas due to the thickness of the finger electrode of IDT and fine roughness were present on the device surface. As a result of investigating the influence of the surface structure on the amplitude deviation, it was found that the convex areas did not influence the electromechanical coupling coefficient; but they reduced the acoustic velocity of SAW. In addition, it was clarified that the convex areas due to the finger electrode and fine roughness on the ZnO surface caused deviations in the acoustic velocity of SAW and reflections of SAW, causing a deviation in the amplitude characteristics.