Closed Regular Electrode Structure for SAW Resonators

A new closed regular electrode structure for future surface acoustic wave (SAW) devices is described. The structure consists of an interdigital transducer in the form of a ring placed on the Z cut of a hexagonal piezoelectric crystal. Finite thickness electrodes produce the known slowing effect for a SAW in comparison with this SAW on a free surface. So, the "slow" electrode region with the "fast" surrounding region forms an open waveguide structure with the acoustic field concentrated in the electrode region. Because of a finite curvature of the waveguide, the propagating SAW mode has unavoidable losses of its energy due to radiation into the surrounding region. However, these losses are decreased exponentially with increasing of waveguide radius, and an acceptable level of radiation losses can be reached. Estimations of the quality factor Q in the case of Al electrodes with 5% thickness/wavelength ratio and with five wavelength aperture placed on an A1N substrate show the changing of Q from 2 X 10² to 2 x 10⁸ with waveguide radius changing from 50 to 300 wavelengths, respectively. The new structure can be named as ring waveguide resonator (RWR) on SAW. Due to its regularity the electrical admittance of RWR does not have sidelobes, which are typical for usual SAW resonators composed of different electrode gratings with gaps.