Improvement of shape factor and loss of surface acoustic wave resonator filter composed of SiO₂/high-density-electrode/LiTaO₃

Radio frequency (RF) filters in high frequencies using surface acoustic waves (SAWs), such as MediaFLO™, time division synchronous code division multiple access (TD-SCDMA) in China's handy phone system, and the global positioning system (GPS) in cars, require a narrow bandwidth. Thus, the SAW substrates for their RF filters also require an excellent temperature coefficient of frequency (TCF) and an optimum electromechanical coupling factor. The authors reported an RF SAW filter for MediaFLO™ using a shear horizontal (SH) leaky SAW (LSAW) on a flattened SiO₂ film/high-density metal electrode/36-48°Y·X-LiTa_O3 substrate. Although it had a good TCF and a large attenuation out of the pass band, it had a slightly large loss at the pass band only at room temperature compared with that of the conventional Al-electrode/42°Y·X-LiTaO ₃ in the previous report. In this study, calculation using the coupling-of-modes (COM) theory showed the effect of a new phase inverse method of obtaining a steep slope at the right side of the filter frequency characteristic, although the previous paper showed only the measured frequency characteristics. In addition, an RF SAW filter with a lower loss at the pass band and a better TCF than that of the previous report has been realized.