Surface acoustic wave filter in high frequency range with narrow bandwidth and excellent temperature property

Radio frequency (RF) filters in high frequency using surface acoustic wave (SAW) such as MediaFLO^TM, time division synchronous code division multiple access (TDSCDMA) in China handy phone system, and global positioning system (GPS) in cars require a narrow bandwidth. So, the SAW substrates for their RF filters are also required to have a very excellent temperature coefficient of frequency (TCF) and optimum electro-mechanical coupling factor. Rayleigh wave on a ZnO film/Al-electrode/quartz structures and shear horizontal (SH) type of leaky SAW (LSAW) on a flattened SiO₂/high density metal electrodes/36deg-48degYX-LiTaO₃ one have good TCF and suitable coupling factor to realize them. On the other hand, large reflection coefficient is required to realize the multimode resonator filter. Though the latter has the large reflection coefficient, the farmer has small one. So, in the case of the former structure, authors also attempted to apply a high density metal electrode such as Au electrode instead of Al electrode to realize the large reflection coefficient. As the result, the multi-mode resonator filters composed of a ZnO/high density electrode/quartz and a flattened SiO₂/high density electrode/LiTaO₃ with good TCF for the RF range have been realized for the first time.