SiO₂/grooved Al-electrodes/LiTaO₃ structures having large reflection coefficient, large coupling factor, and excellent temperature characteristics even with thick Al electrode

A surface acoustic wave (SAW) substrate for a duplexer such as Personal Communication Service handy phone system in US is required to have a good temperature coefficient of frequency (TCF), an optimum electromechanical coupling factor, and a large reflection coefficient. However, there have been no SAW substrates satisfying all of them. Because the duplexer is composed of resonator-type devices such as a ladder filter or a mul-timode resonator filter, thick Al electrodes for interdigi-tal transducer (IDT) and grating reflectors on a substrate are used in order to achieve the large reflection coefficient. The TCF of the SAW substrate such as LiTaO₃ or LiNbO₃ for a transversal SAW filter using a thin Al electrode is negative so that a SiO₂ film having a positive TCF is often deposited on the substrate to improve the TCF. But, when a thick SiO₂ film was deposited on a resonator and a longitudinally coupled multimode resonator filter composing of thick Al electrodes on 36°Y·X-LiTaO₃ substrates to improve the TCF, their frequency characteristics remarkably deteriorated, because of large convex portions (as thick as Al electrodes) yielded on a surface of the SiO₂ film and the small coupling factor decreased due to their convex portions. The authors have considered that thick SiO₂/grooved Al-electrodes/LiTaO₃ structures SAW substrate would have an excellent TCF, a large reflection coefficient, and an optimum coupling factor even if using thick Al electrodes. The results of the experiment and the calculation clarified that resonators consisting of the newly developed SAW structure had good frequency characteristics, the large reflection coefficient, the optimum coupling coefficient, and the excellent TCF (-3ppm/°C).