High-Frequency Resonators with Excellent Temperature Characteristics using Edge Reflection

Currently, some surface acoustic wave (SAW) devices, especially duplexers, are required to have a good temperature coefficient of frequency (TCF), because the bands between a transmission band (Tx) and a receiving band (Rx) of the duplexer such as personal communication services (PCS) mobile phone in US is very narrow (about 1%). However, a 36-48degYX-LiTaO₃ substrate, which is used for most SAW radio frequency (RF) filters and most SAW duplexers in mobile phone systems, has an optimum electromechanical coupling factor for their applications but does not have a good TCF. It was reported that the TCF of a transversal SAW filter using thin Al electrodes on LiTaO₃ and LiNbO₃ substrates with negative TCF was improved by depositing thick SiO₂ film with positive TCF on them. However, most R,F SAW filters and most duplexers are composed of resonator type SAW devices such as ladder-type or multi-mode resonator type filters using thick Al-electrodes on the substrate to obtain a large reflection coefficient. When the thick SiO₂ films were deposited on the SAW resonator and the multi-mode SAW resonator filter consisting of thick Al-electrodes/36deg YX-LiTaO₃ to improve their TCF, their frequency characteristics were markedly deteriorated. Because their coupling factor becomes smaller and their propagation loss larger due to large periodic convex portions as thick as the Al-electrodes produced on the SiO₂ surface. To avoid influence due to these large periodic convex portions, a SiO₂ with thin convex/thin Al-electrodes/LiTaO₃ and a flattened SiO₂ without convex/thick Al-electrodes/LiTaO₃ structures were examined, but they didn't show good characteristics because of the small reflection at the grating reflectors. On the other hand, it is considered that the reflection of a shear horizontal (SH) wave at a substrate edge of the SiO₂/Al-electrodes/LiTaO₃ structure is large regardless of Al or SiO₂ thickness, because the SH wave completely reflects at the free substrate edge. But, it has been considered that it is difficult to form a fine substrate edge for a high-frequency resonator using the edge reflections. This time, by developing of forming method for a fine substrate edge, high-frequency edge reflection type resonators with a good TCF and an excellent frequency characteristic, which were composed of a thick SiO₂ with thin convex/thin Al-electrodes/36degYX-LiTaO₃ and a flattened thick SiO₂ without convex/thick Al-electrodes/36degYX-LiTaO₃, have been realized for the first time.