P2N-2 High Frequency Resonators with Excellent Temperature Characteristic Using Edge Reflection

Most surface acoustic wave (SAW) radio frequency (RF) filters and duplexers have the Al-electrodes/36-48degYX-LiTaO₃ substrate structure, which has an optimum coupling factor and an optimum reflection coefficient but does not have a good temperature coefficient of frequency (TCF). The TCF can be improved by depositing SiO₂ film with positive TCF on a transversal SAW filter consisting of a substrate with negative TCF such as LiTaO₃ and LiNbO₃. However, resonator-type SAW devices combining SiO ₂ film, Al electrodes, and LiTaO₃ substrate do not show a good frequency characteristics regardless of Al thickness. Because both (a) a SiO₂/thin Al-electrodes/LiTaO₃ structure with small convex portions on the SiO₂surface and (b) a flattened SiO₂/Al-electrodes/LiTaO₃ structure have a small reflection coefficient, and (c) a SiO₂/thick Al-electrodes/LiTaO₃ structure with large convex portions has a small coupling factor. It is considered as a counter-measure that a resonator using the reflection of a shear horizontal (SH) wave at substrate edges of their structures shows a good frequency characteristic because its reflection coefficient is very large regardless of Al thickness. It has been difficult to obtain a high-frequency edge reflection resonator because it requires too fine substrate edges for ordinary machining techniques. However, a high-frequency edge reflection resonator with a good TCF and an excellent frequency characteristic has been realized using our newly developed method of obtaining fine edges