Combination of ZnO film and quartz to realize large coupling factor and excellent temperature coefficient for SAW devices

A good temperature coefficient of frequency (TCF) and an appropriate electromechanical coupling factor (k_s) are required for applications of surface acoustic wave (SAW) devices requiring a specific bandwidth. An ST cut X propagation quartz substrate for Rayleigh SAW devices is known for its good TCF; however, its electromechanical coupling factor is as small as 0.0014 at k_s ², so it is often unsuitable for applications of SAW devices requiring a specific bandwidth. Generally, most single crystals and thin films have a negative TCF. Only a few materials such as a quartz substrate having a specific cutting angle or a propagating direction and SiO₂ films have a positive TCF. Many investigations aimed at improving the TCF by depositing a SiO₂ film having a positive TCF on a substrate having a negative TCF have been reported on. However, since the absolute value of the TCF of such substrates is large, a relatively thick film of SiO₂ is required for compensation of its TCF. The author has theoretically and experimentally studied in detail the opposite combination, that is, the combination of a ZnO film having a negative TCF and a quartz substrate having an appropriate positive TCF, with selection of appropriate cutting and propagation angles of the substrate. As the result, the author could realize SAW substrates having an appropriate electromechanical coupling factor (k _s²=0.011), a good TCF (≈Oppm/°C), and a zero power flow angle in structures of IDT/ZnO/quartz and IDT/ZnO/shorted-plane/quartz using an ST-cut 35°X propagation quartz substrates (a 29°45´ rotated Y and a 42°45´ rotated Y plates 35°X propagation quartz) for the first time