Surface acoustic wave characteristics of a ZnO/quartz substrate structure having a large electromechanical coupling factor and a small temperature coefficient

A substrate having a suitable electromechanical coupling factor (k(s)) value and a good temperature coefficient of frequency (TCF) is required for an applications of resonator surface acoustic wave (SAW) filters and rudder-type SAW filters requiring a specific bandwidth, because their bandwidth largely depends on the coupling factor of the substrate. Although a quartz substrate for SAW devices provides a good TCF, the electromechanical coupling factor obtained is small. The author speculated that an excellent TCF and an appropriate electromechanical coupling factor could be realized by combination of a quartz substrate having a positive TCF and a ZnO film having a negative TCF. As a result of theoretical and experimental investigations performed using a 29 degrees 45' rotated Y and a 42 degrees 45' rotated Y 35 degrees X propagating quartz substrate, an excellent TCF (approximate to 0 ppm/degrees C) and an appropriate coupling factor (k(s) = 0.1-0.11; k(s)(2) = 0.01-0.0121) were realized with a zero degree power now angle on the structure of IDT/ZnO/quartz or IDT/ZnO/shorted-plane/quartz for the first lime.