The improvement of temperature coefficient of frequency in thin film bulk acoustic wave resonator using secondary harmonics

We have succeeded in making an 870MHz-range thin film bulk acoustic wave (BAW) resonator that has a small temperature coefficient of frequency (TCF) using secondary harmonies. The 870MHz-range BAW resonator has been requested to have nearly zero TCF, because it will be used in an oscillator for remote keyless entry systems. The BAW resonator has composite structure that consists of Al electrodes and ZnO/SiO2. We directed our attention to the fact that ZnO and Al have negative TCF, and SiO2 has a positive one, It. is theoretically possible to. make zero TCF BAW resonators by optimizing the thickness ratio of ZnO and SiO2. However, using fundamental resonance, TCF is so sensitive to the thickness ratio that it cannot be easily controlled by MEMS techniques. We found in finite element method simulation and confirmed by experiment that the TCF of secondary harmonics has a local minimum when changing the ZnO/SiO2 thickness ratio. As the result, a nearly zero TCF resonator without strict control of ZnO/SiO2, thickness ratio has been realized by adopting Al/Zno/SiO2/ZnO/Al/SiO2 structure @and combining thermal oxidized Si and sputtered SiO2. The resonator has the TCF of - 1.86ppm/degree in the range of -40 to 85 degrees centigrade.