Control of temperature coefficient of frequency in zinc oxide thin film bulk acoustic wave resonators at various frequency ranges

Two types of piezoelectric bulk acoustic wave resonators have been fabricated at various frequency ranges using zinc oxide (ZnO) piezoelectric thin films with negative temperature coefficient of frequency (TCF) and substrates with positive TCF. One is a 3.58 MHz resonator made of ZnO thin film on ELINVAR, which is known to be able to control its TCF by heat-annealing conditions. The other is a 200 MHz resonator made of ZnO thin film on a membrane of SiO2 which has positive TCF. The TCF of the 3.58 MHz resonator is controlled by heat-annealing temperature of ELINVAR alloy, and that of the 200 MHz resonator is also controlled by the thickness ratio between ZnO and SiO2. The TCFs of both resonators are optimized to about 2 ppm/degreesC though methods to control TCF are different. As a result of the experiments, it is clarified that TCF of piezoelectric bulk acoustic wave resonators can be controlled in the range of MHz to several hundreds of MHz range combining some kinds of materials that have different TCF values. (C) 2002 Elsevier Science Ltd. All rights reserved.