Stress Adjustment and Characteristics Improvement in a 1.8GHz Range Film Bulk Acoustic Wave Resonator by Using Multi-layer Structure of ZnO/Al_2O_3/SiO_2

摘要

Stress adjustment and improvement of electrical characteristics in film bulk acoustic wave resonator (BAW) have been successfully carried out in l.8GHz range by using a multi-layer structure of ZnO / Al_2O_3 / SiO_2. The BAW resonator, designed on secondary harmonics at about 1.8GHz, has Al / ZnO / Al / Al_2O_3 / SiO_2 structure. ZnO and SiO_2 thin films have a negative and a positive temperature coefficient of sound velocity, respectively. So temperature coefficient of frequency (TCF) of the BAW resonator can be controlled by the thickness ratio of ZnO and SiO_2 thin films. Since both ZnO and SiO_2 have compressive stress, and Al_2O_3 has tensile one, the stress of the membrane is reduced by combining these thin films so that the membrane can avoid deformation. The BAW resonator, with thickness of ZnO /Al_2O_3 / SiO_2=1.2 /0.45/1.25 microns, was designed by finite element method (FEM) simulation and fabricated. The value of quality factor (Q factor) and the TCF of the BAW resonator were realized over 1000 and -20 ppm/degree C, respectively. The Q of ZnO / Al_2O_3 / SiO_2 structure was higher than that of ZnO / SiO_2 one with keeping small TCF.