Resonance characteristic improvements in SMR-type FBAR devices for RF filter application

Abstract

The film bulk acoustic resonator (FBAR) has attracted a great attention as a promising technology to fabricate the next-Generation radio frequency (RF) filters. This is mainly because the FBAR technology can be integrated fully with the current silicon process technology, eventually realizing the microwave monolithic integrated circuits. FBAR filter can be designed by connecting FBARs in series and parallel. To realize an FBAR filter of high performance, the resonance characteristic of each FBAR needs to be improved. Consequently, in this thesis, we present three new approaches to more effectively improve the resonance characteristic of the solidly mounted resonator (SMR)-type FBAR for RF FBAR filter application. The SMR, one of the FBAR structures, has a Bragg reflector, consisting of alternately deposited multi-layer films. Three critical factors that determine the resonance characteristics of the SMR-type FBAR are the performance of the piezoelectric property of piezoelectric material (ZnO) and Bragg reflector layer, and the selection of bottom electrode matched with ZnO film because the quality of the ZnO film depends highly on the bottom electrode film quality. Firstly, to improve the performance of the piezoelectric property of ZnO film, we suggest the new approach, which is about the effects of the deposition temperature on the preferred orientation or grain growth behaviors of the ZnO deposited on an Al bottom electrode The ZnO films were deposited using an RF sputtering method by varying the deposition temperature from room temperature to 350$\deg$C. The growth characteristics of the deposited ZnO films are shown to have a strong dependence on the deposition temperature. Overall, the ZnO films deposited at/below 200$\deg$C exhibited reasonably good columnar grain structures with a highly preferred c-axis orientation while those above 200$\deg$C showed very poor columnar grain structures with a mixed-axis orientation. This temperature dependence stu...