EXPERIMENTAL STUDY OF ALUMINIUM NITRIDE THIN FILM MICROWAVE BULK ACOUSTIC WAVE RESONATORS, ACOUSTICALLY ISOLATED FROM SUBSTRATE

摘要

The interest to the thin film microwave bulk acoustic wave resonators acoustically isolated from substrate is stimulated by modern trends in the development of communication systems such as the handling the more high frequencies achievement of higher reliability, improving of electrical parameters and miniaturization. Thin film resonators (TFR) well fit to these requirements. This report is devoted to the systematic study of TFR’s made of AlN piezoelectric films with metal electrodes acoustically isolated from the substrate by a set of quarter wavelength layers made of SiO_2 and W having a big contrast in their acoustic impedances. Such sets of layers deposited onto rather thick well polished silicon substrate are working like a Bragg mirror effectively reflecting acoustic waves. Various resonator configurations were studied. Half-wavelength and quarter wavelength in thickness TFR’s were investigated with the Bragg structures designed as the reflectors providing zero and pi phase shift correspondingly. The dependence of the resonator properties on the number of layers in the mirror (5, 6, 7, 9) was investigated near the resonant frequencies 1.7, 2.3, and 5.4, 5.5 GHz. A good agreement between experimental and calculated data was obtained when experimentally found values of K~2 and directly measured thickness of the layers (by SEM) composing the structure had been used. It was found that the best agreement between data was regularly achieved for smaller (than table data) values of sound velocity in AlN films and higher values of the attenuation coefficients for layers. It means that in the case of a small thickness of the AlN and other films their mechanical properties may differ from the data for bulk material.