Characterization of surface acoustic wave (SAW) filters grown on aluminum nitride and zinc oxide thin films

Abstract

The study on the influences of the aluminum nitride (AlN) and zinc oxide (ZnO) film quality on the SAW filter performances were conducted. In our studies the phase velocity (Vp) and electromechanical coupling coefficient (K2) are the most important considerations. Both AlN and ZnO piezoelectric thin films were selected for their high Vp and K2. The AlN and ZnO thin films were grown by rf magnetron sputtering technique and the influences of different sputtering conditions on the structure and the surface morphology of the thin films were investigated. Sputtering power and substrate temperature were found to be the most important factors affecting the crystal structure and the surface roughness of the thin films respectively. To further study the relationship between the quality and the surface roughness of the thin films and the SAW filter performances, AlN and ZnO thin films, grown with different sputtering powers and substrate temperatures, were fabricated into SAW filters and the devices were systematically characterized. Detailed characterizations of the frequency response, K2, phase noise and insertion loss of the SAW filters were conducted as a function of the full width half maximum of the x-ray diffraction pattern and the surface roughness of both the AlN and ZnO thin films. From the experiment results, Vp, K2, insertion loss and phase noise of the SAW filters were strongly affected by the sputtering power and the substrate temperature at which the films were grown. In the second part of this thesis, MBE technique was used to further improve the material quality. From the measurement results, AlN SAW filters grown by MBE technique were much better than those grown by rf magnetron sputtering. This clearly demonstrates the significant influence of the material quality of the thin film on device performance.