Effects of electric bias and O2 content on properties of ZnO films and characterization of ZnO-based film bulk acoustic resonators

The amount of oxygen, which is used as an additive gas in preparing ZnO films by RF magnetron sputtering, has been changed during deposition in the range of O₂/(Ar+O₂) = 10-90 % and its influence on film properties has been examined. X-ray diffraction (XRD) and atomic force microscope (AFM) studies present an optimal condition for oxygen addition at which ZnO films with desirable properties, such as highly preferred [002] orientation, excellent crystallinity, and smooth film surface, can be grown. The effect of electric bias (DC) applied to the substrate (from -80 V to +80 V) has also been analyzed in terms of the crystal quality and surface roughness of ZnO films. In addition, film bulk acoustic resonators (FBARs) with configuration of Al/ZnO/Al/Si(111) are fabricated and the influence of electric bias on their frequency response characteristics. The result shows that DC substrate biasing may improve the crystal quality of deposited ZnO films, and consequently enhance the electromechanical coupling capability of ZnO-based FBARs.