Shear mode excitation in thin piezoelectric films has gained a lot of interest in recent years mainly in view of high frequency in-liquid resonator operation and hence such films are very attractive for the fabrication of highly sensitive biochemical sensors. In a short period of time shear mode thin film electroacoustic resonators have reached a level of performance comparable to or better than the established QCM sensor technology. Shear mode excitation in wurtzite thin piezoelectric films (AlN, ZnO, etc) requires a film texture with a nonzero c-axis mean tilt.
This presentation describes a method for the deposition of thin piezoelectric AlN films with a nonzero c-axis mean tilt. The process is implemented in a standard sputtering system without any hardware modification. In essence, the process consists of two stages. In stage one, a seed or nucleation layer, is grown at a relatively high process pressure (20 mTorr) to a thickness of around 100 nm. This seed layer is fundamental to the process and is characterized with a relatively poor texture and a large population of (103) nano-crystallites with a random in-plane orientation of the c-axis. This layer sets the conditions for phase 2 of the process which is done at a pressure of 2 mTorr, where the condensing Al atoms experience negligible energy losses in gas phase collisions, so that growth proceeds in the so called competitive growth regime. This means that the main growth is along the fastest growing planes, which are the c-planes of the (103) grains facing the mean flux direction at the point under consideration. Thus, tilted growth in stage 2 is achieved through the directionality of the deposition flux at the substrate surface, which for a finite target is asymmetric for all points on the substrate except at the center of the wafer. The mean c-axis tilt ranges from 25 to 30 degrees over the wafer (c-axis leaning towards the center), excluding a minor circular area in the center. The film texture and morphology has been characterized by XRD, TEM and AFM. Tilted AlN thin film bulk acoustic wave resonators (FBAR) operating at 1.2 GHz have been fabricated exhibiting an electromechanical coupling coefficient of around 1.8% and an excellent Q value of 200 when operated in contact with pure water.
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