Quantitative Ultrasonic Characterization of c-Axis Oriented Polycrystalline AlN Thin Film for Smart Device Application

Generation of surface acoustic waves (SAW) on polycrystalline piezoelectric materials is gaining popularity due to possibility of manufacturing ultrathin film of such materials that have diverse applications, e.g. flow separation, particle guiding etc. It can be done by controlling the potential pattern in the electrodes placed on the surfaces of the thin film. Piezoelectric Aluminum Nitride (AlN) thin films have been used to fabricate a variety of radio frequency (RF) resonators and filters, contour mode resonators, bulk acoustic resonators and Lamb wave resonators. In this paper the polycrystalline AlN films for applications in high frequency SAW devices were prepared by rf-magnetron sputtering of a high purity aluminum target material in nitrogen atmosphere. Thickness of the film was 6.6 μm with preferential c-axis orientation. However, for appropriate application of such film it is paramount to compute Lamb wave dispersion and the bulk wave angular dispersion relationship in AlN thin film nondestructively, which is the central goal of this paper. Such study is very scarce in the literature and was attempted in this paper using quantitative ultrasonic imaging and characterization (QUIC) technique. The surface dominated elastic modulus and hardness of AlN film were evaluated using QUIC and using nanoindentation, respectively. AlN film was evaluated at different depths using an acoustic microscope and the respective material signatures were recorded. The surface and bulk wave velocities in the film were calculated using QUIC method. These values were determined to be ∼11065 m/s, ∼6046 m/s and ∼5545 m/s, for the longitudinal, transverse and surface acoustic waves, respectively. Subsequently to achieve our central goal, Lamb wave dispersion and bulk angular dispersion relationship were computed from the derived material properties and thickness of the AlN film, which can be used as an easy read out tool for the device manufacturing usage.