Impact of sputter deposition parameters on the microstructural and piezoelectric properties of CrxAl1 −xN thin films

In the field of MEMS, aluminum nitride (AlN) in the wurtzite phase is an important functional material due to its piezoelectricity. To further enhance device performance, transition metal alloying of AlN is proposed to achieve a higher piezoelectric response. In this work, the effect of moderate chromium (Cr) doping (up to x = (5.0 ± 0.6) at.%) as well as the influence of changing deposition pressures and gas composition are investigated, focusing on the microstructure, morphology and piezoelectric response of sputter deposited CrxAl1 −xN thin films. X-ray diffraction analyses reveal a wurtzite type structure with highest degree in c-axis orientation for lowest deposition pressures and Cr concentrations. With increasing deposition pressure, a deterioration in crystal quality and the formation of other crystal orientations within the films are observed. The presence of crystallite orientations away from the 〈002〉 orientation is linked to the presence of randomly oriented, conical-shaped grains which are embedded in the fine grainy thin film surface topography as indicated by atomic force microscopy. By using piezo force microscopy, these grains are identified to cause a reduction in the effective piezoelectric response. Furthermore, we show that combining both the analysis of orientation distribution and surface morphology, is by far more reliable for estimating the piezoelectric response of thin films than the sole use of one method.