Optimization of co-sputtered Cr_xAl_1−xN thin films for piezoelectric MEMS devices

CrAlN alloys can play an important role in the improvement of next generation piezoelectric MEMS devices. However, enhanced piezoelectric constants require high degree of uniaxial orientation in the polycrystalline thin film. In this work, Cr_xAl_1−xN thin films with varying compositions were deposited at different substrate temperatures by reactive DC co-sputtering technique and compared with respect to their microstructure and optical properties. The relationship between the atomic composition of the layers and the plasma powers over the Al and Cr targets during co-sputtering was revealed accurately by Rutherford backscattering spectrometry. As it was found by X-ray and selective area electron diffraction methods, thin films in the range of x = 0–0.23 show hexagonal wurtzite-type phase, which changes to cubic rock-salt-type structure between 0.23 < x < 0.31. At lower Cr cation concentration, like x = 0.12 the wurtzite-type polycrystalline film indicates uniaxial texture, whereas it is almost randomly oriented at x = 0.23. Among the compared compositions, Cr_0.12Al_0.88N showed the strongest c-axis orientation, whereas the optimal substrate temperature was found to be T = 350 °C. While the refractive index measured by ellipsometry increases monotonously from 2.07 to 2.64 with increasing Cr cation concentration in the range of x = 0–0.31, the optical bandgap shrinks from 2.65 to 2.23 eV. These optical data can provide references for future contactless wafer-scale optical monitoring process.