Texture development during growth of Ti_1-xAl_xN thin films studied by in-situ x-ray diffraction

Ti_1-xAl_xN coatings are of common use for a vast variety of applications. For each of them, controlling the microstructure is crucial because it determines usefulness, performance and lifetime. Thus, literature on the relationship between deposition parameters, microstructure, and performance of Ti_1-xAl_xN coatings is large. However, little is known regarding the atomistic mechanisms for these observed relationships. Our approach in understanding those mechanisms is in-situ x-ray diffraction during the growth of Ti_1-xAl_xN films using a deposition chamber installed at the Rossendorf beam line BM20 at the European Synchrotron Radiation Facility in Grenoble, France. All films were deposited by reactive co-sputtering from Ti and Al targets; one series at constant x = 0.06 varying substrate temperature, bias voltage, and nitrogen partial pressure and thus growth rate. In another series, x was systematically varied from 0 to 0.73 while keeping all other parameters constant. Values of x < 0.15 and high deposition rates lead to a typical cross-over behavior between initial (002) and final (111) preferred orientation. Reducing the deposition rate leads to (002) preferred orientation practically independent of film thickness and substrate temperature. Yet, suppressing collisionally-induced atomic N on the sample surface by applying a positive bias voltage, brings back a (111) preferred orientation. Those observations are consistent with proposed atomistic models. Keeping the deposition rates low, (111) preferred orientation can also be induced by increasing x above 0.15, which in the presence of atomic N can be explained by its higher adatom mobility. Increasing x towards the AlN segregation threshold at x = 0.60 leads to hard nano-composite TiAlN/AlN structures, and pushing x further to 0.73 leads to highly stressed AlN with an a-axis off-plane texture.