Structure, stress, and mechanical properties of Mo-Al-N thin films deposited by dc reactive magnetron cosputtering: Role of point defects

Abstract

In this work, the structural and mechanical properties of ternary Mo-Al-N alloys are investigated by combining thin film growth experiments and density functional theory (DFT) calculations. Mo1−xAlxNy thin films (∼300 nm thick), with various Al fractions ranging from x = 0 to 0.5 and nitrogen-to-metal (Al + Mo) ratio ranging from y = 0.78 to 1.38, were deposited by direct-current reactive magnetron cosputtering technique from elemental Mo and Al targets under Ar + N2 plasma discharges. The Al content was varied by changing the respective Mo and Al target powers, at a fixed N2 (20 SCCM) and Ar (25 SCCM) flow rate, and using two different substrate temperatures Ts = 350 and 500 °C. The elemental composition, mass density, crystal structure, residual stress state, and intrinsic (growth) stress were examined by wavelength dispersive x-ray spectroscopy, x-ray reflectivity, x-ray diffraction, including pole figure and sin2ψ measurements, and real-time in situ wafer curvature. Nanoindentation tests were carried out to determine film hardness H and elastic modulus EIT, while the shear elastic constant C44 was measured selectively by surface Brillouin light spectroscopy. All deposited Mo1−xAlxNy films have a cubic rock-salt crystal structure and exhibit a fiber-texture with a [001] preferred orientation. The incorporation of Al is accompanied by a rise in nitrogen content from 44 to 58 at. %, resulting in a significant increase (2%) in the lattice parameter when x increases from 0 to 0.27. This trend is opposite to what DFT calculations predict for cubic defect-free stoichiometric Mo1−xAlxN compounds and is attributed to variation in point defect concentration (nitrogen and metal vacancies) when Al substitutes for Mo. Increasing Ts from 350 to 500 °C has a minimal effect on the structural properties and phase composition of the ternary alloys but concurs to an appreciable reduction of the compressive stress from −5 to −4 GPa. A continuous increase and decrease in transverse sound velocity and mass density, respectively, lead to a moderate stiffening of the shear elastic constant from 130 to 144 GPa with increasing Al fraction up to x = 0.50, and a complex and nonmonotonous variation of H and EIT is observed. The maximum hardness of ∼33 GPa is found for the Mo0.81Al0.19N1.13 film, with nitrogen content close to the stoichiometric composition. The experimental findings are explained based on structural and elastic constant values computed from DFT for defect-free and metal- or nitrogen-deficient rock-salt MoAlN compounds.