Modulation periodicity dependent structure, stress, and hardness in NbN/W2N nanostructured multilayer films

Abstract

NbN/W2N nano-multilayer films with a modulation periodicity, Λ, ranging from 5.1 to 157.4 nm have been deposited on a Si(100) substrate by reactive magnetron sputtering in Ar/N2 mixtures. The Λ dependent structural and mechanical properties for the resulting NbN/W2N multilayers have been evaluated by means of low-angle x-ray reflectivity, x-ray diffraction, high-resolution transmission electron microscope, and nanoindentation measurements. The finding is that for films with Λ ≤ 10.6 nm, fcc NbN layers are coherent with cubic W2N layers, resulting in NbN layers and W2N layers that are in the compressive and tensile states, respectively. In contrast, as Λ is larger than 10.6 nm, a phase transition from W2N to W occurs in the W2N layer, which is a result of the coherent interface strain relaxation. For this case, all layers are in the compressive state, and the coherent interface disappears. The intrinsic compressive stress evolution with Λ can be interpreted in terms of interface stress. The formation of coherent interface at small Λ (≤10.6 nm) is helpful for releasing point defects in layers, leading to a low compressive stress (≤1.1 GPa). The hardness for the obtained multilayer film increases with decreasing Λ, and approaches a maximum value of 43.7 GPa when Λ is 7.4 nm. The maximum strengthen at lower Λ is mainly attributed to coherent interface stresses and the modulus difference between the NbN and W2N layers. The increase in hardness with a decrease in Λ is interpreted by the Lehoczky model.