Stress field in sputtered thin films: Ion irradiation as a tool to induce relaxation and investigate the origin of growth stress

Abstract

The stress state of sputtered Mo thin films was studied, and a detailed analysis of elastic strains, using x-ray diffraction and the “sin2 Ψ method,” was performed. The evolution of the lattice parameter under ion irradiation showed that the usual assumption of a biaxial stress state is not adequate to determine the true stress-free lattice parameter a0 of the film. An original stress model, including a hydrostatic component linked to volume distortions induced by point defects, is required. This model, which describes a triaxial stress field, allows a reliable determination of a0. Furthermore, ion irradiation was shown to be a powerful method for stress relaxation, providing a stress-free lattice parameter solely linked to chemical effects.