Ion-irradiation induced stress relaxation in metallic thin films and multilayers grown by ion beam sputtering

The stress state of Mo layers in Ni/Mo multilayers was investigated and its evolution under ion beam irradiation was monitored using the “sin2 ψ” method. The growth stress in the Mo sub-layers was found to be consistent with a hydrostatic state of stress, accounting for the local deformations due to point defects induced during the sputter growth process. The hydrostatic stress is also responsible for a biaxial stress component appearing in the multilayer that is attached to the substrate, an additional stress component which is superimposed to the coherency stress developed due to epitaxial growth of the multilayers. Ion-irradiation of multilayers results in partial stress relaxation at low fluences, as the growth stress can be almost fully relaxed, while the coherency stresses remain unchanged. That is due to the system’s state, the growth stress is far from thermodynamic equilibrium, the coherency stress is close-to-equilibrium. The employed method, combining X-ray diffraction strain analysis and ion irradiation-induced relaxation, in addition to identifying the chemical effects contribution, proved to be a unique tool for recognising and distinguishing stress contributions.