Toughening of thin films by high energy ion beams

Refractory metal films tend to grow with a high degree of intrinsic tensile stress. As the film thickness increases, the integrated stress becomes high enough to cause loss of adhesion. In the present study, when chromium films on the order of 1 μm were irradiated with Ni ions of energy 75 MeV, a reduction in strain as measured by X-ray diffraction was observed. Scratch Adhesion measurements demonstrated that this is accompanied by a large improvement in coating to substrate adherence. A new model is proposed to account for the observed effects and provides a basis on which to engineer stresses in thin films using ion beams. The model pictures mobile interstitials that escape from displacement cascades as nucleating in to loops in preferred orientations. The preferential orientation is a result of the lattice strain. As these aligned loops grow, they introduce a strain of their own which cancels out the initial strain. A mathematical formulation enables prediction of strain with irradiation dose for a given initial stress