Intrinsic stress evolution in laser deposited thin films

Abstract

In situ stress measurements were performed on polycrystalline Permalloy and Ag thin films laser deposited in ultrahigh vacuum (UHV) and at different Ar gas pressures. In UHV, when the kinetic energy of the particles is high (about 100 eV), in the initial growth stage the stress is dominated by the surface energy and intermixing effects. With increasing deposition time, capillary-induced compressive growth stress is observed. Additionally, the film stress is strongly influenced by the growth mode (island growth or layer-by-layer growth). In the case of Volmer–Weber growth, island zipping generates tensile stress, as soon as island impingement and coalescence occurs. In the late stages, compressive stress due to shot-peening and implantation dominates the measurements, similar as in sputtered films. The depth of influence of the impinging particles is determined to be about 3 nm. With increasing Ar pressure (or at low laser fluence) the impinging particles are slowed down and implantation or intermixing effects are diminished. This is accompanied by changes in the film morphology and texture. At high Ar pressures a compressive-to-tensile transition occurs and the laser deposited films become more comparable to evaporated samples with an open structure. These results can be understood by a combination of stress formation and relaxation effects below the film surface.