Plastic flow in ion-assisted deposition of refractory metals

Ion-assisted deposition of refractory metals (Fe, Mo, Nb, Ta, W, Pt) was studied using an unbalanced magnetron source to provide both the depositing atom flux and the argon ion flux at ion-to-atom flux ratios in the range 2--10. The films were characterized by x-ray diffraction and the measurement of room-temperature electrical resistivity. As the ion bombardment energy was increased, the stress in the films as measured by the lattice parameter in the growth direction became more compressive for Fe, Nb, Mo, and β-Ta before decreasing, and decreased for α-Ta, W, and Pt. The results are interpreted in terms of the elastic and plastic response to the stress produced by the bombardment. The metals Fe, Nb, and Mo were deposited at a low stress, which became more compressive with bombardment energy until the metal plastically flowed. The metals W, Ta, and Pt were deposited with a level of stress above that required for plastic flow, and increasing the bombardment energy just caused more plastic flow. The compressive stress and resulting plastic flow could be an important mechanism in producing the low porosity and other behavior of ion-assisted thin films