Modification of niobium film stress by low-energy ion bombardment during deposition

Abstract

The effect of ion bombardment on film stress was studied by evaporating Nb thin films in the presence of a controlled ion beam from a Kaufman ion source. Films were deposited at 4 Å/s by electron-beam evaporation with ion bombardment over the energy range 100–800 eV, at ion current densities of 0.001 to 1.1 mA/cm2. Film stress was measured by an x-ray bending-beam technique on Si substrates. Stress values were found to depend strongly on both ion flux and sample temperature. For films deposited at 400 °C, film stress is tensile in the absence of ion bombardment. Increasing argon ion flux causes a change toward compressive stress, in some cases passing through zero. For films deposited at room temperature, film stress is compressive in the absence of ion bombardment, due to incorporation of oxygen. Argon ion bombardment causes a change toward tensile stress, correlating with an improvement in film purity by preferential resputtering of oxygen. With 100 eV argon ions, an ion-to-atom ratio of only 3–10 % is sufficient to cause reversal of the sign of film stress in most cases. Results are interpreted as a combination of cleaning by resputtering and annealing or compacting the film microstructure. In addition, nitrogen ion bombardment is shown to produce superconducting NbN films with a superconducting transition temperature up to 14.5 K.