Thickness Modulated Niobium Nanoconstrictions by Focused Ion Beam and Anodization

We have fabricated microstrips of Nb thin films by dc magnetron sputtering and lift-off. Focused ion beam (FIB) has been used to mill the surface of the films in a nanometer-sized region across the width of the strip. FIB has the side effect of implanting Ga+ ions on the exposed surface of Nb, damaging the film and modifying its superconducting properties. To overcome this problem, the surface of the FIB-milled Nb films has been passivated by anodization, including the damaged region of the strip into the oxidized layer. Anodization also allows for finely tuning the thickness of the nanoconstrictions after the milling process. We report here the effect of FIB exposure and anodization voltage on the superconducting transition temperature and on the temperature dependence of the resistance of the Nb nanoconstrictions. We also show some preliminary results suggesting that these devices behave as overdamped SNS-like Josephson junctions.