Inhibiting effect of thermal oxides on the growth of rf plasma oxides for Pb-alloy Josephson junctions

The effect of the thermal oxide existing on a Pb-alloy base electrode on rf-plasma oxidation is investigated. This is carried out by removing the initial thermal oxide progressively and also by forming the thermal oxide after the complete removal of the initial thermal oxide. The electrical measurement of current-voltage characteristics and ellipsometric observation are primarily used to evaluate the junction characteristics and oxide thickness. It is found that the thicker thermal oxide, either the initial thermal oxide removed only slightly or thermal oxide formed by the exposure of the base electrode to oxygen for a longer period, brings about the smaller normal tunnel resistance. This result indicates that the thermal oxide inhibits the growth of the rf-plasma oxide; ion transport during rf-plasma oxidation is presumably inhibited by the presence of the thermal oxide.