Josephson junctions and superconducting quantum interference devices made by local oxidation of niobium ultrathin films

We present a method for fabricating Josephson junctions and superconducting quantum interference devices (SQUIDs) which is based on the local anodization of niobium strip lines 3-6.5 nm thick under the voltage-biased tip of an atomic force microscope. Microbridge junctions and SQUID loops are obtained either by partial or total oxidation of the niobium layer. Two types of weak link geometries are fabricated: lateral constriction (Dayem bridges) and variable thickness bridges. SQUIDs based on both geometries show a modulation of the maximum Josephson current with a magnetic flux periodic with respect to the superconducting flux quantum h/2e. They persist up to 4 K. The modulation shape and depth of SQUIDs based on variable thickness bridges indicate that the weak link size becomes comparable to the superconducting film coherence length xi which is of the order of 10 nm. (C) 2001 American Institute of Physics.