Submicrometer-Scale Patterning of Superconducting Nb Films Using Focused Ion Beam

We have developed a novel patterning technique for superconducting Nb films with submicrometer scales, using a focused ion beam (FIB). The technique makes use of a phenomenon wherein the plasma etching rate of Nb films exposed to a Ga+ focused ion beam is much lower than that of unexposed Nb films, when they are etched by reactive ion etching (RIE) in CF4 plasma. The etching rate of films exposed to 100 keV Ga+ FIB at a dose >2×1016/cm2 was less than one-quarter of that of unexposed films. Using this technique, we have fabricated 0.1-µm-wide Nb strips with the critical current density of 1×107 A/cm2 at 4.2 K. This value is almost equal to that of bare films.