High quality all refractory Josephson tunnel junctions for SQUID applications

We report on Nb based all-refractory Josephson tunnel junctions made using a combination of UHV getter sputter deposition, CF4plasma etching and ion beam etching/deposition. Like the SNEP process developed by Gurvitch et al. the junction forming steps are performed without breaking vacuum, producing a whole wafer Nb-Al-AlOx-Al-Nb sandwich. Two routes have been used to produce 20μm×20μm junctions from the sandwich structure, one of which includes a novel liftoff stage. The Nb layers are deposited at 700°C for the base electrode and 100-400°C for the top electrode. The Al layers are deposited and oxidised at 80-180°C. Both randomly oriented and 'R'-plane single crystal sapphire substrates are used, the latter enabling epitaxial single crystal Nb to be grown. We have made a preliminary study to characterise the junctions with respect to the temperature of oxidation and of the top electrode deposition.