Stable performance Nb variable thickness microbridge type Josephson junctions: A reproducible fabrication technique

A novel method of fabrication, as described in the following, was developed that ensures a good reproducibility of the junction. Namely, poly-Si was deposited over a Si substrate whose surface had been previously thermally oxidized. The poly-Si overlaid surface was then thermally oxidized. The bridge configuration of the junction was microprocessed by means of electron beam lithography. The fabricated configuration served as a mask in depositing the Nb layer on the substrate by means of rf sputtering to conclude the fabrication of a microbridge Josephson junction. The final process of the formation of the variable thickness microbridge configuration was readily carried out by placing another properly shaped mask over the masking bridge or by forming an SiO2 layer over the poly-Si layer. The SiO2 layer was processed by selective plasma etching so that it would act as a mask over the bridge portion. The stabilization of the characteristics of the junction has been achieved due to the passivation which takes place during the heat treatment in atmosphere by forced oxidation of the Nb film surface and to the annealing of the film itself.