Characterization of the fabrication process of Nb/Al$ndash$AlNx/Nb tunnel junctions with low RnA values up to 1 Ω μm2

We discuss and characterize the fabrication process of superconductor–insulator–superconductor (SIS) junctions based on a Nb/Al–AlN x /Nb tri-layer. Utilization of the AlN x tunnel barrier, produced by Al nitridation in a nitrogen glow discharge, enables us to produce high-quality SIS junctions with low R n A values (a product of junction resistance and area). We characterize the tunnel barrier formation and investigate the correlation of plasma characteristics and junction properties. The experiment shows that an increase in nitridation time and applied power results in an increase in junction resistance, while variation in nitrogen pressure has almost no influence on the junction characteristics. Analysing the correlation of junction resistance and plasma properties, it is concluded that the mechanism of tunnel barrier formation is based on nitrogen implantation into the Al layer with subsequent diffusion of nitrogen, stimulated by plasma heating.