Analysis of the fabrication process of Nb/Al-AlNx/Nb tunnel junctions with low RnA values for SIS mixers

We characterize the fabrication process of superconductor-insulator-superconductor junctions (SIS) 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_nA values (the product of junction resistance and area). Analyzing 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. The latter process plays a dominant role since R_nA values are well correlated with the power dissipated on the substrate surface. An SIS mixer using this technology and electron-beam lithography has been successfully fabricated.