Fabrication and characterization of Nb/Al-AlN /Nb superconducting tunnel junctions
We report a Nb/Al-AlN /Nb superconducting tunnel junction process in which the AlN barrier is formed by plasma nitridation using a compact microwave electron-cyclotron-resonance (ECR) nitrogen plasma source integrated into a standard sputter cluster. This enables growth of uniform tunnel barriers across a broad range of specific resistances, with $R_n A$ down to $\approx 3,\Omega,\mu\mathrm{m}^2$. Junctions maintain excellent quality, exhibiting $R_j/R_n \ge 25$ at the highest barrier transparencies. We characterize resistivity, specific capacitance, and the evolution of junction parameters under room-temperature aging and thermal annealing. A consistent calibration of the junction specific capacitance $C_s$ versus $R_n A$ is established and independently validated by the performance of demonstrator SIS mixers designed using the extracted $C_s$.