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Nb/Al-AlNx/Nb SIS junctions have been fabricated under various conditions of plasma nitridation. Junctions with low RN A of ~ 10 Omega mum2 have been fabricated in pure nitrogen plasma, although this process exhibited low reproducibility. A process with higher reproducibility was realized by adopting a two-step-ignition nitridation process in a composite plasma of N2 and Ar. An exponential correlation between the nitridation time and the junction RN A was found for the junctions with barriers formed in this new process, which suggests a linear time dependence of the barrier thickness during nitridation. SIS junctions with RN A ~ 10 Omega mum2 can be repeatedly produced using this process. Preliminary results from X-ray photoelectron spectroscopy (XPS) observations of the nitride barrier are also presented. While the N 1s XPS signal was apparently detected in the junctions with high RN A, their intensity was reduced as RN A the decreases.