A self-aligned nano-fabrication process for vertical NbN–MgO–NbN Josephson junctions

We present a new process for fabricating vertical NbN–MgO–NbN Josephson junctions using self-aligned silicon nitride spacers. It allows for a wide range of junction areas from 0.02 to several 100 μm². At the same time, it is suited for the implementation of complex microwave circuits with transmission line impedances ranging from $<1\Omega$ to $>1k\Omega$. The constituent thin films and the finished junctions are characterized. The latter are shown to have high gap voltages ($>4mV$) and low sub-gap leakage currents. (paper)