Fabrication and characterization of sub-micron high current density NbN Josephson junctions for large-scale digital circuits

We have developed a fabrication process based on Chemical Mechanical Polishing (CMP) for the sub-micron NbN Josephson junctions on single-crystal MgO substrates. The NbN junctions were fabricated with epitaxial NbN/AlN/NbN trilayers deposited by reactive DC magnetron sputtering. The top electrode of the junction was exposed using CMP to contact the NbN wire without opening the submicron via through the dielectric. It has produced submicron NbN junctions with areas as small as 0.09 μm². We have fabricated NbN tunnel junctions with a wide range of critical current density J_c. The NbN junctions show excellent Josephson tunneling characteristics, the gap voltage V_g is 5.8 mV and the quality factor R_sg/R_n is 12 for the junctions with a J_c of 10.7 kA/cm², and V_g is 5.3 mV and R_sg/R_n is 4 for the junctions with a J_c of 150.5 kA/cm². The 1000-junction arrays were fabricated and their I-V characteristics were measured, critical current uniformity was obtained and the standard deviation is about 5%. The high J_c sub-micron NbN junctions will be helpful in developing large-scale NbN SFQ circuits with high operating frequency.