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

Abstract

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 Jc. The NbN junctions show excellent Josephson tunneling characteristics, the gap voltage Vg is 5.8 mV and the quality factor Rsg/Rn is 12 for the junctions with a Jc of 10.7 kA/cm2, and Vg is 5.3 mV and Rsg/Rn is 4 for the junctions with a Jc 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 Jc sub-micron NbN junctions will be helpful in developing large-scale NbN SFQ circuits with high operating frequency.