Micron and submicron Nb/Al-AlOx/Nb tunnel junctions with high critical current densities

To increase superconducting IC speed and density, it is necessary to reduce junction size and increase critical current density. We describe the fabrication and properties of high critical current density micron and submicron Nb/Al-AlO_x/Nb tunnel junctions. Using a 10:1 reduction wafer stepper with I-line photoresist, we obtained a minimum linewidth of 0.6 μm and junctions as small as 0.3 μm² . The critical current densities can be as high as 20 kA/cm² still with low subgap currents. The measured critical current spreads are small. This is due to the use of low-temperature, low-stress ECR (Electron Cyclotron Resonance)-based PECVD (Plasma Enhanced Chemical Vapor Deposition) SiO₂ insulation layers and light anodization around junction areas. The junctions have potential applications in very high-speed superconducting digital circuits and submillimeter microwave devices