Superconductor integrated circuit fabrication technology

Today's superconductor integrated circuit processes are capable of fabricating large digital logic chips with more than 10 K gates/cm/sup 2/. Recent advances in process technology have come from a variety of industrial foundries and university research efforts. These advances in processing have reduced critical current spreads and increased circuit speed, density, and yield. On-chip clock speeds of 60 GHz for complex digital logic and 750 GHz for a static divider (toggle flip-flop) have been demonstrated. Large digital logic circuits, with Josephson junction counts greater than 60 k, have also been fabricated using advanced foundry processes. Circuit yield is limited by defect density, not by parameter spreads. The present level of integration is limited largely by wiring and interconnect density and not by junction density. The addition of more wiring layers is key to the future development of this technology. We describe the process technologies and fabrication methodologies for digital superconductor integrated circuits and discuss the key developments required for the next generation of 100-GHz logic circuits.