Preparation of overdamped NbTiN Josephson junctions with bilayered Ti–TiN barriers

Overdamped NbTiN Josephson junctions with Ti–TiN bilayered normal-metal barrier are proposed for metrological applications. Binary arrays consisting of 32 768 NbTiN/Ti–TiN/NbTiN junctions were fabricated for quantum voltage standards, and a Shapiro step at 1 V with the step height of about 0.5 mA was generated at around 8 K. The Ti layer was added to protect the base NbTiN electrode from nitrogen plasma during the deposition of the TiN layer. While the critical current Ic and the junction resistance Rn are mainly dominated by the thickness of the TiN barrier, Ic can be also adjusted by changing the thickness of the Ti barrier to maximize the operating margin for voltage standard circuits. An optimization of the NbTiN film in terms of film stress is also described for fabrication of circuits containing such a large number of junctions.