All refractory NbN integrated circuit process

The authors describe the fabrication and electrical performance of an eight mask step, 2.5- mu m*2.5- mu m minimum junction size, all refractory NbN integrated circuit process. NbN/MgO/NbN trilayers sputtered in situ are patterned by reactive ion etching to form Josephson tunnel junctions. A two-level dielectric process has been developed to ensure low defect densities. Sputtered molybdenum films form a resistor layer. NbN wire J/sub c/ enhancement and improved step coverage have been achieved. This process has been used to successfully fabricate SQUID amplifier circuits, digital modified variable threshold logic circuits, arrays of 256 SQUIDs, SFQ counters, and 4000 junction strings. NbN logic circuit operation above 10 K was demonstrated.<
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