AN ALL REFRACTORY NBN JOSEPHSON JUNCTION MEDIUM SCALE INTEGRATED-CIRCUIT PROCESS

The fabrication and electrical performance of an all refractory, eight mask step, NbN medium scale integrated circuit process are discussed. In situ rf sputter deposited trilayers of NbN/MgO/NbN are plasma etched to fabricate Josephson junctions. A novel low temperature, plasma enhanced chemical vapor deposited SiO2 film is used for wiring and resistor insulation. Sputter deposited molybdenum films are used for resistors. Tunnel junctions fabricated with this process have Vm=61 mV at jc=1100 A/cm2, and Vg = 5.1-5.2 mV at 4.2 K. Critical currents are uniform to within ±3% for 101 junction strings and are within ±25% over all die on 3 in. wafers. This process has been used to fabricate 8-bit single flux quantum counter circuits, squid magnetometer circuits, 870 junction strings, and arrays of 256 squids. Preliminary circuit testing indicates operation at temperatures within the range of small closed-cycle refrigerators.