A Josephson binary counter using single-flux quanta transitions of dc SQUID's has been fabricated using an eight-level NbN-based process. High-speed binary division has been demonstrated at 4.2 K, with single-cell counting observed at 60 GHz using the Josephson voltage-to-frequency relationship. Count rate was primarily limited by conservative process and design rules. The counter was designed for operation at 4.2 K. At 8-10 K, the beta-L of the SQUID's would not allow operation, though the junction characteristics were good.

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.

Underlayers of MgO as thin as 8.0 nm have been used in the fabrication of NbN/MgO/NbN Josephson tunnel junctions. NbN/MgO/NbN trilayers with and without MgO underlayers were deposited on thermally oxidized Si substrates at 100°C using RF magnetron sputtering in a semi-UHV (ultrahigh vacuum) load lock vacuum system. Sputtering parameters were first optimized to produce NbN with Tc =14.7 K on SiO2/Si substrates, and then thin MgO underlayers were used to enhance the Tc of the trilayers to 15.7 K. X-ray diffraction of NbN films indicates that thin MgO underlayers of 8.0 nm are capable of almost completely removing the NbN (111) diffraction peak found in lower Tc films and enhancing the NbN (200) peak. MgO underlayers were found to be oriented in the (100) direction when sputtered in an atmosphere of Ar and N2 and randomly oriented when sputtered in Ar alone. The authors present details for the preparation and analysis of NbN and MgO films as well as the fabrication and electrical performance of tunnel junctions with and without MgO underlayers

A laser ablation process was developed to pattern thin-film superconducting devices with 1- mu m resolution. The process was utilized to fabricate bridge structures in high T/sub c/(>90 K) YBaCuO superconducting thin films sputtered on various substrates. For bridges patterned on thin films with a high degree of polycrystallinity, the authors observed weak-link Josephson behaviour, from 4.2 K up t...