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IEEE Transactions on Applied Superconductivity

Niobium nitride Josephson junction process development

James M. Murduck, J. DiMond, C.Y. Dang, Huang-Tian Chan

Published Mar 1, 1993• 7 citations
Publisher

Abstract

NbN/MgO/NbN tunnel junction fabrication has been developed using statistical experimental design for insertion into a Josephson LSI (large-scale integration) fabrication technology. Junction deposition conditions have been studied and correlated with structural characterization and junction I-V measurements. Josephson junctions with sum gaps as large as 5.0 mV and junction quality factor of 47 mV have been produced. Current density uniformity, a critical parameter in circuit design, has also been investigated. I/sub c/(O) uniformity at 4.2 K of a 4000 junction array was within sigma =3.1% and was +or-10% across 1 in/sup 2/. The utility of this junction process has been demonstrated by insertion into a standard circuit fabrication process that has produced logic circuit operation above 10 K.>

DOI
10.1109/77.233566

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NbN/MgO/NbN tunnel junction fabrication has been developed using statistical experimental design for insertion into a Josephson LSI (large-scale integration) fabrication technology. Junction deposition conditions have been studied and correlated with structural characterization and junction I-V measurements. Josephson junctions with sum gaps as large as 5.0 mV and junction quality factor of 47 mV have been produced. Current density uniformity, a critical parameter in circuit design, has also been investigated. I/sub c/(O) uniformity at 4.2 K of a 4000 junction array was within sigma =3.1% and was +or-10% across 1 in/sup 2/. The utility of this junction process has been demonstrated by insertion into a standard circuit fabrication process that has produced logic circuit operation above 10 K.>
We present the latest development of an Nb/Al-AlOx/Nb Josephson junction fabrication process for superconducting integrated circuits at SIMIT. Our fabrication process has four niobium superconducting layers and one Ti/Pd resistor layer with 1.0 Ω/square sheet resistor for junction shunting and circuit biasing. Critical current density Jcof the junctions is controlled from 30 A/cm2to 10 kA/cm2by exposing aluminum layer to pure or mixed O2atmosphere. Correlation between measured Jcand exposure conditions has been established. Average quality factor (Rsg/Rn) of the junctions in our process is larger than 20 with subgap leakage factor (Vm) beyond 30 mV. Critical current uniformity is obtained according to the switching current (Is) distribution of a 1000-JJ array. Measured standard deviation sigma of Isis less than 4% over the 100-mm wafer. Plasma resonance in a large Josephson junction is used to measure specific capacitance (Cs) of the junction. Sheet inductances of superconducting niobium strip-lines are extracted by a SQUID diagnose circuit and the measured results show good agreement with our design values.
A novel process suitable for fabrication superconducting circuitry based on the all refactory material NbN is described. In this process, and in-situ trilayer film composed of NbN/MgO/NbN is used to fabricate Josephson tunnel junctions. Reactive ion etching processes are used to delineate devices and pattern insulators and metallizations. Fabricated junctions have yielded good tunneling characteristics with reasonable current density uniformity and reproducibility. Devices with gap voltages close to 3mV have been achieved for high quality junctions (Vm 20 mV). In addition to the trilayer, there are two wiring layers, two resistor depositions, and two insulation layers, constituting a full NbN based fabrication technology. Using this process, we fabricated and successfully tested thin film DC SQUID and time domain reflectometer (TDR) circuits. Preliminary measurements suggest that the critical temperature of these circuits is well within the operating temperature of commercial two-stage closed cycle refrigerators. (Author)
Niobium nitride-niobium nitride Josephson tunnel junctions have been fabricated using amorphous magnesium oxide (a-MgO) films as barriers. These junctions have excellent tunneling characteristics. For example, a large gap voltage (Vg=5.1 mV), a large product of the maximum critical current and the normal tunneling resistance (IcRn=3.25 mV), and a small subgap leakage current (Vm=45 mV, measured at 3 mV) have been obtained for a NbN/a-MgO/NbN junction. The critical current of this junction remains finite up to 14.5 K.
All-NbN Josephson tunnel junctions with sputter-deposited magnesium oxide barriers have been fabricated using single-crystal NbN films for the base electrodes. Fabricated Josephson junctions have shown good tunneling characteristics with large gap voltages (5.6-5.8 mV), narrow gap widths (0.1-0.2 mV, from 30 to 70%), and small subgap leakage currents (V/sub m/=20-30 mV, measured at 3 mV). The results of a measurement of a subgap structure for a fabricated junction suggest that the excess leakage currents of fabricated junctions are due to multiparticle tunneling through locally thin areas in the MgO barriers.
A novel process suitable for fabricating superconducting circuitry based on the all refractory material NbN is described. In this process, an in-situ trilayer film composed of NbN/MgO/NbN is used to fabricate Josephson tunnel junctions. Reactive ion etching processes are used to delineate devices and pattern insulators and metallizations. Fabricated junctions have yielded good tunneling characteristics with reasonable current density uniformity and reproducibility. Devices with gap voltages close to 5mV have been achieved for high quality junctions. In addition to the trilayer, there are two wiring layers, two resistor depositions, and two insulation layers, constituting a full NbN based fabrication technology. Using this process, we fabricated and successfully tested thin film DC SQUID and time domain reflectometer (TDR) circuits. Preliminary measurements suggest that the critical temperature of these circuits is well within the operating temperature of commercial two-stage closed cycle refrigerators.