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Final Report

Development of NbN Josephson Junction technology

Masoud Radparvar

Published Jul 24, 1986• 0 citations
OpenAlex

Abstract

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)

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IEEE Transactions on Magnetics•Mar 1, 1991•25 citations
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)
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.
We have developed a novel process for fabricating all refractory Josephson tunnel junctions whose base and counter electrodes are composed of (Nb-NbN) double-layered films. In this process, tunnel junctions have been isolated using a reactive ion etching from a junction sandwich which had been formed on an entire silicon wafer, and also the reactive ion etching has been used for patterning electrodes and insulation layers in devices. Fabricated junctions have yielded good tunneling characteristics with excellent uniformity and reproducibility. The maximum critical current density was widely changed from 300 A/cm2to 8 kA/cm2. All refractory single flux quantum(SFQ) memory cells in a 32×32 arrangement have been integrated with a 2.5 μm minimum linewidth on a chip using this process, and an SFQ cell in the memory array has been successfully operated. The maximum deviation of critical currents in the memory array was measured to be ±6.5%.
All hard Josephson tunnel junctions, whose base and counter electrodes are composed of double-layered niobium nitride (NbN) and niobium (Nb) films, have been successfully fabricated by isolating a junction sandwich formed on a whole silicon wafer with a reactive ion etching technique. The reactive ion etching technique has been used for patterning both base and counterelectrodes, and self-aligning definition of junction areas has been performed. The fabricated junctions show good quality single-particle tunneling characteristics and excellent uniformity in critical currents.
We have developed a new integration method for Josephson tunnel junctions based on high Tc superconducting compound materials such as NbN. In this method, a sputtered ZnO film has been employed as a resist mask which can be used in high substrate temperatures during the film deposition. It has been found that fine patterns of the refractory superconducting film can be obtained by this method and the patterning processes have no interference with other integration processes. We have applied this method to patterning base electrode of NbN and formed high quality NbN-oxide-Pb Josephson tunnel junctions by using an rf plasma oxidation. The fabricated junctions have been found to have a relatively large gap ( ∼ 4.0 mV ) and low current leak in the subgap region, and to be quite stable for thermal cyclings and storage. We have also fabricated quantum interference memory devices by integrating these junctions and the operation has been successfully demonstrated.
We report the fabrication of all-refractory superconductor-insulator-superconductor tunnel junctions of the form NbN/MgO/NbN. The MgO insulating barrier was deposited by e-beam evaporation. High quality junctions were fabricated with sum gaps of 5.2 meV, and a small subgap leakage parameter (Vm=30 mV, measured at 3mV). These devices are for eventual use as quasiparticle mixer elements in millimeter/submiltimeter wave heterodyne receivers. Fabrication techniques and current-voltage characteristics are discussed. We also propose a new growth mode for MgO films on NbN.
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.