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Applied Physics Letters

New fabrication process for Josephson tunnel junctions with (niobium nitride, niobium) double-layered electrodes

Akira Shoji, Fujitoshi Shinoki, S. Kosaka, Masahiro Aoyagi, Hisao Hayakawa

Published Dec 1, 1982• 70 citations
Publisher

Abstract

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.

DOI
10.1063/1.93378

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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 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%.
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.
A new structure of the base-electrode for Josephson devices have been developed in which niobium (Nb) and niobium nitride (NbN) films were sequentially sputtered to form a double-layered structure. Owing to this structure, it is possible not only to reduce the kinetic inductance of the base-electrode due to the small penetration depth of Nb film, but also to obtain good tunneling characteristics with a small subgap conductance due to the surface stability of NbN film. Single-flux-quantum (SFQ) memory cells with this structure have been fabricated and it has been observed that the kinetic inductance is much decreased comparing with that of the SFQ cell with a base-electrode of NbN single layer.
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 technique for the fabrication of Josephson tunnel junctions without breaking vacuum and without the use of masks is presented. No chemical processing is done on either of the junction electrode surfaces. Hence, no chemical or reactive cleaning is required before the formation of the tunneling barrier. Nevertheless, standard photolithography and chemical or plasma etching can be employed for defining patterns. I-V characteristics of Nb-NbOx-Pb junctions fabricated by this method have not exhibited a ’’knee’’ at the onset of the quasiparticle current.