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IEEE Transactions on Magnetics

Nb/Nb oxide/Pb-alloy Josephson tunnel junctions

Stanley I. Raider, R.E. Drake

Published Jan 1, 1981• 41 citations

Abstract

A procedure is described for fabricating thin film Nb/Nb oxide/Pb-alloy Josephson tunnel junctions that satisfies the principal requirements for integrated circuit design and fabrication. A deposited Nb film, evaporated from an e-gun heated source, was patterned by chemical etching to form a base electrode. A junction was completed by plasma etching and plasma oxidizing the Nb junction area to form a tunnel barrier and by depositing a Pb alloy counterelectrode. Josephson tunnel junctions with Nb/Nb oxide/Pb-Au-In structures were prepared with low excess subgap currents in the current-voltage (I-V) curve and with reproducible and stable I-V characteristics. Variations in junction current density from run-run were ± 15%. Seven 3-junction interferomeeters out of a population of 50,000 were shorted (99.99% yield) for causes not immediately attributable to photoresist-related defects. No changes in Josephson current were detected after thermal cycling 17,000 interferometers 1,800 × between room temperature and 4.2°K, after storing them for 2 years at 5°C, or after annealing 5,000 interferometers for 4.5 hours at 70°C.

DOI: 10.1109/tmag.1981.1060970

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A procedure is described for fabricating thin film Nb/Nb oxide/Pb-alloy Josephson tunnel junctions that satisfies the principal requirements for integrated circuit design and fabrication. A deposited Nb film, evaporated from an e-gun heated source, was patterned by chemical etching to form a base electrode. A junction was completed by plasma etching and plasma oxidizing the Nb junction area to form a tunnel barrier and by depositing a Pb alloy counterelectrode. Josephson tunnel junctions with Nb/Nb oxide/Pb-Au-In structures were prepared with low excess subgap currents in the current-voltage (I-V) curve and with reproducible and stable I-V characteristics. Variations in junction current density from run-run were ± 15%. Seven 3-junction interferomeeters out of a population of 50,000 were shorted (99.99% yield) for causes not immediately attributable to photoresist-related defects. No changes in Josephson current were detected after thermal cycling 17,000 interferometers 1,800 × between room temperature and 4.2°K, after storing them for 2 years at 5°C, or after annealing 5,000 interferometers for 4.5 hours at 70°C.

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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.

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.

Niobium/aluminum-oxide/niobium Josephson tunnel junctions have been fabricated using electron beam evaporation to form all-metal films in a high vacuum system. A quality factor, V m (the product of the maximum dc Josephson current and the subgap resistance defined at 2 mV) between 36–37 mV, has been obtained for the best junctions of a 6 nm-thick Al layer by cooling down the surface of the base electrode for 100 minutes under a high background pressure, before the deposition of aluminum, after the formation of the base niobium electrode. An array of 80 series-connected junctions has also been fabricated using the selective niobium anodization process and the planarization process. The current-voltage characteristics of the array showed a standard deviation of the maximum dc Josephson current of 3.3%.

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.