Nb-Oxide-Pb Josephson Tunnel Junctions Fabricated Using CF4 Cleaning Process (II)

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Abstract
The rf plasma cleaning conditions using a CF4 cleaning process (CFCP) giving high-quality junctions are discussed. The C-F radical chemical reaction provides a clean, damage-free Nb surface which is protected from oxygen dissolution by a carbon layer. The carbon layer is possibly effective in forming a high-quality tunnel oxide. The dependence of the maximum dc Josephson current on an external magnetic field shows a uniform current distribution. The total critical current spread was measured in arrays of 50 series-connected junctions with an area of 10×10 µm2 and was found to be within ±2.5% for the best sample at a mean current density of 10.6 kA/cm2.