Bonding of a niobium wire to a niobium thin film

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Abstract
A method for bonding a niobium wire to a niobium thin film is described. The bonds are to be used as superconducting connections between wire-wound gradiometers and thin-film coupling coils on DC SQUIDS. The method is characterized by two steps. Firstly, the hardness of the niobium wire is reduced by a heat treatment. Secondly, the niobium film is covered with a thin layer of palladium to prevent it from oxidizing. Superconducting bonds were realized using an ultrasonic bonding technique. We tested the bonds and measured superconductivity (to a sensitivity level of 6 × 10-15 Ohm) with currents up to 80 mkA, which is equivalent to 10 times the dynamic range of the DC SQUID systems. Even at 80 mkA, limited by the measuring set-op, the critical current of the bonds is not reached.