Thermal stability of Pb-alloy Josephson junction electrode materials. I. Effects of film thickness and grain size of Pb-In-Au base electrodes

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Abstract
Pb‐alloy Josephson devices are attractive for potential use as active elements in high‐speed computers. Failures occur in some Pb‐alloy junctions during repeated thermal cycling between 300 and 4.2 K. The failures are believed to be primarily caused by rupture of the ultrathin tunnel barrier oxide between base and counter electrodes when strain in the electrode films induced by thermal‐expansion‐coefficient mismatch between film and substrate is relaxed. The strain relaxation can cause microstructure changes such as grain rotation and hillock formation that can lead to device failure. The effects of film thickness h and grain size g on the strain behavior and microstructure of Pb‐12 wt. % In‐4wt. % Au films similar to those used for a Josephson junction base electrode were studied by x‐ray diffraction, and by transmission and scanning electron microscopy. Films with various combinations of h and g were prepared by evaporation onto oxidized Si substrates at room or liquid‐nitrogen temperature. In the present Pb‐alloy films, grain size and film thickness were found to be the key factors that control strain relaxation upon cooling from 300 to 4.2 K, as previously observed in pure Pb films. By reducing the average grain size or the film thickness down to ∼0.1 mm, the strain relaxation during cooling to 4.2 K was reduced to near zero. The stress at 4.2 K was calculated from the measured strain, and it was found that the stress level supported in fine‐grained Pb‐alloy films was close to the theoretical shear strength of Pb. It is believed that this stress level is the highest value obtained in soft materials with a film thickness of ∼1 mm. After repeated cycling between 300 and 4.2 K of fine‐grained Pb‐In‐Au films, no evidence of microstructure changes which could cause device failure was found. The use of such fine‐grained Pb‐alloy base‐electrode films has allowed a 100‐times improvement in the ability of Pb‐alloy Josephson devices to withstand repeated thermal cycling to 4.2 K.