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

Journal Article · Sun Mar 01 04:00:00 EST 1981 · J. Appl. Phys.; (United States)

DOI:https://doi.org/10.1063/1.329757· OSTI ID:6447132

Murakami, M

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 approx.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 approx.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 films has allowed a 100-times improvement in the ability of Pb-alloy Josephson devices to withstand repeated thermal cycling to 4.2 K.

• Research Organization: IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598
• OSTI ID: 6447132
• Journal Information: J. Appl. Phys.; (United States), Journal Name: J. Appl. Phys.; (United States) Vol. 52:3; ISSN JAPIA
• Country of Publication: United States
• Language: English

Related Subjects

420201* -- Engineering-- Cryogenic Equipment & Devices
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
ALLOYS
COMPUTERS
CRYSTAL STRUCTURE
DATA
DIMENSIONS
ELECTRODES
ELEMENTS
EXPANSION
FILMS
GOLD
GRAIN SIZE
INDIUM
INFORMATION
JOSEPHSON JUNCTIONS
JUNCTIONS
LEAD ALLOYS
LOW TEMPERATURE
MEDIUM TEMPERATURE
METALS
MICROSTRUCTURE
RELAXATION
SIZE
STABILITY
STRAINS
SUPERCONDUCTING JUNCTIONS
THERMAL EXPANSION
THICKNESS
TRANSITION ELEMENTS
ULTRALOW TEMPERATURE
VERY LOW TEMPERATURE