The Interface Between High Tc Superconductors and Silver Contacts
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THE INTERFACE BETWEEN HIGH Tc SUPERCONDUCTORS AND SILVER CONTACTS
MALCOLM D. MCCONNELL AND WILLIAM G. MORRIS General Electric Corporate Research and Development, 1 River Road, Schenectady, NY 12301
ABSTRACT Silver diffuses into the lattice of YBa2Cu 3 0 7 during the annealing of silver contacts at 500 0 C. Depth profiles using Auger electron spectroscopy, with a focused electron beam, show an extensive diffusion zone, as well as segregation of barium and oxygen to the surface of the contact. There appears to be a slight loss of silver from the surface region, which could be explained by enhanced concentration at grain boundaries. High resolution SEM analysis of a fracture surface did not reveal the presence of silver on the grain boundaries. Diffusion rates were correlated with grain orientation by using magnetically aligned samples, and were found to be greater for the direction parallel to the C axis.
EXPERIMENTAL Rectangular blocks of 97% dense YBa2Cu3O7, made by isostatic pressing of finely ground powders and sintering, were used in these experiments. They were polished on one face to a one micron diamond finish. Subsequently, the blocks were cleaned in hexan with ultrasonic agitation. Silver contact pads or stripes were evaporated onto this polished surface at thicknesses from 500 to 3,000 angstroms. The vacuum during evaporation was in the 10-6 torr range. Auger electron spectroscopy was performed using a scanning Auger microprobe (SAM) operating with a beam current of 150 nA at 5 Kv. The beam was focused in the center of a large grain to reduce the effects of grain boundaries. Sputtering was done with 2 Kv argon ions. On the asevaporated 300-nm contact, an Auger elemental profile was made through the silver and into the underlying ceramic (Fig. 1). The profiles are plotted in atomic percent verses sputtering time. Sputtering rates are approximately 150 angstroms per minute. Peak to peak intensities were corrected using sensitivity factors, calculated from standards whose values were determined by ICP, and then normalized to 100 percent. Interface broadening, resulting from initial surface roughness, was controlled by using highly polished surfaces before deposition of silver. Samples were then annealed at 500 0 C in air for one and four hours and then cooled slowly.
Mat. Res. Soc. Symp. Proc. Vol. 169. '1990 Materials Research Society
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Fig. 1. Auger depth profile through 300-nm Ag layer before annealing In air.
RESULTS As-deposited films have sharp interfaces between the silver and the ceramic. Yet, the silver appears to have diffused slightly into the ceramic under the evaporation conditions. This shows the extreme mobility of the silver. The 300-nm silver contact was then annealed for one hour. The profile shows a broad interface with diffusion of the silver into ceramic and oxygen and barium to the surface of the silver (Fig 2).
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