Interdiffusion-induced surface morphology in lead-alloy Josephson tunnel junction electrodes

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Abstract
Surface morphology changes have been observed in both of the electrode alloys adjacent to the tunneling oxide layer in electrically shorted Pb-alloy Josephson devices. The morphology change appears to be a result of interdiffusion between the base electrode alloy (Pb-In-Au) and the counterelectrode alloy (Pb-Bi) through ruptures in the thin tunnel oxide. By using an electron microprobe, In atoms were found to have diffused into the Pb-Bi film, while Bi atoms diffused in the reverse direction into the Pb-In-Au film. Their diffusivities at 298 K were estimated from concentration profiles to be ∼1×10−10 cm2/s. Grain boundary diffusion is thought to be responsible for such rapid material transport. Morphology changes are explained as a consequence of stress relief in both electrodes, where the stress results from differences in interdiffusion flux. It appears that Bi diffusion flux into the Pb-In-Au layer is larger than the In flux diffusing into the Pb-Bi layer because of the smaller grain size of the Pb-In-Au layer. As a result, Pb-In-Au film is put into compressive stress, which is subsequently relieved by hillock formation, whereas Pb-Bi film is put into tensile stress, causing void formation and/or surface ripples. Morphology changes were also found to be strongly influenced by the nature of the surface coating on the films. Of particular interest, when the Pb-Bi film is uncoated or covered only with KMER (Kodak metal etch resist) photoresist, a set of multiple, concentric rings of voids were observed, each centered at the site of an oxide rupture. This ring formation therefore provides a unique means to locate the site of initial oxide rupture during failure analysis and to distinguish In-diffusion-induced failures from thermal-cycle-induced failures.