Thermal stability of Pb-alloy Josephson junction electrode materials. II. Effects of SiO coating on Pb-In-Au base electrode

The preceding paper pertains to studies which were carried out for Pb-In-Au films without overlying SiO layers. Because use is made of configurations in Josephson junction integrated circuits presently under investigation in which Pb-In-Au base-electrode films are coated by overlying layers of SiO, a study was undertaken of the effects of such layers on the strain behavior of Pb-In-Au films using an x-ray diffraction technique. Pb-12 wt. % In-4 wt. % Au films were prepared at 298 or 77 K to obtain large or fine grains, respectively, and then thermally cycled between 300 and 4.2 K and between 300 and 350 K. In large-grained Pb-In-Au films, the strain relaxation, upon thermal cycling, by dislocation glide or by grain-boundary diffusion creep, was observed to be significantly inhibited by overlying SiO layers. However, in fine-grained films, that was not observed. For junctions in which the tunnel oxide is formed within an opening in an overlying SiO layer, it is believed, e.g., that upon heating, a compressive strain gradient develops in the large-grained base-electrode films between the junction area and its surrounding, leading to base-electrode deformation inside the opening that could rupture the tunnel oxide. This study indicates that the strain gradient should be reduced by reducing the average grain size of the base-electrode films. These results agree well with the previous experimental results in which no hillocks were detected by scanning electron microscopy inside similar SiO-layer openings above fine-grained Pb-In-Au films during repeated thermal cycling between 300 and 4.2 K or upon heating from 300 to 343 K.