Effects of intrinsic stress on submicrometer Nb/AlOx/Nb Josephson junctions

The intrinsic stress and its relaxation process are discussed for sputtered Nb films used for the electrodes in Nb/AlO_x/Nb Josephson junctions. The stress, optically measured for Nb films, depends on the Ar pressure during sputtering, and it changes from compressive to tensile when the Ar pressure is increased. From X-ray diffraction, the shift in the lattice constant was observed to be proportional to the film stress. Changes in the lattice constant were also clearly observed when Nb films were etched to fine patterns. This suggests that the intrinsic stress in Nb films is relaxed at peripheral areas after the patterning process, and that such relaxation is one cause of deterioration in current-voltage characteristics frequently observed in small Josephson junctions. This indicates that stress-free Nb should be used for submicrometer junctions