Relationship of Nb surface morphology and Al coverage to the intrinsic stress of magnetron-sputtered Nb films

To optimize Nb/Al-AlO/sub x//Nb Josephson junctions, atomic force microscopy is used to study the morphology of DC magnetron-sputtered Nb and Al. The Nb film stress was sensitive to Ar pressure, and changed from compressive to tensile as the Ar pressure increased. The roughness of DC magnetron-sputtered Nb films was related to their internal stress. Stress-free Nb films have a smooth surface, without depressions, and a roughness of 14.6 AA. The roughness of the Al surface on Nb film was shown to reflect the roughness of the underlying Nb film, and was least for the stress-free Nb film. Coverage of Al was continuous for Al thicker than 40 AA. To make a fine Nb/Al-AlO/sub x//Nb Josephson junction, one should use a stress-free Nb film and deposit an Al layer at least 40 AA thick on the Nb films.<
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