THE ROLE OF SURFACE-ROUGHNESS IN THE FABRICATION OF STACKED NB AL-ALOX/NB TUNNEL-JUNCTIONS/

The surface roughness of sputtered Nb films was determined with high p recision using x-ray specular reflectivity measurements in the 10 keV range. The roughness of Nb films increased from 0.9 nm for a 70-nm-thi ck film to 1.8 nm for a 210-nm-thick film. The roughness of the Nb sur face strongly influences the tunnel barrier formation and the electric al properties of that barrier. For stacked tunnel junctions each therm al Al oxidation has to be adjusted as a function of the underlying Nb film thickness. Alternatively, one can use an Al or Al/AlOx underlayer in order to obtain stacks with small spread in critical currents. (C) 1996 American Institute of Physics.