Abstract
Nb/Al-AlO x/Nb Josephson junctions are widely used as single or stacked devices for high-frequency applications. The quality of these junctions depends on the morphological and structural properties of the Nb base film, which must be optimized. To this purpose, niobium films (200 nm thick) have been deposited by RF magnetron sputtering at argon pressures between 0.75 and 3 Pa on different substrates. Measurements of the substrate curvature indicate that the stress changes from strongly compressive to zero as the Ar pressure increases. The film surfaces, as reconstructed by scanning tunneling microscopy (STM), show grains with a size varying between 30 and 100 nm. The measured roughness is larger for films deposited at higher Ar pressures because of the presence of larger voids between grains. At low deposition pressures, films show a smooth surface with a roughness close to 1 nm.
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