STRUCTURAL AND SURFACE-PROPERTIES OF SPUTTERED NB FILMS FOR MULTILAYER DEVICES

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