Properties of Rf-sputtered Niobium Thin-films for Metrological Applications

Niobium superconductive films for metrological applications (refractory Josephson tunnel junctions, superconductive bolometers and radiation sensors), have been fabricated via r.f. magnetron sputtering. Their electrical and structural properties have been measured on a wide range of film thicknesses (10-1000 nm). The data indicate the growth of a first layer at the substrate interface, which has a different behaviour with respect to the remaining of the film. The electrical data of resistivity at various temperatures show the relevant role of this layer at thicknesses < 200 nm and the effect of pre-deposition treatments on the reduction of this interfacial layer. Experimental resistivity data can be fitted for these thicknesses only using a modified version of the Mayadas-Shatzkes model, which keeps into account not only the reflection at grain boundaries but also the altered composition layer.