Microwave surface resistance of Nb films

The surface resistance, R_s, of niobium (Nb) films has been experimentally investigated as a function of thickness, preparation technique and substrate material at 8.86 GHz. Nb films were prepared by either sputtering or evaporation in the thickness range Between 0.1 μm and 3.0 μm on either copper (Cu) or sapphire substrate. R_s was determined using a cylindrical TE011mode resonant cavity with one removable end-plate which was utilized as the test substrate. The low field R_s at 4.2 K is lower than that of bulk Nb and shows good agreement with BCS calculation which takes into account the effects of mean free path. The temperature dependence of R_s indicates a normalized film gap parameter, Δ(0)/KTc, nearly equivalent to the bulk value for most of the films. At low temperatures, Rsis dominated by residual resistance (R0) which approaches 1 μΩ. The overall characteristics of Nb on Cu (Nb/Cu) indicate that this composite material is potentially useful in applications requiring high rf field as well as high thermal stability