Properties of DC magnetron sputtered Nb and NbN films for different source conditions

We have compared the quality of Nb and NbN films obtained by DC magnetron sputtering from a new and a fully eroded Nb target. Since current superconducting electronic devices such as SIS mixers, RSFQ digital circuits and hot electron bolometers are produced by reactive sputtering, we are interested in optimum source operating conditions over the target lifetime. We find that stress-free Nb films can, at any state of the target, be obtained under the same Ar pressure and DC power applied to the sputtering source. We show that this approach also works for NbN reactive sputtering if the nitrogen flow rate is maintained proportional to the deposition rate of Nb. In both cases the zero-stress point shifts to lower cathode DC voltage as the target erodes. Additionally we find that the effectiveness of the magnetic trap of the magnetron influences the normal state resistivity of the NbN films.