Structural and superconducting properties of sputter-deposited niobium films for applications in RF accelerating cavities

The present work presents the results of a systematic study of superconducting and structural properties of niobium films sputter deposited onto the inner walls of radiofrequency copper resonators. The measured superconducting quantities include the surface resistance, the critical temperature, the penetration depth and the upper and lower critical fields. In addition to films grown with different discharge gases (Xe, Kr, Ar, Ne and Ar-Ne mixtures) and to films grown on substrates prepared under different conditions, the study also includes massive niobium cavities. The surface resistance is analysed in terms of its dependence on the temperature and on the rf field amplitude and, when possible, compared to theoretical predictions. In general, good agreement with BCS theory is observed. All experimental results are presented in the form of a simple, but adequate parameterisation. The residual resistance is observed to be essentially uncorrelated with the other variables, but strongly dependent on the macroscopic defect density of the copper substrate. Several mechanisms, leading to the generation of a non-zero residual resistance in niobium films are discussed. Electrochemical polishing of the copper resonators before sputter deposition of niobium gives reliably low values of the residual resistance. These values are entirely comparable with those obtained on bulk resonators, defeating any argumentation against the applicability of film cavities for particle accelerators. By comparing the relative merits of the bulk and film technologies, the latter should be preferred in future large-scale applications using superconducting cavities. Knowledge about the film structure is necessary, when discussing the differences of the superconducting properties observed between different types of films. Therefore, niobium films cut from different sites of 1.5GHz resonators, and films, deposited under different conditions on small copper samples are analysed by X-ray diffraction. The films are characterised by measuring the resistivity, the lattice parameter, the texture, the stress and the microstrain. The strong dependence of these properties, especially of the resistivity and the film strain, on the amount of implanted noble gas is evidenced, and the effect of sputter cleaning before deposition is evaluated. It is found that stresses and microstrains of niobium films deposited on oxidised copper are considerably higher than those of films grown on sputter cleaned copper. The dependence of stress on the nature of the substrate can be interpreted by a combined consideration of growth and misfit stresses. The critical temperature is found to be higher than the nominal value of 9.3K of bulk niobium and is observed to depend on the amount of macrostresses present in the material. Films, which had been removed from their substrates, release their elastic stresses and show a transition temperature close to that of the bulk material. The residual resistivity is shown to be determined by the grain size and the presence of microstrains representing crystal imperfections, such as dislocations and embedded noble gas atoms. The mean free path is calculated for different films from the experimentally determined residual resistivity ratio and from measured values of the penetration depth. Excellent agreement between both sets of data is obtained. The upper critical field of the films is independent of the amount of stresses present in their interior and found to take values similar to those measured on bulk niobium with a comparable mean free path.