A systematic study of superconducting properties of niobium films sputtered on the inner wall of radiofrequency cavities is presented. The measured quantities include in particular the response to 1.5 GHz microwaves, the critical temperature, the penetration depth and the magnetic penetration field. In addition to films grown in different gas discharges (Xe, Kr, Ar and Ar/Ne mixtures) and to films grown on substrates prepared under different conditions, the study includes also bulk niobium cavities. The surface resistance is analysed in terms of its dependence on temperature, on RF field and, when relevant, on the density of trapped fluxons. A simple parameterisation is found to give a good fit to the data. Once allowance for the presence of impurities and defects is made by means of a single parameter, the electron mean free path, good agreement with BCS theory is observed. The fluxon-induced losses are studied in detail and their dependence on RF field, on temperature and on the density of trapped fluxons is analysed. The residual resistance is observed to be essentially uncorrelated with the other variables, suggesting that it is dominantly extragranular. In occasions very low residual resistances, in the nΩ range, have been maintained over a broad range of RF field, indicating the absence of significant fundamental limitations specific to the film technology in practical applications such as the production of accelerating cavities for particle accelerators.

Potential contributions to the residual surface resistance of niobium films exposed to 1.5 GHz microwaves are reviewed and studied. These include the oxidation of the film surface, the formation of hydride precipitates, the contamination by noble gas atoms and the presence of macroscopic film defects such as those resulting from the roughness of the substrate. Particular attention is given to the dependence of the residual resistance on the amplitude of the microwave. Results similar to those obtained for bulk niobium provide strong evidence against the conjecture that the small size of the film grains should be a fundamental limitation to the production of films having a low residual resistance.

Resistive losses induced by the presence of trapped magnetic flux in niobium superconducting films have been studied using 1.5 GHz microwaves. They are measured to span a very broad spectrum depending on the film-substrate interface and on the gas used in the sputtering discharge. An interpretation in terms of pinning by noble gas clusters is considered.

Evidence is presented for niobium film cavities performing as well as niobium bulk cavities, at variance with a widespread belief that their much smaller grain size should be a fundamental limitation preventing high quality factors to be maintained over a wide range of accelerating fields. By comparing the relative merits of the bulk and film technologies, a strong case is presented in favour of the latter.

An energetic vacuum deposition system has been used to study deposition energy effects on the properties of niobium thin films on copper and sapphire substrates. The absence of a working gas avoids the gaseous inclusions commonly seen with sputtering deposition. A biased substrate holder controls the deposition energy. Transition temperature and residual resistivity ratio of the niobium thin films at several deposition energies are obtained together with surface morphology and crystal orientation measurements by atomic force microscope inspection, X-ray diffraction analysis and transmitted electron microscope (TEM) analysis. The results show that niobium thin films on a sapphire substrate exhibit the best cryogenic properties at a deposition energy of around 123 eV. The TEM analysis revealed that epitaxial growth of film was evident when the deposition energy reached 163 eV for a sapphire substrate. Similarly, niobium thin films on copper substrates show that the film grows more oriented with higher deposition energy and the grain size reaches the scale of the film thickness at a deposition energy of around 153 eV. D