Quality measurement of niobium thin films for Nb/Cu Superconducting RF cavities

Niobium-coated copper resonator cavities are an interesting alternative to bulk niobium ones for superconducting RF (SCRF) applications to particle accelerators. Magnetron sputtering is the technology developed at CERN for depositing niobium films and applied over the past 20 years. Unfortunately, the observed degradation of the quality factor with increasing cavity voltage, not completely understood, prevents their use in future large accelerators designed to work at gradients higher than 30 MV m with quality factors in the 10 10 (or higher) region. At the beginning of the new millennium, new deposition techniques were proposed to overcome the difficulties encountered with magnetron sputtering. This paper presents the main analysis applied to the niobium films to investigate their quality and the obtained results. The correlations between the niobium film properties and the RF performances of accelerating cavities are summarized. The properties of niobium films obtained by magnetron sputtering and by cathodic arc deposition in ultra-high vacuum (UHVCA) are compared. The UHVCA-produced Nb films have structural and transport properties closer to the bulk ones, providing a promising alternative for niobium coated, high voltage, high Q, copper RF cavities with respect to the standard magnetron sputtering technique.