Structural and RF properties of niobium films deposited onto annealed niobium resonators

Studies have been performed on the properties of niobium thin films sputtered onto solid niobium TM010 resonators at 1.5 GHz. The purpose of the work is to study the behaviour of the film's RF and str uctural properties as a function of heat treatment temperature in order to determine if and at what treatment temperature the properties of the films merge with those of the bulk. Niobium resonators h ave been heat treated at temperatures up to 1100°C in a vacuum furnace inside a niobium box surrounded by a titanium gettering protection. Subsequently, they have been sputter coated with a niobium fi lm. Following RF measurements of the coated resonators, the cavities have undergone heat treatments as described above at 800°C, 900°C, 1000°C and 1100°C, each time followed by RF measurements. Before heat treatment, the RF response of the film was similar to that of a film coated on a copper substrate. A marked transition towards bulk-like RF behaviour was observed after the 900°C treatment. The c hanges include a sharp variation of the BCS resistance and of the sensitivity to externally applied magnetic field, quantities believed to be closely linked to the amount and nature of defects in the coating.