Experimental evaluation of niobium film pinholes [for LEP cavities]

To assess the quality of vacuum-deposited films, well-defined areas of free-hanging thin film may be obtained by chemically dissolving the substrate on a fraction of its surface. A global evaluation of the thin film pinholes is then possible by measuring the gas conductance across the film. To do so, copper discs clamped between Con-Flat flanges have been used to separate a chamber filled with helium gas from another chamber where the helium throughput was measured. This method has been used to compare films produced under dust-free and dusty conditions, of various surface roughnesses and at different deposition incidence angles.