Improved film density for coatings at grazing angle of incidence in high power impulse magnetron sputtering with positive pulse

The production of a dense and void-free thin film on large and complex substrates is still a challenge in Physical Vapor Deposition. High Power Impulse Magnetron Sputtering (HiPIMS) with voltage inversion (positive pulse) after the main negative pulse is an attractive alternative to a negatively biased substrate to improve the film properties. In this manuscript, the properties of Nb thin-films deposited on flat Cu samples at different incidence angles with respect to the sputtered Nb target are investigated for various coating techniques. In particular, the results obtained using HiPIMS with the application of a positive voltage are compared with those resulting from negatively biased substrates, using as a reference films coated with the consolidated technique of Direct Current Magnetron Sputtering. Images of the film cross section obtained with a Focused Ion Beam-Scanning Electron Microscope enable to assess film morphology and local thickness, which is compared with the value obtained from X-ray Fluorescence measurements. Differences in the film morphology are highlighted for samples placed perpendicularly to the surface of the sputtered target. A significant densification for HiPIMS in the presence of a positive pulse is observed. The application of this approach to the coatings of superconducting radio-frequency cavities is discussed.