The Future Circular Collider (FCC) study aims to develop designs for a new research infrastructure for the next generation of high-performance particle colliders, in order to extend the research currently being conducted at the Large Hadron Collider (LHC). The goal of the FCC is to push the energy and intensity frontiers of particle colliders, with the aim of reaching collision energies of 100 TeV that will aid in the search for new physics.

The FCC places high demands on the requirements for high-gradient accelerating structures used to accelerate the particle beam. While niobium-coated copper cavities are being considered for some of the FCC machine variants, like the FCC-ee, further optimization is required to reach the ambitious energy targets of the project, as their RF performance deteriorates with increasing accelerating fields. This phenomenon, termed Q-slope, is not seen on the more expensive bulk Nb competitors. The mechanisms behind the Q-slope are not yet completely understood but can be correlated to the microstructure of the superconducting layer. As an alternative to the traditionally used Nb, ongoing studies also focus on Nb3Sn, a type-II high temperature superconductor within its A-15 phase. Its high critical temperature twice the one of Nb and predicted superheating field motivate its application on SRF accelerating cavities.

The key to increasing the RF performance of these materials is a deep understanding of the impact of the coating parameters on the resulting microstructure, which ultimately determines the macroscopic performance. Here we present the ongoing activities at CERN that revolve around the manufacturing and characterisation of DC-biased High Power Impulse Magnetron Sputtering (HiPIMS) Nb and bipolar-HiPIMS Nb3Sn coatings. A particular focus is laid upon the microstructural characterisation by means of advanced microscopy techniques such as high sensitivity EDS, SEM and FIB and diffraction techniques such as high resolution XRD and EBSD in order to characterise the phase evolutions depending on the deposition parameters.

For the Nb coatings, investigations were conducted to study the effect of film thickness on dislocation density and cross-correlate it to the critical current density of the films, as the latter is a good metric for evaluating the quality of a superconductor from a defect content point of view. In the case of Nb3Sn, the discussed topics include challenges of A15 phase formation related to the correct Nb3Sn stoichiometry and relationship between deposition parameters and Sn percentage, as well as the adverse effects of film stress on the resultant critical temperature. Additionally, the deposition of Ta interlayers using various parameters is discussed as well as the Ta phase formation, in order to alleviate the presence of surface Cu contamination, while also achieving the required phases.