Thin Films Development For Superconducting RF Applications
Significant progress has recently been made in the development of niobium (Nb) films for Superconducting RF (SRF) applications using energetic condensation of ions extracted from plasma generated by Electron Cyclotron Resonance. Nb films with RRR comparable to bulk values are readily produced on both metallic and insulating substrates. The controlled incoming ion energy enables a number of processes such as desorption of adsorbed species, enhanced mobility of surface atoms and sub-implantation of impinging ions, thus producing improved film structures at lower process temperatures. Particular attention is given to the nucleation conditions to create a favorable template for growing the final surface exposed to SRF fields. The influence of the deposition energy is investigated with the characterization of the film surface, structure, superconducting properties and RF performance. With a similar approach, High Power Impulse Magnetron Sputtering (HiPIMS) is investigated for the development of multilayer SIS (superconductor-insulator-superconductor) structures which have the theoretical prospect to reach RF performance beyond bulk Nb. This paper presents results on the correlation of the surface and material characteristics with the superconducting properties and RF surface impedance for Nb and multilayered NbTiN-based SIS films produced on a variety of substrates.