Oxygen desorption during niobium sputtering for superconducting RF accelerators

Superconducting niobium RF resonators are an integral part of the various moderm accelerators — from low energy LINACs for heavy ions to electron-positron colliders with energy up to hundreds of GeV. Traditionally, the resonators have been made from high purity sheet niobium, but recently this design started to give way to copper substructures onto which a thin niobium film is deposited. tors with sputtered niobium are less expensive and higher acceleration fields can be achieved because the high thermoconductivity of copper eliminates thermal quenches. Unfortunately, that advantage is counterbalanced by degradation of the quality factor, Q, with increasing accelerating field. It is generally believed that Q degradation is caused by non-metallic impurities embedded into the niobium film during discharge sputtering. However, neither the amount and composition nor sources of the impurities are known with any certainty. aper examines the hypothesis that the mechanism of film contamination is the decomposition of the surface oxides on the copper substrate and the niobium cathode when they are exposed to the sputtering discharge. Quantitative connections between the impurity concentrations, the pressure of active gases during sputtering and the residual resistivity ratio of niobium are established and applied to the experimental data published by CERN. Calculations show that, with ignition of the sputtering discharge, the residual pressure of oxygen-containing gases increases by two orders of magnitude. uce the contamination of niobium films during sputtering, outgassing of the sputtering system in helium discharge is proposed.