OXYGEN DESORPTION DURING NIOBIUM SPUTTERING FOR SUPERCONDUCTING RF ACCELERATORS

Superconducting niobium RF resonators are an integral part of the vari ous modern accelerators - from low energy LINACs for heavy ions to ele ctron-positron colliders with energy up to hundreds of GeV. Traditiona lly, 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.Resonators with sputtered niob ium are less expensive and higher acceleration fields can be achieved because the high thermoconductivity of copper eliminates thermal quenc hes. Unfortunately, that advantage is counterbalanced by degradation o f the quality factor, Q, with increasing accelerating field. It is gen erally believed that Q degradation is caused by non-metallic impuritie s embedded into the niobium Nm during discharge sputtering. However, n either the amount and composition nor sources of the impurities are kn own with any certainty. This paper examines the hypothesis that the me chanism of film contamination is the decomposition of the surface oxid es on the copper substrate and the niobium cathode when they are expos ed to the sputtering discharge. Quantitative connections between the i mpurity concentrations, the pressure of active gases during sputtering and the residual resistivity ratio of niobium are established and app lied to the experimental data published by CERN. Calculations show tha t, with ignition of the sputtering discharge, the residual pressure of oxygen-containing gases increases by two orders of magnitude. To redu ce the contamination of niobium films during sputtering, outgassing of the sputtering system in helium discharge is proposed.