Md. Malev et Dc. Weisser, OXYGEN DESORPTION DURING NIOBIUM SPUTTERING FOR SUPERCONDUCTING RF ACCELERATORS, Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment, 364(3), 1995, pp. 409-415
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.