Niobium films produced by magnetron sputtering using an Ar-He mixture as discharge gas

Superconducting RF accelerating cavities have been produced at CERN by sputter-coating, with a thin niobium layer, cavities made of copper. In the present work, the discharge behaviour and niobium film properties have been investigated when part of the argon sputtering gas is replaced with helium. Helium is chosen because of its low mass, which reduces the energy lost by the niobium atoms colliding with the sputter gas atoms. The higher niobium atom energy should lead to higher adatom mobility on the substrate and, hence, to a larger grain size, a feature which is highly desirable to reduce the cavity surface resistance. It has been found that helium addition effectively helps to maintain the discharge at considerably lower argon pressures, via metastable-neutral ionisation and high secondary electron yield. However, a large amount of helium is trapped in the film, amount which is proportional to the helium partial pressure during the discharge, resulting in a reduction of both Residual Resistivity Ratio and grain size.