Niobium Films on Copper-Its Adherence and Superconducting Rf Properties

The application of niobium films on copper for superconducting rf cavities is expected. We have developed a method for depositing adhesive niobium films on copper which were stable below 4.2 K. Niobium films were deposited on the polished copper substrates with rf magnetron sputtering. The niobium films deposited directly on the mechanically, chemically or electrolytically polished copper substrates exfoliated or blistered. We installed, therefore, a titanium layer as an intermediate layer to deposit adhesive niobium films on copper. Thus deposited niobium were very adhesive under the thermal cyclic test from 2.5 K to 300 K, and showed as good superconducting rf properties as those for a niobium sheet.