Influence of coating temperature on niobium films

The coating of niobium on copper is the technology successfully used for the production of LEP accelerating cavities. A good understanding of the influence of the different coating parameters on the film properties can contribute to improve the RF performance of such cavities. Several copper samples were coated with a 1.5 mu m thick niobium film in a cylindrical magnetron sputtering system, using argon as discharge gas. To study the mere effect of the coating temperature, a 500 MHz cavity was equipped with three sample-holders on the equatorial region. The latter were kept at different temperatures during the baking and the simultaneous coating (150 degrees C, 250 degrees C and 350 degrees C). The films were characterised by measuring the RRR, critical temperature, total Ar content, lattice parameter. Films deposited at higher temperatures show higher RRR and lower Ar content. The film lattice parameter and, consequently, the critical temperature change with the coating temperature. The results are interpreted in terms of the film bombardment during the growth, of higher niobium surface mobility at higher temperature and of the different thermal expansion coefficients between the niobium film and the substrate.