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 f ilm properties can contribute to improve the RF performance of such cavities. Several copper samples were coated with a 1.5 mm thick niobium film in a cylindrical magnetron sputtering system, using a rgon as discharge gas. To study the effect of the coating temperature only, a 500 MHz cavity was equipped with three sample-holders on the equatorial region. The latter were kept at different temperat ures during the baking and the simultaneous coating (150ºC, 250ºC and 35ºC). The films were characterised by measuring the RRR, critical temperature, total Ar content and lattice parameter. Films depo sited 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 interpret ed 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 sub strate.