Influence of coating temperature on niobium films

The coating of niobium on copper is the technology successfully used for th e production of LEP accelerating cavities. A good understanding of the infl uence of the different coating parameters on the film properties can contri bute 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 s tudy the mere effect of the coating temperature, a 500 MHz cavity was equip ped with three sample-holders on the equatorial region. The latter were kep t at different temperatures during the baking and the simultaneous coating (150 degrees C, 250 degrees C and 350 degrees C). The films were characteri sed by measuring the RRR, critical temperature, total Ar content, lattice p arameter. Films deposited at higher temperatures show higher RRR and lower Ar content. The film lattice parameter and, consequently, the critical temp erature 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 coef ficients between the niobium film and the substrate.