TITANIUM NITRIDE COATING AS A MULTIPACTOR SUPPRESSOR ON RF COUPLER CERAMIC WINDOWS*

LAL-Orsay is developing an important effort on R&D and technology studies on RF power couplers for supraconductive cavities. One of the most critical components of those devices is the ceramic RF window that allows the power flux to be injected in the coaxial line. The presence of a dielectric window on a high power RF line has a strong influence on the multipactor phenomena. The most important method to reduce the multipactor is to decrease the secondary emission yield of the ceramic window. Due to its low secondary electron emission coefficient, TiN thin film is used as a multipactor suppressor coating on RF ceramic coupler windows. In this frame work, a TiN sputtering bench has been developed in LAL. The reactive sputtering of TiN needs the optimisation of the gas flow parameters, as well as the electrical one, to obtain stoichiometric deposit. XRD analysis was performed to control the film composition. The obtained measurements point out how the Nitrogen vacancy on the film can be controlled very well acting on the N2 flow during the reacting sputtering process. As the coating thickness should be optimized so that the TiN effectively reduces the SEE coefficient but does not cause excessive heating due to ohmic loss, and before multipactor level breakdown and resistance measurements for different deposit thicknesses, a good control of film thickness is essential. In-situ deposit thickness measurements given by quartz crystal microbalance was compared to X-Ray reflectivity one. A very good agreement was found between those two methods pointing out an efficient thickness and deposition rate monitoring. Once those steeps accomplished, an optimisation of deposition rate was performed in stoichiometric gas flow rate and electrical conditions