Outgassing and surface properties of TiN coated SNS ring vacuum chambers

The stainless steel vacuum chambers of the 248 m Spallation Neutron Source (SNS) accumulator ring are being coated with ∼ 100 nm of titanium nitride (TiN) to reduce the secondary electron yield (SEY). The coating is produced by DC magnetron sputtering using a long cathode imbedded with permanent magnets. The outgassing rates of several SNS half-cell chambers, with and without TiN coating, and before and after in-situ bake, were measured. The SEY of the coated chamber coupons were also measured with primary electron energy of 50-3000 eV. By varying the coating parameters, films of different surface roughness were produced and analyzed by scanning electron microscopy. It was found that the outgassing rate varies as a function of surface roughness of the TiN layer, and chambers with a rougher surface have a lower SEY.