A capping layer for a semiconductor structure is described. The capping layer is deposited over a silicide-forming metal and has a composition such that nitrogen diffusion therefrom is insufficient to cause formation of an oxynitride from an oxide layer on the underlying silicon. The capping layer may be a metal layer from which no N diffusion occurs, or one or more layers including Ti and/or TiN arranged so that N atoms do not reach the oxide layer. A method is also described for forming the Ti and TiN layers. It is advantageous to deposit non-stoichiometric TiN deficient in N, by sputtering from a Ti target in a nitrogen flow insufficient to cause formation of a nitride on the target.