Single layers of Mo and W thin films and Mo/Si/Mo and W/Si/W tri-layers have been deposited by r.f. sputtering on c-Si substrates as precursor to the fabrication of Mo/Si and W/Si multilayer X-ray mirrors. The single layer thin films have been characterized primarily by phase modulated spectroscopic ellipsometry (SE) and information have been derived regarding the thickness and volume fraction of voids present in the layers. The results obtained by fitting the SE data have been verified by complementary techniques, viz. grazing-incidence X-ray reflectivity, Rutherford back-scattering and atomic force microscopy. With the optimized values of the sputtering parameters, Mo/Si/Mo and W/Si/W tri-layer structures have been deposited on c-Si substrates and have been characterized by secondary ion mass spectrometry (SIMS) technique using sputtering by a Cs+ ion source. The depth profile data obtained from the SIMS measurements have been analyzed to get the inter-diffusion at the metal-Si interfaces. Efforts have been given to correlate the interface diffusion at the interfaces to the thickness of the surface layers of the metal films. (C) 2003 Elsevier Science B.V. All rights reserved.