Non-destructive XPS depth-profiling of ta:C-films by variable excitation energy

Introduction Since hard coatings such as tetrahedral amorphous carbon (ta:C) are of growing interest for protecting surfaces of industrial machines and high capacity tools against wear and friction or for biomedical applications more information about the structure-to-property relation is required. Therefore core level photo emission and other spectral features in the x-ray photoelectron spectra (XPS) of carbon materials were used to derive “fingerprints” for selected carbon species. Besides the surface properties which can be gained by constant-energy XPS, information of different surface regions is required to receive information on the chemical structure of the material in different depths to understand the deposition process. Non-destructive depth profiling is possible by using photoelectron spectroscopy exploiting the variation of excitation energy to get information from several monolayers in depth down to the minimum of monolayer resolution. This is possible because there is a correlation of the effective attenuation length (EAL) of electrons in a solid with their kinetic energy [1]. A variation of the energy of the exciting X-ray radiation provides the possibility to adjust the kinetic energy of the photoelectrons as well as the EAL. The minimum EAL of C 1s photoelectrons should occur at roughly 350 eV Xray photon energy.