Composition of tantalum nitride thin films grown by low-energy nitrogen implantation:: a factor analysis study of the Ta 4f XPS core level

Tantalum nitride thin films have been grown by in situ nitrogen implantation of metallic tantalum at room temperature over the energy range of 0.5-5 keV. X-ray photoelectron spectroscopy and factor analysis (FA) have been used to characterize the chemical composition of the films. The number of the different Ta-N phases formed during nitrogen implantation, as well as their spectral shapes and concentrations, have been obtained using principal component analysis and iterative target transformation factor analysis, without any prior assumptions. According to FA results, the composition of the tantalum nitride films depends on both the ion dose and the ion energy, and is mainly formed by a mixture of metallic tantalum, beta-TaN0.05 , gamma-Ta2N and cubic/hexagonal TaN phases. The kinetics of tantalum nitridation is characterized by two stages. In the first stage, the formation of beta-TaN0.05 species leads to a strong attenuation of the metallic tantalum signal. During the second stage, beta-TaN0.05 transforms into gamma-Ta2N and cubic/hexagonal TaN species. For intermediate ion doses, the concentration of gamma-Ta2N reaches a maximum, subsequently decreasing because of its transformation into cubic/hexagonal TaN phases with increasing ion dose up to saturation. At saturation, the films are mainly composed of a mixture of gamma-Ta2N and cubic/hexagonal TaN phases, but small Ta-0 and beta-TaN0.05 signals are also observed. They should be attributed to preferential sputtering of nitrogen and/or to the limited thickness of the film. Comparison of the experimental nitrogen concentration with that obtained using TRIDYN simulations suggests that, in addition to nitrogen implantation and atomic mixing, other mechanisms, like ion beam enhanced diffusion or the chemical reactivity of the tantalum substrate towards nitrogen, should also be taken into account at higher ion-beam energies.