This article reports on the use of real-time spectroscopic ellipsometry (RTSE) to (1) understand the growth process of ultrathin (10 nm thick) Ta-Si-N diffusion barriers and to (2) monitor their thermal stability up to a temperature of 800 degrees C. Thin films of Ta-Si-N diffusion barriers and Cu overlayers, were deposited on Si(111) substrates using reactive unbalanced magnetron sputtering. In order to reduce roughness and interdiffusion between consecutive surfaces, a modulated low energy and high flux ion assistance was utilized. The initial part of the films (2 nm) of each layer was deposited with a high flux of low energy ions (< 10 eV) to reduce intermixing, while higher energies (between 40 and 130 eV) were utilized for the remainder of the layer to decrease the percolation thickness. RTSE data were simulated using the Drude-Lorentz model to obtain information about the growth mechanism and the conduction electron transport properties for these structures. The films were annealed at 800 degrees C and the diffusion of copper into silicon was evaluated by monitoring changes in the optical properties of the bilayers. The pseudodielectric function of the films was found to be altered whenever diffusion proceeded. Thermal stability at 800 degrees C was achieved for samples produced using the ion-assistance technique. The results deduced from RTSE were verified by characterizing the elemental composition of the as-deposited and heat-treated films using Rutherford backscattering and time-of-flight secondary ion mass spectrometry. (c) 2005 American Vacuum Society.