The objective of this work is to monitor the growth process and the thermal stability of ultrathin tantalum nitride barrier nanostructures against copper diffusion in integrated circuits using real-time spectroscopic ellipsometry (RTSE). Single layers of copper and bilayer films of copper and tantalum nitride were produced on Si(111) substrates using unbalanced magnetron sputtering. The RTSE data was simulated using the Bruggeman effective medium approximation and a combined Drude-Lorentz model to obtain information about the growth process, film architecture, interface quality, and the conduction electron transport properties for these structures. The results deduced from the RTSE were verified by characterizing the structural and the chemical properties of the fabricated films using x-ray diffraction, Auger electron spectroscopy, and Rutherford backscattering. The effectiveness of the tantalum nitride barrier to stop the diffusion of copper into silicon was evaluated, monitoring their optical properties when annealed at 720 degreesC. The dielectric function of the films changed from a metallic to an insulating character when the diffusion proceeded. Also, the RTSE provided valuable information about the microstructure and the kinetics of the phase transformations that occur during heat treatment. (C) 2004 American Institute of Physics.