Spectrally extended, near-surface refractive index determination: atomic layer deposited TiO₂ on Si

The optical performance of very thin films has always been difficult to measure, and it has been a gray area due to interface and coating imperfections. Atomic layer deposition has produced pinhole-free, continuous, and extremely pure Titania films, with precise control at monolayer level, on native oxide/Si substrates, whose atomic force microscopy imaged nano-topology is approximately followed. For interface probing, the Abelès method is extended to the visible spectrum, and Cauchy dispersion curves are fitted to the refractive index experimental data, showing excellent compatibility with ellipsometric analysis. This is possible with decreasing optical thickness until ∼1/10 quarterwave is reached, which sets a limit for the usual optical film behavior.