Ellipsometric porosimetry on pore-controlled TiO₂ layers

Highlights: • We investigate the porosity and optical properties of porous TiO₂ layers. • With a new preparation technique, the layer porosity, pore size, and pore number is controllable. • By a combination of spectroscopic ellipsometry and EPMA, the porosity can be independently determined. • A new method for determining model uncertainties for spectroscopic ellipsometry is used. • The goal is a better understanding of metrology of complex material parameters with ellipsometry. - Abstract: The practical performance of surface coatings in applications like catalysis, water splitting or batteries depends critically on the coating materials' porosity. Determining the porosity in a fast and non-destructive way is still an unsolved problem for industrial thin-films technology. As a contribution to calibrated, non-destructive, optical layer characterisation, we present a multi-method comparison study on porous TiO₂ films deposited by sol-gel synthesis on Si wafers. The ellipsometric data were collected on a range of samples with different TiO₂ layer thickness and different porosity values. These samples were produced by templated sol-gel synthesis resulting in layers with a well-defined pore size and pore density. The ellipsometry measurement data were analysed by means of a Bruggeman effective medium approximation (BEMA), with the aim to determine the mixture ratio of void and matrix material by a multi-sample analysis strategy. This analysis yielded porosities and layer thicknesses for all samples as well as the dielectric function for the matrix material. Following the idea of multi-method techniques in metrology, the data was referenced to imaging by electron microscopy (SEM) and to a new EPMA (electron probe microanalysis) porosity approach for thin film analysis. This work might lead to a better metrological understanding of optical porosimetry and also to better-qualified characterisation methods for nano-porous layer systems.