Multilayer model for determining the thickness and refractive index of sol–gel coatings via laser ellipsometry

The optical properties of sol–gel silica coatings on silicon wafers are investigated. During the sol–gel process the samples are subjected to heat treatment with a range of different temperatures on which the resultant film thicknesses and refractive indices are dependent. The morphology of the coating is also dependent on the heat treatment, particularly at higher temperatures where precipitates can form and delamination can occur. We used a model of the coating that not only accounts for the sol–gel film but includes a thin crust on the surface and a layer of oxide in between the coating and substrate. The multilayer model is then used to determine the refractive index of the film by least-squares fitting the measured change in polarization versus the incident angle. ► Thin sol–gel coatings are characterized by a multilayer model. ► Refractive index increases, thickness decreases with heat treatment temperature. ► Ellipsometry enables characterization of complete multilayer model. ► Findings confirmed by refractometry, reflectometry and calotte grinding.