Use of optical spacers to enhance infrared Mueller ellipsometry sensitivity: application to the characterization of...

Mueller ellipsometry in the mid-infrared (IR) spectral range can be used to obtain information about chemical composition through the vibrational spectra of samples. In the case of very thin films (<100  nm), the ellipsometric spectral features due to vibrational absorption are in general quite weak, and sometimes they are hidden by the noise in the measured data. In this work, we present one method based on the use of optical spacers as a tool to enhance the sensitivity of IR Mueller ellipsometry. An optical spacer is a thin film made of a known material which is between the substrate and the layer of interest. We show that, when the thickness of the two layers fulfills a given condition, the spectral features due to vibrational absorptions are enhanced. We explain the enhancement effect in terms of the Airy formula. The theoretical discussion is illustrated with two examples. We analyzed polystyrene thin films deposited on silicon wafers. Some of the wafers were covered by a thin film of thermal silicon dioxide (SiO₂), which was used as a spacer. The results show the suitability of the proposed technique to overcome the lack of sensitivity in ellipsometric measurements when it comes to working with either very thin films or materials with low absorption.