Characterization of interfacial layers and surface roughness using spectroscopic reflectance, spectroscopic ellipsometry, and atomic force microscopy

Abstract

In modeling the optical properties of thin films, incorporation of roughness or interfacial layers is often required in the analysis of spectroscopic ellipsometric and spectroscopic reflectance data in order to achieve good agreement between the model and experimental data. The location of the roughness or interfacial layer is usually discernable from the spectroscopic ellipsometric data; however, their location is not always unambiguous from spectroscopic reflectance data. In the current work, we have explored how the spectroscopic determination of the interfacial and surface roughness layers correlates with direct measurements of the surface using atomic force microscopy (AFM). Spectroscopic reflectance and subsequent analysis of several thick films demonstrate the difficulty in placement of a roughness or interfacial layer in the optical model. The samples involved in this study have films deposited on metal substrates and include stainless steel and aluminum. We have used AFM to directly measure the surface roughness in order to improve the optical characterization and model development. As an example, we have examined an 8,000 nm silicon dioxide film on stainless steel. Models with the placement of an interfacial layer between the substrate and film, or placement of a roughness layer at the surface produce fits with nearly equivalent mean squared error values; however, the surface roughness layer is nearly an order of magnitude larger than that of the interfacial layer. Analysis using AFM shows a surface topography consistent with the magnitude of the interfacial roughness layer. In this example, the silicon dioxide layer was too thick for standard spectroscopic ellipsometry and spectroscopic reflectance was used exclusively in the analysis. For several samples with silicon dioxide on an aluminum substrate, an interfacial layer was necessary to produce a good model fit with the experimental data. These films of 500-1,000 nm thickness were analyzed using both spectroscopic ellipsometry and spectroscopic reflectance. The analysis for all films shows good agreement between the interfacial roughnesses calculated using an effective medium approximation (EMA) with AFM measurements, indicating the transfer or correlation of the substrate roughness to the surface.