Thickness analysis of double-layered thin-film sample using spectral reflectometry

A multi-layered structure is being extensively applied in the high-tech devices fabrication in the semiconductor and display industries. For measuring the thin-film thicknesses of the multi-layered structure, various techniques like spectral reflectometry, spectral ellipsometry, and SEM/TEM have been used depending on the application fields. Among them, the spectral reflectometry is being widely used because of the advantages of simple configuration, non-destructive characteristics, and high-speed measurement. In spectral reflectometry, the reliability of the reflectance model is very important, because the higher the agreement of the modeled reflectance to the measured one, the lower the measurement uncertainty of the thin-film thicknesses determined by the reflectance model. In case of the single-layer thin-film sample, the thickness can be verified using a certified reference material, but the multi-layer thicknesses are not easy to be verified unlike single-layer case. In this study, to check the reliability of the multi-layer reflectance model, two different methods were used; (1) the extension of the single-layer model and (2) the multi-layer model based on the transfer matrix. The first one is to sequentially determine the thin-film thicknesses from layer to layer. The second one is to simultaneously determine all the thin-film thicknesses of a multi-layer structure. By applying two methods to double-layered thin-film sample(SiO₂-SiN), the thin-film thicknesses of both layers were determined and compared to each other by considering the measurement uncertainty. The applicability of the theoretical reflectance model can be confirmed according to whether the thin-film thickness measurement results are agreed within the uncertainty.