Optical method for simultaneous thickness measurements of two layers with a significant thickness difference

In this study, an optical method that allows simultaneous thickness measurements of two different layers distributed over a broad thickness range from several tens of nanometers to a few millimeters based on the integration of a spectroscopic reflectometer and a spectral-domain interferometer is proposed. Regarding the optical configuration of the integrated system, various factors, such as the operating spectral band, the measurement beam paths, and the illumination beam type, were considered to match the measurement positions and effectively separate two measurement signals acquired using both measurement techniques. Furthermore, for the thickness measurement algorithm, a model-based analysis method for high-precision substrate thickness measurements in thin-film specimens was designed to minimize the measurement error caused by thin films, and it was confirmed that the error is decreased significantly to less than 8 nm as compared to that when using a Fourier-transform analysis. The ability to undertake simultaneous thickness measurements of both layers using the proposed system was successfully verified on a specimen consisting of silicon dioxide thin film with nominal thicknesses of 100 nm and 150 nm and a 450 mu m-thick silicon substrate, resulting in the exact separation between the two layers. From measurement uncertainty evaluation of a thin-film, a substrate in a thin-film specimen, and a single substrate, the uncertainties were estimated to be 0.12 nm for the thin-film, 0.094 mu m for the substrate in a thin-film specimen, and 0.076 mu m for the substrate. The measurement performance of thicknesses distributed on multi-scale was verified through comparative measurements using standard measurement equipment for several reference samples. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Automated summary

An optical method for simultaneous thickness measurements of two different layers was proposed in this study, utilizing a model-based analysis method to minimize measurement errors and achieving significant accuracy improvement. The system's ability to separate layers was successfully verified on a silicon dioxide thin film specimen, with uncertainties estimated for each component. The measurement performance of multi-scale thicknesses was validated through comparative measurements with standard equipment on reference samples.