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Optical surface topography measurements sometimes suffer from systematic errors. In order to predict such deviations, modeling of optical profilers is a substantial part of the European project TracOptic (Traceable Optics). Within the framework of this project, we recently developed the UFO (Universal Fourier Optics) model, which simulates virtual CSI measurements of surface topographies that fulfill the requirements of the scalar Kirchhoff approximation. The model enables a fast computation of ‘measured’ surface topographies as it is based on discrete Fourier transforms. It treats the surface under investigation as a two-dimensional phase object assuming a linear dependence of the interference phase on surface height and axial spatial frequency. The scattered light field is transferred to the Fourier domain and multiplied by a partial two-dimensional transfer function (TF) representing a horizontal cross section of the three-dimensional TF at a certain axial spatial frequency or evaluation wavelength, respectively. The TF includes parameters of the interference microscope and the reference field distribution. Inverse Fourier transform enables the reconstruction of the phase object. The coherence peak position of an interference signal results from numerical differentiation with respect to the axial spatial frequency and is generally used to overcome the 2π ambiguity of the phase profile. Parameters affecting final results of reconstructed surface topographies are the central wavelength and the spectral bandwidth of the illuminating light as well as the numerical aperture of the objective lens and the chosen evaluation wavelength. We discuss results of the UFO model with respect to the prediction of systematic deviations of measured surface topographies.
Peter Lehmann,Tobias Pahl, andJörg Riebeling
"Universal Fourier optics model for virtual coherence scanning interferometers", Proc. SPIE 12619, Modeling Aspects in Optical Metrology IX, 126190O (10 August 2023); https://doi.org/10.1117/12.2673292
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Peter Lehmann, Tobias Pahl, Jörg Riebeling, "Universal Fourier optics model for virtual coherence scanning interferometers," Proc. SPIE 12619, Modeling Aspects in Optical Metrology IX, 126190O (10 August 2023); https://doi.org/10.1117/12.2673292