Adaptive high-resolution Linnik interferometry for 3D measurement of microparticles

If high numerical apertures are used in coherence scanning interferometry, an extension of the interference signal’s spectral distribution to lower frequencies can be observed. Depending on the slope of the measured surface interference signal contributions belonging to higher frequencies will vanish. In addition, the high spatial frequency information of a measured surface structure will contribute to the low frequency components of the spectrum of the measured interference signals. These effects can be explained by analyzing both the measuring object as well as the transfer characteristics of the interference microscope in the 3D spatial frequency domain. In this study we analyze the mentioned effects based on Kirchhoff’s diffraction theory in the spatial frequency domain introducing the double foil model. The model explains why the choice of the wavelength, which is used for signal analysis, shows a substantial impact on the reconstructed topography. As a consequence, careful analysis of the 3D transfer function based on the Ewald sphere model enables a better understanding of the measuring process, the lateral resolution capabilities, and the improvement of the measurement results by choosing adequate signal processing parameters.

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Analysis of interference microscopy in the spatial frequency domain

Lehmann

Künne

Pahl

2021

J. Phys. Photonics

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Microsphere-assisted interferometry with high numerical apertures for 3D topography measurements

Hüser

Lehmann

2020

Appl. Opt.

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Improving the lateral resolution is a key focus of the research on optical measuring systems to expand the fields of application for optical metrology. By means of microspheres put on an object in a microscope, and therefore used as a near-field support, it has already been shown that a superresolution of structures below Abbe’s diffraction limit is possible. The following investigations give more detailed theoretical and experimental insight into the physical mechanisms responsible for the transition of near-field information to the far field. In particular, the effects of microspheres as near-field support on the behavior of phase-evaluating interference microscopes close to the optical resolution limit are studied experimentally as well as with numerical simulations. Special attention is drawn to measured data taken with a Linnik microscope of high numerical aperture. Finally, the measurement results of grating structures with a period below Abbe’s diffraction limit are presented.

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Investigation of measurement data of low-coherence interferometry at tilted surfaces in the 3D spatial frequency domain

Künne

Hagemeier

Käkel

et al. 2022

Tm - Technisches Messen

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The 3D transfer characteristics of interference microscopes and their effect on the interference signals occurring at surface slopes are studied. The interference image stacks acquired during a depth scan are 3D Fourier transformed. This allows a comprehensive frequency domain analysis of the interferograms. The double foil model introduced in a previous publication enables the interpretation of the signal spectra and the underlying transfer behavior of the interferometer using the concept of the Ewald sphere, which is limited by the numerical aperture (NA) of the imaging system. Analysis in the 3D spatial frequency domain directly discloses that the lateral dimensions of the transfer function depend on the axial spatial frequency. In this contribution we investigate measuring objects produced by Nanoimprint-Lithography. The corresponding signal spectra bear information that can be utilized to optimize the subsequent signal processing algorithms. These include envelope and phase evaluation procedures of the interference signals. A narrow bandpass filter is used to actively select certain frequency components in order to improve the robustness of the estimation of the envelope position. Although the shape and width of the envelope are affected, this procedure increases the reliability of the evaluation process and improves the accuracy of the measured topography especially at steeper surface slopes.

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Adaptive high-resolution Linnik interferometry for 3D measurement of microparticles

Cited by 4 publications

References 14 publications

Analysis of interference microscopy in the spatial frequency domain

Analysis of interference microscopy in the spatial frequency domain

Microsphere-assisted interferometry with high numerical apertures for 3D topography measurements

Investigation of measurement data of low-coherence interferometry at tilted surfaces in the 3D spatial frequency domain

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