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.
Interference signals in coherence scanning interferometry at high numerical apertures and narrow bandwidth illumination are spectrally broadened. This enables phase analysis within a spectral range much wider than the spectral distribution of the light emitted by the light source. Consequently, different surface features can be resolved depending on the wavelength used for phase analysis of the interference signals. In addition, the surface topography itself affects the spectral composition of interference signals in different ways. Signals related to tilted surfaces or step height structures show special spectral characteristics. Thus, spectral amplitude and phase analysis enables a better understanding of the underlying physical mechanisms and gives hints how to improve the measurement accuracy.
In order to push the limitations of optical measurement technology further and to measure finer structures, various systems have been published in current research. It has been shown that microspheres applied in the near-field can improve the resolution of an interferometric measuring system and make structures below Abbe’s resolution limit visible. In this study, measurement results obtained with a high-resolution Linnik interferometer show structures with lateral dimensions below the resolution limit. Furthermore, the analysis of the transfer behavior in the spatial frequency domain provides information about the mechanisms in nearfield-assisted interference microscopy by microspheres.
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