Coherence scanning and phase imaging optical interference microscopy at the lateral resolution limit.

To get physical insight into the 3D transfer characteristics of interference microscopy at high numerical apertures we study reflecting rectangular grating structures. In general, the height obtained from phase information seems to be reduced, whereas height values resulting from coherence scanning sometimes seem to be systematically overestimated. Increasing the numerical aperture of an interference microscope broadens the spectra of the resulting interference signals, thus offering a broad variety of wavelength contributions to be analyzed. If phase analysis of a measured far-field interference wavefront is performed at very short wavelengths the periodical profiles obtained from coherence scanning and phase shifting analysis differ only by the measured amplitude. However, at longer wavelength there is a 180° phase shift of the measured profiles obtained from phase analysis compared to coherence peak analysis. Increasing the evaluation wavelength improves the lateral resolution since the long wavelength contributions are related to electromagnetic waves of high angles of incidence. This behavior is to the best of our knowledge not documented in literature so far. It was first observed experimentally and could be confirmed by simulation results obtained from either Kirchhoff diffraction theory or an extended Richards-Wolf model developed in our group. Compared to original input profiles used for the simulation the profiles obtained from phase evaluation correspond quite well at longer wavelength, whereas the results obtained from coherence peak analysis are typically inverted with respect to height.