Signal formation in depth-scanning 3D interference microscopy at high numerical apertures

摘要

Increasing capabilities in precision manufacturing and micro technology are accompanied by increasing demands for high precision industrial metrology systems. With respect to optical metrology especially the lateral resolution capabilities of an optical profiler gains in importance. If, in addition, nanometer height resolution is needed interferometers seem to be the most promising instruments. This contribution focuses on interference microscopes using objective lenses of high numerical apertures in order to reach high lateral resolution. Increasing the numerical aperture influences both, the envelope as well as the phase of interference signals obtained by a so-called depth scan, i. e. changing the distance between the measuring object and the interference microscope. The depth of focus of a high numerical aperture objective limits the width of the signal envelope simultaneously increasing the fringe spacing which results in a longer effective wavelength. We demonstrate the practical consequences of these effects using a self-assembled Linnik interferometer of 0.9 numerical aperture. Phenomena resulting from concrete measuring objects will be discussed: Step height structures may lead to a further change of the effective wavelength as a consequence of changes in the signal spectrum due to interference phenomena within a single Airy disk. This may influence the lateral resolution of an interference microscope for a specific measurement task. In addition, a strong dependence of the measurement results on either TE or TM polarization occurs if step height structures are measured. Modeling the polarization dependence requires to consider the angle dependence of Fresnel reflection coefficients and edge diffraction phenomena. Although the maximum measurable surface slope of a tilted surface can be increased by increasing the numerical aperture there is a limitation due to the fringe density compared to the optical resolution of the microscope as it will be demonstrated by measurement results obtained from a chirp-shaped surface standard.