Optical measurement techniques are widely applied in high-resolution contour, topography and roughness measurement. In this context vertical scanning white-light interferometers and confocal microscopes have become mature instruments over the last decades. The accuracy of measurement results is highly related not only to the type and physical properties of the measuring instruments, but also to the measurement object itself. This contribution focuses on measurement effects occurring at edges and height steps using white-light interferometers of different numerical apertures. If the edge is perfectly perpendicular, batwing effects appear at height steps. These batwings show maximum height if the height-to-wavelength-ratio (HWR) is about one forth or three forth, and they disappear if the HWR value is about an integer multiple of one half. The wavelength that is relevant in this context is the effective wavelength, i.e. the center wavelength of the illuminating light multiplied by a correction factor known as the numerical aperture correction. However, in practice the edges are usually not perfectly perpendicular. In this case, the measurement results depend also on the derivative of the surface height function and they may differ from theory and the prediction according to the HWR value. Measurements of such steps show systematical effects depending on the lateral resolution of the instrument. In this context, a Linnik interferometer with a magnification of 100x and NA = 0.9 is used to characterize the three dimensional topography of more or less rectangular calibration specimens and quasi-perpendicular structures produced by the nanoimprint technology. The Linnik interferometer is equipped with LED light sources emitting at different wavelengths, so that the HWR value can be changed. This is possible since the high NA objective lenses show a rather limited depth of focus such that the temporal coherence gating may be replaced by focal gating in this particular instrument. In addition, measurement results are compared with those achieved by a Mirau interferometer of NA = 0.55. A commercial confocal microscope with NA of 0.95 serves as a reference instrument for further comparison. Numerical simulations considering diffraction effects are carried out in order to explain the experimental results obtained by the different white and colored light interferometers

Date Published: 26 April 2016
PDF: 13 pages
Proc. SPIE 9890, Optical Micro- and Nanometrology VI, 98900X (26 April 2016); doi: 10.1117/12.2228307

Published in SPIE Proceedings Vol. 9890:
Optical Micro- and Nanometrology VI
Christophe Gorecki; Anand Krishna Asundi; Wolfgang Osten, Editor(s)