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Characterization of edge effects in precision low-coherence interferometry using broadband light sources

: Taudt, Christopher; Baselt, Tobias; Nelsen, Bryan L.; Aßmann, Heiko; Greiner, Andreas; Koch, Edmund; Hartmann, Peter


Lehmann, Peter H. (Ed.) ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Optical Measurement Systems for Industrial Inspection X : Munich, Germany; 25 June 2017
Bellingham, WA: SPIE, 2017 (Proceedings of SPIE 10329)
ISBN: 978-1-5106-1103-0
Paper 1032932
Conference "Optical Measurement Systems for Industrial Inspection" <10, 2017, Munich>
Fraunhofer IWS ()
Optical metrology; Interferometric measurement; dispersion-encoded measurement; Low-coherence interferometry; Surface profilometry; batwing effect; Diffraction; edge effect

Within this work an alternative approach to precision surface profilometry based on a low-coherence interferometer is presented. Special emphasis is placed on the characterization of edge effects, which influence the measurement result on sharp edges and steep slopes. In contrast to other works, this examination focuses on the comparison of very broadband light sources such as a supercontinuum white-light source (SC; 380 - 1100 nm) and a laser-driven plasma light source (LDP; 200 - 1100 nm) and their influence on the formation of these effects. The interferometer is equipped with one of these broadband light sources and a defined dispersion over a given spectral range. The spectral width of the light sources in combination with the dispersive element defines the possible measurement range and resolution. Instead of detecting the signals only in a one-dimensional manner, an imaging spectrometer on the basis of a high resolution CMOS-camera is set-up. Through the introduction of a defined dispersion, a controlled phase variation in the spectral domain is created. This phase variation is dependent on the optical path difference between both arms and can therefore be used as a measure for the height of a structure which is present in one arm. The results of measurements on a 100 nm height standard with both selected light sources have been compared. Under consideration of the coherence length of both light sources of 1.58 μm for the SC source and 1.81 m for the LDP source differences could be recorded. Especially at sharp edges, the LDP light source could record height changes with slopes twice as steep as the SC source. Furthermore, it became obvious, that measurements with the SC source tend to show edge effects like batwings due to diffraction. Additional effects on the measured roughness and the flatness of the profile were investigated and discussed.