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One-shot roughness measurements based on dispersion-encoded low coherence interferometry

: Taudt, Christopher; Baselt, Tobias; Nelsen, Bryan; Koch, Edmund; Hartmann, Peter


Groot, Peter de (Editor) ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Optics and Photonics for Advanced Dimensional Metrology : 6-10 April 2020, Online Only, France
Bellingham, WA: SPIE, 2020 (Proceedings of SPIE 11352)
ISBN: 978-1-5106-3476-3
ISBN: 978-1-5106-3477-0
Paper 1135212, 7 S.
Conference "Optics and Photonics for Advanced Dimensional Metrology" <2020, Online>
Fraunhofer IWS ()
interferometry; aluminum; mirrors; data acquisition; surface roughness; coating; glasses

The acquisition of surface information such as height, roughness and waviness is crucial in production accompanying metrology. This work aims to demonstrate an alternative approach to gather surface roughness information of profiles with millimeter lengths while having sub-nm resolution based on a low-coherence interferometer. The surface height information is encoded by spectral dispersion with a well-defined phase minimum of the interference data. By applying an imaging approach, the captured information on the surface profiles allows an assessment without any scanning along one lateral dimension. The axial resolution is dependent but not limited by the dispersive element. The combination of the determination of the phase minimum and fitting of spectral interference data allows for sub-nm resolution while the axial measurement range is several ten micrometers. This results in a significantly higher aspect ratio than comparable approaches. During this work, initial experiments were performed on a calibrated surface roughness standard from the German national metrology institute PTB. It could be shown that a roughness of Ra = (21.15 ± 0.8) nm and Rq = (26.58 ± 1.0) nm was measurable on a lateral measurement range of 1.5 mm. Due to the application of advanced analysis methods, such as auto-convolution function analysis it was proven that these values correspond well to the values measured for calibration on a tactile profilometer. Additionally, investigations on a polished glass substrate with an aluminum mirror coating are presented. With these measurements roughness differences of Ra = 0.1 nm could be determined within one measurement between different parts of the sample. All data acquisition was carried out in a one-shot fashion.