900-channel single-shot surface roughness measurement using hyperspectral interferometry

A recently developed hyperspectral interferometer has been applied, for the first time, to the single-shot measurement of surface roughness. Traditional optical surface profiling techniques, such as coherence scanning interferometry (CSI) or focus variation microscopy, require long scan times and mechanical motion of the imaging objectives, making them vulnerable to environmental disturbances and thus inappropriate as an embedded metrology tool for production. Hyperspectral interferometry (HSI) answers these problems by capturing spatial and spectral information to reconstruct the surface profile, from 900 locations measured in parallel, in a single shot. We report here measurement results with both HSI and CSI from 12 individual roughness samples taken from Rubert roughness gauges covering the roughness range π‘…π‘…π‘Žπ‘Ž = 0.025 βˆ’ 50 πœ‡πœ‡πœ‡πœ‡. Very good agreement was obtained for π‘†π‘†π‘Žπ‘Ž values in the range 3 – 30 Β΅m, where the HSI π‘†π‘†π‘Žπ‘Ž values were mostly within a few per cent of (and at worst 24% away from) those measured by CSI π‘†π‘†π‘Žπ‘Ž. An alternative measure of roughness, based on averaging the widths of the Fourier transform peak of the individual HSI interference signals, is shown to be a statistically reliable measure of local roughness in the medium to high roughness regime, i.e. for π‘†π‘†π‘Žπ‘Žvalues of 0.8 Β΅m and above. The results thus demonstrate the technique’s potential for real-time surface quality inspection in manufacturing.