Surface measurement errors using commercial scanning white light interferometers

TLDR

The study evaluates how commercial scanning white light interferometers perform across various measurement tasks. The authors used a step‑height artefact and sinusoidal/rectangular gratings to probe instrument response to discontinuities, gradients, and spatial frequencies, comparing results with tapping‑mode AFM and analyzing discrepancies against known error mechanisms. They found that instruments exhibit errors near discontinuities or steep gradients, and also systematic errors at smaller gradients that, while below the mean wavelength, exceed the instruments’ vertical resolution, indicating caution for sub‑wavelength accuracy.

Abstract

This paper examines the performance of commercial scanning white light interferometers in a range of measurement tasks. A step height artefact is used to investigate the response of the instruments at a discontinuity, while gratings with sinusoidal and rectangular profiles are used to investigate the effects of surface gradient and spatial frequency. Results are compared with measurements made with tapping mode atomic force microscopy and discrepancies are discussed with reference to error mechanisms put forward in the published literature. As expected, it is found that most instruments report errors when used in regions close to a discontinuity or those with a surface gradient that is large compared to the acceptance angle of the objective lens. Amongst other findings, however, we report systematic errors that are observed when the surface gradient is considerably smaller. Although these errors are typically less than the mean wavelength, they are significant compared to the vertical resolution of the instrument and indicate that current scanning white light interferometers should be used with some caution if sub-wavelength accuracy is required.