V-groove measurement with white light interferometry

Abstract

White Light Interferometry (WLI) provides a fast and convenient means to measure surface profile. With sub-nanometre resolution and non-contacting operation, WLI is also a promising means to estimate surface roughness parameters. In recent years, however, several articles have concluded that great care must be taken to interpret the output of this type of instrument. For example the measurement of step artefacts often show a systematic error of a few tens of nanometres or so in magnitude, when the step height is less than the coherence length of the source, and surface roughness is generally overestimated by a similar value. Other error sources exist but for the most part systematic errors in WLI are less than, or of the order of, half a wavelength. In this paper, we report for the first time, that WLI should also be used with caution when it is used to measure the profile of V-grooves. We show that an inverted measurement profile with an error magnitude of several hundred microns is possible in this instance. Using a simple ray based model we show that this effect can be attributed to multiple reflections and can have a magnitude. Although, this experiment is somewhat contrived, we maintain that similar discrepancies exist in the measurement of abraded surfaces and discuss the implications of this type of error in the estimation of surface roughness