V-groove measurement with white light interferometry

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.