Fundamental aspects of resolution and precision in vertical scanning white-light interferometry

Wediscuss the height and lateral resolution that can be achieved in vertical scanningwhite-light interferometry (SWLI).With respect to interferometric height resolution, phase-shifting interferometry (PSI) is assumed to provide the highest accuracy. However, if the noise dependence of SWLI phase evaluation and PSI algorithms is considered, SWLImeasurements can be shown to bemore precise.With respect to lateral resolution, the determination of the coherence peak position of SWLI signals seems to lead to better results compared to phase based-interferometricmeasurements. This can be attributed to thewell-known batwing effect. Since batwing is a nonlinear effect applying nonlinear filters, e.g. amedian filter, it reduces them significantly. Iffiltering is applied prior to the fringe order determination and phase evaluation, the number of artefacts known as ghost steps can be eliminatedwithout changing themodulus of the phase. Finally, we discuss the dependence of measured height values on surface slope.We show that in interferencemicroscopy there are additional limitationswhich aremore rigid compared to themaximum surface slope angle resulting from the numerical aperture of the objective lens. As a consequence, themeasurement precision breaks down at slope changes of steeper flanks even if themodulation depth of the interference signals is still good enough for signal analysis.