Statistical analysis of phase values for the determination of step heights in multi-wavelength interferometry

To determine step heights between a few nanometers and several micrometers, we present a statistical evaluation procedure which overcomes the limitations of the unambiguity range of conventional multi-wavelength interferometry. The experimental setup consists of a shear interferometer with two different wavelengths to measure the phase difference between light reflected from an object with regions of two different heights. The statistical averaging over a large area of both regions of the object for both wavelengths generates a pair of phase difference values. A diagram of the space of phase values converts the pair of phase difference values into a step height. This statistical method enables the determination of step heights greater than the synthetic wavelength with an accuracy of a few nanometers even with small tilts of the object.