Step height measurement using two-wavelength phase-shifting interferometry

Abstract

Two-wavelength phase-shifting interferometry is applied to an interference phase-measuring microscope enabling the measurement of step features. The surface is effectively tested at a synthesized equivalent wavelength lambda(eq) = lambda(a)lambda(b)/| lambda(a) - lambda(b)| by subtracting phase measurements made at visible wavelengths lambda(a) and lambda(b). The rms repeatability of the technique is lambda/1000 at the equivalent wavelength. To improve the precision of the data, the phase ambiguities in the single-wavelength data are removed using the equivalent wavelength results to determine fringe orders. When this correction is made, a measurement dynamic range (feature height/rms repeatability) of 10(4) is obtainable. Results using this technique are shown for the measurement of an optical waveguide and a deeply modulated grating.