Thickness-profile measurement of transparent thin-film layers by spectrally resolved phase-shifting interferometry

Abstract

Spectrally resolved white-light phase-shifting interferometry has been used for accurate measurements of the spectral phase of the wave reflected from a micromachined surface. The phase is linearly related to the wave number, and the slope of the graph of the phase vs. the wave number, for any point on the test surface, gives the absolute value of the optical path difference at this point. These values can be used to generate a line profile of the test surface. However, if the test surface is coated with a transparent thin film, multiple reflections affect the phase of the reflected wave. The values obtained for the phase then depend on the thickness and the refractive index of the film and exhibit an additional nonlinear variation with the wave number, which can be modeled using thin-film theory. We show that this additional nonlinear phase can be measured directly using spectrally resolved white-light interferometry. The thickness profile of the film can then be obtained by a least-squares fit to the experimental phase data.