Three-step phase-shifting imaging ellipsometry to measure nanofilm thickness profiles

Three-step phase-shifting imaging ellipsometry is proposed for the dynamic measurement of a nanofilm's thickness profile in two dimensions. The designed apparatus consists of an ellipsometric optical configuration and an image-processing unit to perform phase-shifting imaging ellipsometry measurements, a key technique used to achieve dynamic and two-dimensional measurements. The uncertainties of the ellipsometric parameters ψ and Δ were evaluated based on the developed optical system and the proposed three-step phase-shifting technique. The thickness profile of a SiO₂ nanofilm on a silicon substrate was measured to calibrate the proposed apparatus; results were comparable to those obtained by a commercial spectroscopic ellipsometer. The measured thickness profiles are almost flat over the area of 1.10 mm × 2.21 mm with the spatial resolutions of 1.58 and 4.62 μm in the horizontal and vertical directions, respectively. The differences of average thickness between the proposed apparatus and a commercial spectroscopic ellipsometer were less than 3 nm. Furthermore, measurement precision was validated by obtaining a standard deviation of less than 2.5 nm.