Phase-shift imaging ellipsometer for measuring thin-film thickness

A phase-shift imaging ellipsometer (PSIE) is developed for two-dimensional ellipsometric measurement. In the optical setup of the PSIE, a liquid crystal variable retarder (LCVR) is employed as an optical phase modulator for inducing a phase difference between the p- and s-polarized components of the incident light. The phase retardation adjustments of an LCVR are highly correlated with measurement accuracy in the PSIE. Since the phase retardation is dependent upon the applied voltage of the modulation signal, a calibration procedure is proposed in order to accurately determine an appropriate voltage magnitude, which in turn allows the LCVR to induce exact phase retardations. Consequently, the spatial distribution of the ellipsometric parameters can be measured with respect to the phase-shifted intensity images. Thus, the PSIE is capable of two-dimensional measurement of the physical properties of a thin-film material. We provide an experimental demonstration, which entails measuring the thickness distribution of a SiO₂ thin film on a Si substrate. These experimental results were in strong agreement with previously reported values.