Method of measuring phase difference and apparatus for carrying out the same

摘要

A phase difference introduced by a half-tone mask including transparent portions and phase shift portions is measured by projecting two light fluxes onto these portions and forming an interference image by combining the two light fluxes. A light flux emitted by a light source is converted by a rotatable polarizer into a linearly polarized light flux having a polarizing direction which can be adjusted at will by rotating the polarizer, and then the linearly polarized light flux is transmitted through a birefringent prism to obtain two light fluxes polarized in different directions. Intensities of these light fluxes can be adjusted in opposite senses by rotating the polarized such that intensities of light fluxes emanating from the photomask become substantially identical with each other. One of the two light fluxes is made incident upon the transparent portion and the other light flux is made incident upon the phase shift portion. The two light fluxes transmitted through the photomask are combined by a birefringent prism and a phase compensating plate to obtain an interference image. By detecting an intensity of the interference image, the phase difference introduced by the photomask can be measured.