| 摘要 |
PURPOSE:To remove the adverse effect of the reflection from the upper surface of a mask and to improve reliability in detecting a periodic length, by matching an incident angle at the upper surface of the mask with a Brewster angle, making the incident light to be the polarized light of a TM wave, and forming interference fringes. CONSTITUTION:A polarizing element 10 is provided in the light path of laser light 3. The incident light is made to be polarized light 3a of a TM wave. An incident angle at an upper surface 2b of a mask is matched with a Brewster angle thetai. The reflection from the upper surface 2b of the mask becomes zero. Therefore, the reflected lights are only the reflected light 4a from an upper surface 1a of a wafer and reflected light 5a from a lower surface 2a of the mask. Interference fringes 7a are formed at a light receiving plane 6. The interference fringes are not associated with the reflection from the upper surface 2b of the mask. The interference fringes are formed with the interference of two luminous fluxes based on only the reflected lights 4a and 5a. A beat phenomenon is removed. A desired periodic length X can be detected with high reliability. |
