| 摘要 |
PURPOSE:To execute a stable detection operation for many hours by a method wherein, instead of a galvanomirror, a scanning slit is used for a scanning optical system. CONSTITUTION:A mask 1 and a wafer 2 are arranged by keeping an interval of a prescribed gap S; a linear Fresnel's zone plate(LFZP) 3 whose focal distance is equal to the gap S is installed on the mask 1; a linear diffraction grating 4 is installed on the water 2. A laser beam 5 is magnified by using a lens 6; after that, it is condensed by using a condensing lens 7; the LFZP 3 is irradiated with the laser beam; the laser beam which has been reflected and diffracted by the linear diffraction grating 4 on the wafer 2 and has been passed through the LFZP 3 again is separated from an incident optical axis by using a half- mirror 8; the laser beam is passed through a rotary slit 9 and is detected by using a photosensor 10. In addition, a reference light source 11 and a photosensor 12 for reference use are installed so as to sandwich the rotary slit 9; an output (a) of the photosensor 10 is detected synchronously with an output (b) of the photosensor 12 for reference use by using a synchronous detector 13; a dislocation signal (c) is obtained. Thereby, it is possible to stably detect a derivation with high accuracy. |
