| 摘要 |
PURPOSE:To widen the good image area of a corrected image height and to increase the printing quantity of a wafer per unit time by using a back-surface convex mirror and making at least either of the reflecting surface and refracting surface of the back-surface mirror aspherical. CONSTITUTION:Astigmatism and curvature of field are compensated by an aspherical lens L1. Consequently, the shape of the aspherical lens L1 is so deterimined that all of a main light beam parallel to the optical axis O of each image height in a compensation area (h) is incident on the center 02 of the convex mirror M2 and each main light beam reflected at the center 02 of the convex mirror M2 is all incident on an image plane is parallel. Consequently, the astigmatism is compensated and neither coma aberration nor distortion is caused because of the symmetry of the optical system, but when higher performance is required, chromatic aberration and other aberrations due to the glass thickness of the aspehrical lens L1 can not be disregarded. For the purpose, the convex mirror M2 uses the back-surface mirror and its surface is made aspherical to compensate chromatic aberration and lateral aberration due to the introduction of the aspherical lens L1. |
