| 摘要 |
<p>PURPOSE:To enhance the detection accuracy by a method wherein a base where an optical system such as two TV cameras is mounted and a prism are composed of an identical material so that the displacement of a focal point can be corrected with high accuracy. CONSTITUTION:A reflector 3 is a removable mirror; when an exposure beam is illuminated, the reflector retreats from an exposure area quickly. That is to say, the reflector 3 is actuated whenever an exposure operation is executed; an exposure light path does not interfere with a detection optical axis. When an alignment operation is executed, two images of an alignment mark are treated separately; when a gap between a wafer and a reticle is changed and a focal point is displaced, relay lenses 6, 6 are moved slightly in an optical axis direction in order to correct the displacement of the focal point and is then fixed. A relative position of two TV cameras with reference to an optical axis is also shifted by variation amount delta of the relative position due to an indefinite factor. Whenever this variation amount delta exceeds a definite value, it is detected and corrected; accordingly, it is possible to always suppress the variation amount delta to less than the definite value. By this setup, it is possible to enhance the detection accuracy; because a prism and a mounting base of the TV cameras are composed of an identical material, it is possible to enhance the detection accuracy.</p> |
