| 摘要 |
<p>Position detectors are disclosed for reliable, high-accuracy detection of alignment marks on a wafer, including alignment marks consisting of small phase steps. The position detector comprises a phase plate located conjugate to an aperture stop. The phase plate produces a phase shift between the undiffracted light flux from the alignment marks and the diffracted light flux and a phase-contrast image is of the alignment marks is formed. The phase plate is switchable or insertable so that a bright-field image is also obtainable. Bright-field images and phase-contrast images are directly viewable or are received by one or more image sensor. A signal processor receives image signals from the image sensors and determines the position of the alignment marks. The sensor then provides a position signal that is used to move the wafer and reticle into alignment. <IMAGE></p> |
