Importance

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Abstract

In order to facilitate the alignment of a mask with a substrate on which the mask is to be imaged, a rectangular window on the mask is projected through a semitransparent mirror and an optical objective upon an area of the substrate carrying a line mark parallel to two edges of the window. The image of the window is retroprojected through the same objective, along with the associated line mark, and is focused via the semitransparent mirror upon a detection plane of an evaluator where the resulting composite image is scanned in a direction perpendicular to the line mark to determine the position of that mark relative to the window edges. The same scan, on traversing each window edge, yields a signal representing sharpness of focus in terms of the time required for passing between two predetermined thresholds of light intensity; the latter signal is used for adjusting the relative distance of the object and image surfaces respectively constituted by the mask and the substrate.

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First Claim