International Business Machines Corporation

David A Nelson, Jr

Warren D Grobman

John M Warlaumont

Fine alignment of mask and wafer, using Fresnel zone plates is achieved. Light is focused on the wafer by a zone plate in the mask. Light diffracted from a zone plate on the wafer is received by a sensor. The received light is coded (analog or digital) to indicate alignment. For analog coding the wafer zone plate diffracts light to the sensor from an area of the wafer zone plate which is indicative of alignment. For digital coding, the wafer zone plate is digitally encoded as a function of alignment to similarly code the diffracted light. To eliminate ambiguity, the mask zone plate is formed from a plurality of 'elements', each of which is itself a Fresnel zone plate. The focal length of the elemental Fresnel zone plate can be related to the mask/wafer separation distance, whereas the focal length of the macro zone plate (made up of a plurality of the elemental zone plates) is related to the distance between mask and light sensor.

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Carmel, NY, United States of America

David A Nelson, Jr