At&t Bell Laboratories

Chin-Chin Chen

Martin Barry Feldman

Herbert A Waggener

Conventional alignment procedures in electron beam (e-beam) direct write systems typically use an e-beam exposure tool as a scanning electron microscope (SEM) to imagine wafer alignment marks. However, electrical charging of the wafer surface by the electron beam can typically result in image distortions which generally can lead to alignment inaccuracies. The inventive method and apparatus advantageously overcome the alignment inaccuracies associated with the charging effects, by optically aligning the wafer to a reference axis of the electron beam. In a preferred embodiment of this invention, light is focused on a diffraction grating on the wafer, used as an alignment mark, and the diffracted light is spatially filtered and detected. Spatially filtering the diffracted light, eliminating the 0th order of the diffracted light, provides increased depth of focus. Also in this particular embodiment, travel of the wafer stage for optical alignment remains below the electron lens column by mounting an optical head (used for directing light onto the wafer, for spatially filtering the diffracted light, and for directing the spatially filtered light to the detection means) to the pole tip of the electron lens column. Significantly, the optical head comprises a lens section cut from a relatively larger lens providing a large working distance and numerical aperture without restricting movement, below the pole tip, of the wafer stage with a wafer mounted thereon.

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Scotch Plains, NJ, United States of America

Chin-Chin Chen