Optical projection lithography using gradation stitching of the exposure fields

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Optical projection lithography is one of the key technologies for fabricating advanced semiconductor devices. In order to obtaining highly integrated devices, chip sizes should be enlarged. However, it becomes very difficult to realize higher resolution for a larger exposure field. For this reason, a unique method for stitching small exposure fields smoothly and realizing large chip exposure was investigated in this research. In the new method, scan exposure is utilized, and either side of the fields are exposed with intensity gradation. Gradation of the neighborhood field side is controlled inversely to the initial field to be stitched. Thus, the total intensity at the stitched parts is kept constant and equals to the one at the non-stitched parts. Effectiveness of this gradation stitching exposure is demonstrated using a laboratory-built exposure system. After investigating convenient slit shapes, trapezoidal slits with an apex angle of 80゜ were adopted. As the first experimental sta … More ge, exposure dose uniformity and continuity of the stitched parts are evaluated at the reticle plane in order to eliminate the complicated influence of the projection optics on patterns printed on wafers. Exposure and development of a special resist the dissolved depth of which after development changes almost linearly with the exposure dose is utilized for detecting partial irregular dose distribution in the narrow stitched parts. It is clarified that the dose inequality in the stitched parts is successfully improved by adopting the new method. Moreover, it is proved that the changes of top-view profiles and line widths are also very small and negligible. As the second stage, pattern stitching for scan projection exposure is investigated. It is demonstrated that patterns in the stitched parts don't degrade even if the fields are stitched with certain intentional errors of positions. Error margin of the stitching positions becomes very large. It will become difficult to develop a high-resolution projection optics. with a large field for shorter wavelengths hereafter. Applying the gradation stitching exposure, large LSI chips will be replicated favorably by stitching two small scan fields. If a small field size is admissible, resolution and/or depth of focus will also be considerably improved. Less