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Application of serifs in microlithography helps to extend and maintain more aggressive groundrules. Furthermore, this method of mask correction can be implemented in a conventional mask making environment. It puts no new demands on lithographic printing tools, and helps to extend their useful lifetime. Images whose dimensions are near the resolution limit of the imaging system are recorded in photoresist with size and shape different than predicted by geometrical optics. This difference is referred to as print bias. For small 2-dimensional features it can be characterized as shrinking of lateral dimensions and a corner rounding effect. It results in the loss of area, perimeter, and aspect ratio of printed features and makes it difficult to simultaneously maintain a desired tolerance for all features in the design. Modelling of optical projection printers is used to characterize printability of rectangles in a wide range of sizes and aspect ratios, with printed area being the primary parameter of concern. Motivation for use of the serif-like correction of reticle shapes is examined and a simple design procedure using experimental data on printability of small squares is developed. This procedure yields a serif design which, when added to reticle features, results in the area printed more closely to the nominal value and provides an optimal compromise between the compensation of the corner rounding effect and the associated process variability. A single size square serif is shown to correct a broad range of shapes and sizes. Improvements in terms of reduced radius of corner rounding, reduced area loss, and better preservation of aspect ratio of rectangles are shown. Use of serifs is also shown to result in a larger range of focus conditions for which, given a set exposure variation, all design features print with defined tolerances, i.e., a larger common process window. SEM microphotographs of resist images illustrate the use of serifs to improve lithography of small rectangles. Experiment confirms a substantial reduction of corner rounding and compensation for the loss of area, and aspect ratio in rectangles. An example of application to a dense pattern with typical circuit characteristics is also given.
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