<title>Partial Coherence In Projection Printing</title>

Abstract

AbstractA mathematical model for partially coherent imaging in projection printing is presented using Fouriertransform methods. Some interesting two -dimensional examples are offered which illustrate the dramatic effectsof partial coherence in the mask illumination on the edge contours in the image. Optical simulation techniquesare discussed with possible application to system evaluation and design.IntroductionThe effects of partial coherence in projection printing have received considerable attention in recentyears because of the improvement in edge contrast as predicted by theory and substantiated by the user of pro-jection aligners.(1) Though only a few commercial aligners presently allow user control of the illuminationcoherence, an increased awareness of the importance of optimized illumination will undoubtedly shape the nextgeneration of optical projection printers.The mercury and xenon arc lamps, presently dominating the optical lithography industry, may soon give wayto rapidly improving industrial laser sources. Unlike conventional radiators, the collimated laser outputallows considerably more freedom in controlling partial coherence without sacrificing throughput and provideseither a single or multi -line spectral source in the near ultraviolet.This paper attempts to briefly explore some of the properties of partially coherent imaging with more com-plex mask geometries than generally attempted in the literature, which thus far has concentrated on one- dimen-sional profiles of isolated lines and edges. The examples offered serve mainly to dramatize the two- dimen-sional effects of partial coherence on binary image structures. For more complex mask geometries and operat-ing conditions, optical simulation techniques offer a practical tool for real -time studies.A Model for Partial Coherence in Projection PrintingA schematic of a unit magnification projection printer with Kohler's illumination is shown in Figure 1.The collimating lens L1 provides uniform illumination over the mask and, in combination with L2, images thesource into the aperture stop P. The projection optics are represented by lenses L2 and L3, which image themask onto the wafer. With the aperture stop located midway between L2 and L3, the entrance and exit pupilslie at infinity, characteristic of modern telecentric designs for unit magnification projection printers.