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14 September 2001 Study of obscuration in catadioptric lenses
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In this paper we will examine the effect of obscuration upon the various features we desired to image with a 157 nm microstepper utilizing a catadioptric lens. We will show the effect the obscuration has upon imaging when using not only conventional illumination and binary masks, but also when using a range of enhancement techniques such as off-axis illumination and phase-shifting masks. We will show how use of a large obscuration, while enhancing the signals for the densest features, actually degrades the signal for more isolated features. The level of obscuration must also take into account cross duty-ratio effects, i.e. the distribution of diffraction energy, for phase shifted features of various sizes. In this situation where a small sigma would be used a large level of obscuration can significantly increase biases. The choice of obscuration can have a major effect upon the imaging capabilities of a tool. In future, when the use of catadioptric lenses may be more widespread (for example this may happen at 157 nm) it may be desirable to have the option to vary this obscuration dependant upon the pattern being imaged.
© (2001) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Martin McCallum, Bruce W. Smith, Steve Bassett, Julian S. Cashmore, and James E. Webb "Study of obscuration in catadioptric lenses", Proc. SPIE 4346, Optical Microlithography XIV, (14 September 2001);

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