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17 May 1994 Innovative image formation: coherency controlled imaging
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Two types of innovative imaging systems are studied. One is the system with non-incoherent effective source and the other is the system with non-coherent pupil function. When the optics are configured in such a way that a certain pattern would be illuminated by a group of waves diffracted by the pattern itself or an exactly the same pattern, the effective source can be made no longer incoherent. Then the coherence of the effective source can be a new parameter to improve image quality. The simulation shows that the contrast of L/S pattern imaging under the off-axis illumination, for instance, can be boosted up to 99.5% with this system, while to only 90.6% with conventional optics. A new pupil filter is proposed; the area of which is divided into some annular zones to be made mutually incoherent. Then the phase difference becomes less crucial and large DOF can be achieved. The simulation shows that this filter is effective for the exposure of contact hole patterns.
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Koichi Matsumoto, Naomasa Shiraishi, Yuichiro Takeuchi, and Shigeru Hirukawa "Innovative image formation: coherency controlled imaging", Proc. SPIE 2197, Optical/Laser Microlithography VII, (17 May 1994);

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