1 July 2002 Aspherical mirror measurement using a point diffraction interferometer
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An point diffraction interferometry (PDI) system is used for measurement of EUV aspherical mirrors, because diffracted light by a small aperture has a nearly ideal spherical wavefront and EUV projection systems is designed with mild aspheres so that the mirrors can be tested at the center curvature without null optics. An advanced point diffraction interferometer has been developed and its precision and accuracy performance tested with a spherical mirror have been reported in last year1. After that, the diameter of the pinhole employed in the PDI system is switched from 1.0mm to 0.5mm in anticipation of measurement accuracy improvement. An aspherical mirror is measured, and the system error is estimated from the aspherical measurement data. In this system error estimation, an aspherical mirror designed for a four-mirrors EUV projection optics is used.
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Kazuya Ota, Takahiro Yamamoto, Yusuke Fukuda, Katsura Otaki, Iwao Nishiyama, Shinji Okazaki, "Aspherical mirror measurement using a point diffraction interferometer", Proc. SPIE 4688, Emerging Lithographic Technologies VI, (1 July 2002); doi: 10.1117/12.472343; https://doi.org/10.1117/12.472343

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