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25 June 1999 Optical vernier interferometry for aspheric metrology
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The ease of manufacture and testing spherical optical surfaces has made them the default choice for optical systems. Optical designs could greatly benefit form aspheric surfaces; the use of aspherics for projection photolithography in particular puts increasingly greater demands on optical manufacturing. Extreme UV (EUV) lithography requires all reflective elements, some of which will likely be strong aspheres. Modeling software and manufacturing have outpaced aspheric metrology, and we must be able to measure an optical component to have any hope of fabricating it. We seek to extend the dynamic range of optical interferometry to include aspheric surface metrology. We employ two wavelengths to create a vernier effect, allowing the measurement of larger departures without fundamentally sacrificing measurement accuracy. Such large departures impose more rigorous specifications on the interferometer. We explain the new challenges in the acquisition and interpretation of aspheric surface data, and compare to conventional spherical testing. The interferometer optical components are modeled using OSLO SIX design tools. Preliminary experimental results confirm the theory of operation. Some obstacles to practical implementation were also observed, and will be addressed.
© (1999) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Paul E. Murphy, Thomas G. Brown, and Duncan T. Moore "Optical vernier interferometry for aspheric metrology", Proc. SPIE 3676, Emerging Lithographic Technologies III, (25 June 1999);

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