26 May 1995 Dual-wavelength interferometer for testing projection lenses
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Abstract
A custom-built phase shifting interferometer is described for measuring the wavefront quality of Ultratech Stepper 1X projection lenses over a field of 25 mm X 50 mm using wavelengths of 363.8 nm (for i-line systems) and 442.0 nm (for gh-systems). Over an N.A. of 0.42 the wavefront is determined at 240 X 240 points with a calibrated accuracy of (lambda) /20 peak-to-valley (at (lambda) equals 363.8 nm). The interferometer is an unequal path Twyman-Green interferometer with an Argon-laser and a HeCd-laser as light sources. The interferometer test system is integrated on a 10 ft X 4 ft vibration isolation table together with an xy-stage to position the projection lens at different field points. The safe handling of the projection lens is supported by a load/unload mechanism on the table. The measurement wavefront maps at various field points of the tested projection lens are used to optimize the state of alignment of the projection lens.
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Klaus R. Freischlad, Klaus R. Freischlad, Chunsheng Huang, Chunsheng Huang, "Dual-wavelength interferometer for testing projection lenses", Proc. SPIE 2440, Optical/Laser Microlithography VIII, (26 May 1995); doi: 10.1117/12.209301; https://doi.org/10.1117/12.209301
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