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Lithographic lens systems are continually being designed to work at shorter wavelengths and higher numerical apertures. The prospect of 157 nm F2 excimer-based lithography presents many demanding new challenges to lithographic lens manufacturers. Lens fabricators must re-orient themselves to handling and finishing more delicate optical materials such as calcium fluoride to unprecedented surface requirements. Thin film engineers are pressed to deliver a multitude of new optical coatings, but with a dramatically limited selection of raw materials. And optical test engineers are presented with new testing challenges: among them is at-wavelength interferometric testing of lithographic objectives using an F2 excimer laser source. Requirements for constructing such an interferometer dictate a design containing several nitrogen-purged beam paths and a camera capable of detecting 157 nm radiation. These contribute to an interferometer that is cumbersome and expensive when applied to production testing of lithographic lens assemblies. In addition, complications emerge in the interferometer design due to the relatively poor coherence in the 157 nm F2 excimer source. Fortunately, off-wavelength sources (usually at a 'user-friendly' longer wavelength) can be applied to transmitted wavefront testing of lithographic objectives designed for shorter wavelengths, while still providing nearly perfect and predictable at-wavelength imagery. This testing approach requires additional null optics to correct for off-wavelength spherochromatism effects. We have successfully used off-wavelength 248 nm interferometer testing to characterize 193 nm ArF lens systems, and this approach has been extended to the 157 nm regime by incorporating a well-characterized null corrector. We explain methods to perform null corrector characterization: We describe a technique to separate the non-rotationally symmetric errors introduced by a multi-element null corrector from the errors in the lithographic lens under test. We also discuss methods to characterize the rotationally symmetric errors introduced by this null corrector. In addition, we describe a method to cascade the error separation algorithm such that additional non-rotationally symmetric errors are also isolated. Test results are included and discussed.
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