Surface Figure Metrology for X-Ray Optics,
NATIONAL INST OF STANDARDS AND TECHNOLOGY GAITHERSBURG MD
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A soft-X-ray projection lithography system will require diffraction limited performance at wavelengths near 13 nm. A typical conceptual design for such a system consists of an x-ray source, a reflecting mask, and a series of normal incidence, multilayer coated mirrors used to image the mask upon a resist-coated wafer. System throughput and image field flatness demands will require aspheric mirrors with figure accuracies of A200 or better at 633 nm. Commercial phase measuring interferometers PMIs offer A1000 resolution and, with care, A 300 repeatability. For flats and spheres, absolute figure accuracy is limited by ones knowledge of the reference optics, since such PMIs are differential devices. At the National Institute of Standards and Technology NIST our present absolute figure uncertainty is about A20 for flats up to 150 mm in diameter and no better than A10 or so for spheres. For an asphere, compared with a reference sphere of no more than a few waves of figure departure, we can perhaps attain an absolute accuracy of A5 or so, but we are fundamentally limited by errors in the interferometer system due to non-common mode optical paths of the test and reference beams. At NIST, we are in the initial phases of a five-year program aimed at improving the accuracy of optical figure measurements as a part of an Institute Competence Program in support of X-ray projection lithography.