Surface Figure Metrology for X-Ray Optics,

Abstract : 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 A/200 or better at 633 nm. Commercial phase measuring interferometers (PMIs) offer A/1000 resolution and, with care, A /300 repeatability. For flats and spheres, absolute figure accuracy is limited by one's 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 A/20 for flats up to 150 mm in diameter and no better than A/10 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 A/5 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.