Profile coating has been successfully applied to produce elliptical Kirkpatrick-Baez (KB) mirrors using both cylindrical and flat Si substrates. Previously, focusing widths of 70？nm with 15-keV monochromatic and 80？nm with white beam were achieved using a flat Si substrate. Now, precision elliptical KB mirrors with sub-nm figure errors are produced with both Au and Pt coatings on flat substrates. Recent studies of bare Si-, Au-, and Pt-coated KB mirrors under prolonged synchrotron X-ray radiation and low-temperature vacuum annealing will be discussed in terms of film stress relaxation and Si plastic deformation. 1. Introduction It has long been recognized that mirror figure can be modified by controlled thin-film deposition [1, 2]. We have developed a profile-coating technique to make ultraprecise elliptical mirrors for X-ray microfocusing [3, 4]. Elliptical KB mirrors with sub-nm figure errors can now be produced using flat Si substrates with only two profile coatings. Sub-100-nm focusing has been demonstrated using a synchrotron X-ray beamline [5]. Nested KB mirrors have also been successfully fabricated [6, 7]. One serious concern of X-ray mirrors is their long-term figure stability. We have noticed changes in mirror figure during prolonged X-ray irradiation when the mirror is too thin and narrow. The observed changes are believed to result from plastic deformation of profile-coated KB mirrors. This plastic deformation is a surprise since in our applications there is very little heat load on each KB mirror and the Si substrate has a brittle-ductile transition of 847°C [8]. The plastic deformation may be related to film stress introduced during coatings. The film stress can be relaxed at temperatures as low as a few hundred degrees C [9]. We found that low-temperature vacuum annealing can cause plastic deformation similar to that introduced by prolonged X-ray radiation in profile-coated KB mirrors. To overcome this plastic deformation, we need to optimize the mirror dimensions to improve mechanical stability and to decrease the film stress. 2. Recent Results of Au- and Pt-Coated KB Mirrors Using Flat Si Substrates Precision elliptical KB mirrors with sub-nm figure errors have been fabricated with a predicted focal spot of 38.9？nm (FWHM), compared to the diffraction limit of 37.8？nm for a perfect ellipse [10]. On the 34-ID beamline at the Advanced Photon Source of Argonne National Laboratory, an Au-profile-coated KB pair, with dimensions of 70-mm？L × 9-mm？W × 4.5-mm？H and 40-mm？L × 10-mm？W × 20-mm？H, has produced ~0.60 × 0.40？μm2 focused white beams
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