Performance Limitations Of Imaging Microscopes For Soft X-Ray Applications

Recent advances in the fabrication of nanometer-scale multilayer structures have yielded high-reflectance mirrors operating at near-normal incidence for soft x-ray wavelengths. These developments have stimulated renewed interest in high-resolution soft x-ray microscopy. The design of a Schwarzschild imaging microscope for soft x-ray applications has been reported by Hoover and Shealy. Based upon a geometrical ray-trace analysis of the residual design errors, diffraction-limited performance at a wavelength of 100 angstrom was predicted over an object size (diameter) of 0.4 mm. In this paper we expand upon the previous analysis of the Schwarzschild x-ray microscope design by determining the total image degradation due to diffraction, geometrical aberrations, alignment errors, and realistic assumptions concerning optical fabrication errors. NASA\u27s optical surface analysis code (OSAC) is used to model the image degradation effects of residual surface irregularities over the entire range of relevant spatial frequencies. This includes small angle scattering effects due to mid spatial frequency surface errors falling between the traditional `figure\u27 and `finish\u27 specifications. Performance predictions are presented parametrically to provide some insight into the optical fabrication and alignment tolerances necessary to meet a particular image quality requirement