Abstract

A principle motivation for producing soft x-ray imaging systems is to surpass the resolution of visible-light systems. Such systems have been demonstrated for ~13 nm radiation using multilayer-coated mirrors with normal-incidence reflectivities of approximately 40%. Unique impediments to achieving diffraction-limited resolution arise due to the phased nature of multilayer reflection, as well as the stringency at 13 nm of the λ/8 surface figure requirement. To examine the consequences of the multilayer coating, we performed a Foucault knife-edge test on a spherical optic both before and after a molybdenum/silicon coating was deposited. A 22-period undulator source was sufficiently powerful at 13 nm to provide a measurable reflection even without the multilayer coating. The results of the knife-edge scans were fit by a diffraction model, with the mirror’s effective surface characterized by Zemike polynomials. We noted a slight difference in the image after the multilayer was deposited. This was attributed to a slight decrease in some of the higher-order aberration terms. The difference in effective surface figure was less than λ/8 for the two measurements, indicating that a multilayer coating does not alter the effective surface figure of an optic in a way that is detrimental to the resolutions.

© 1990 Optical Society of America