Abstract

To date there has been little research directed at correlating variations in peak normal-incidence soft x-ray reflectance of multilayer-coated optics with measured surface roughness. This information would be useful to those involved in both the specification and fabrication of figured optics for soft x-ray projection lithography (SXPL). The work presented here is focused on the characterization of the high-spatial frequency topography, which causes reduced soft x-ray reflectance. We have deposited Mo/Si multilayers on polished substrates of various materials obtained from several commercial suppliers. The multilayers consisted of 40 bilayers of Mo/Si, having a period of ~75 Å, appropriate for use at near-normal incidence at a wavelength of around 140 Å. The soft x-ray reflectance of these samples was measured by using a laser plasma-based soft x-ray reflectometer. Surface topography was measured by using both a commercial optical profilometer (Wyko Corp., TOPO-3D, 20× and 40× objectives) and an atomic force microscope (AFM), thereby covering an overlapping range of spatial wavelengths extending from 50 Å to 438 µm. Our results indicate that the soft x-ray reflectance is most sensitive to spatial wavelengths in the range extending from ~50 Å to 10 µm. Most of this range of spatial wavelengths is inaccessible by conventional contact and non-contact profilometers, though it is readily accessible by using the AFM.

© 1992 Optical Society of America