SURFACE FINISH REQUIREMENTS FOR SOFT-X-RAY MIRRORS

We have examined the correlations between direct surface-finish metrol ogy techniques and normal-incidence, soft x-ray reflectance measuremen ts of highly polished x-ray multilayer mirrors. We find that, to maint ain high reflectance, the rms surface roughness of these mirrors must be less than approximately 1 angstrom over the range of spatial freque ncies extending approximately from 1 to 100 mum-1 (i.e., spatial wavel engths from 1 mum to 10 nm). This range of spatial frequencies is acce ssible directly only through scanning-probe metrology. Because the sur face-finish Fourier spectrum of such highly polished mirrors is descri bed approximately by an inverse power law (unlike a conventional surfa ce), bandwidth-limited rms roughness values measured with instruments that are sensitive to only lower spatial frequencies (i.e., optical or stylus profileres) are generally uncorrelated with the soft x-ray ref lectance and can lead to erroneous conclusions regarding the expected performance of substrates for x-ray mirrors.