Status and limitations of multilayer X-ray interference structures

Trends in the performance of X-ray multilayer interference structures with periods ranging from 9 to 130 Angstrom are reviewed. Analysis of near-normal incidence reflectance data versus photon energy reveals that the effective interface width sigma in a static Debye-Waller model, describing interdiffusion and roughness, decreases as the multilayer period decreases, and reaches a lower limit of roughly 2 Angstrom. Specular reflectance and diffuse scattering from uncoated and multilayer-coated substrates having different roughnesses suggest that this lower limit results largely from substrate roughness. The increase in interface width with period thus results from increasing roughness or interdiffusion as the layer thickness increases.