Normal Incidence Multilayer Mirrors for the Wavelength Range 2·3 to 4·6 nm

Near-normal incidence imaging optics for use at wavelengths λ in the range 2·3  λ  4·6 nm may utilize the in-phase reflections from a multilayer stack deposited on a figured substrate. The reflectivity of such a multilayer depends upon the number N of contributing interfaces and the amplitude reflectivity r at each interface, where r ∝ Δδ + iΔκ is determined by the difference in the real and the imaginary parts of the refractive indices of the media defining the interface. Thus r may be predominantly determined by Δδ (class I) or Δκ (class II) or a combination of both (class III). The spectral variation of r has been computed for interfaces formed by a-C (or a-Si) with 54 different elements, which are thereby classified as I, II or III. The normal incidence reflectivity R versus λ of representative multilayers from each class has been calculated as a function of N and layer thickness ratio γ and criteria established for the most suitable choice of layer materials. The influence of the metal layer atomic structure, interfacial roughness and possible interlayer diffusion on the performance of the ‘ideal’ Ni/a-C (or a-Si) multilayer is described. At present the particulate nature of the thin metal film components of the multilayer results in the experimental values of R being much less than the theoretical ones.