A “periodic multilayer” coating, illustrated conceptually in the figure at right, is a film stack comprising a number of identical repetitions of two or more optically dissimilar component layers. Periodic multilayers are typically used in the EUV and soft X-ray bands where they can provide both high reflectance and strong spectral filtering due to their intrinsically narrow spectral response. The thickness of each component layer in a periodic multilayer stack is designed so that the small reflections that occur at each interface in the stack add coherently, in phase. Just as Bragg's law describes the condition for constructive interference of X-rays in a crystal, the same law (with important corrections for refraction in the layers, which we omit here) describes the condition for constructive interference in a multilayer film (operating in first order): λ=2dsinθ, where θ in this equation is the (grazing) incidence angle, λ is the photon wavelength, and d is the multilayer period; in the case of a multilayer containing just two component layers, the multilayer period is equal to the thickness of a bilayer, as in the figure at right. Near normal incidence (θ~90°) Bragg’s law tells us that the multilayer period d is approximately equal to half the photon wavelength, and so each component layer is only a few nanometers thick for operation at EUV wavelengths. By carefully controlling the layer thicknesses during deposition, typically with sub-Å precision, it is possible to tune the peak of the multilayer spectral reflectance curve (i.e., the ‘peak wavelength’) to precisely match the desired wavelength.
A variety of high-performance periodic multilayer coatings comprising stable material combinations have now been developed for use in the EUV and soft X-ray bands. The performance of some of these coatings is shown in Figure 1, for the case of soft X-ray multilayers operating at 45° incidence (i.e., near the Brewster angle, where these coatings also act as highly efficient polarizers), and in Figure 2, for EUV multilayers operating at normal incidence.
Figure 1. Theoretical performance of some soft X-ray multilayers operating at 45° incidence.
Figure 2. Theoretical performance of some EUV
multilayers operating at normal incidence.
High-resolution TEM images of selected EUV and soft X-ray multilayers are shown in Figure 3, illustrating some of the variations in microstructure and interface properties that may be encountered in these thin-film structures.
Figure 3. HR-TEM images of some periodic multilayers.
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