Properties of broadband depth-graded multilayer mirrors for EUV optical systems

Abstract

The optical properties of a-periodic, depth-graded multilayer mirrors operating at 13.5 nm wavelength are investigated using different compositions and designs to provide a constant reflectivity over an essentially wider angular range than periodic multilayers. A reflectivity of up to about 60% is achieved in these calculation in the [0, 18 degrees] range of the angle of incidence for the structures without roughness. The effects of different physical and technological factors (interfacial roughness, natural interlayers, number of bi-layers, minimum layer thickness, inaccuracy of optical constants, and thickness errors) are discussed. The results from an experiment on the fabrication of a depth-graded Mo/Si multilayer mirror with a wide angular bandpass in the [0, 16 degrees] range are presented and analyzed.

Spectral tailoring of nanoscale EUV and soft x-ray multilayer optics

“…This scheme has been extensively applied in the soft x-ray and EUV region to increase both the angular and wavelength bands. [32][33][34][35][36][37][38] Note that the increase of the reflection bandwidth is unavoidably connected to a decrease of the maximum reflectivity, due to the fact that the layer thicknesses do no longer perfectly match the interference conditions. Furthermore, the absorption in the layers can be enhanced for different wavelengths/angles.…”

“…34,35 A realistic layer structure has to be taken into account during the design of such a broadband multilayer, including interlayer formation, a variation of the layer density, effects of local crystallization, etc, in order to achieve the desired optical response. 37 Among these factors, the naturally formed interlayer between the main pair of constituent materials is a dominant issue. These interlayers act effectively as additional layers which can obviously deform the reflectivity profile of the ideal layered structure.…”

“…The experimental reflectivity is very close to the design value. 37 The layer thickness variation of a broadband multilayer should also be minimized during the design. This is not only for easy thickness control in the fabrication, but also to keep the internal layer structure the same over the whole stack, and thus close to the design.…”

Wideband multilayer mirrors with minimal layer thicknesses variation

“…Several successful experiments on fabrication and characterization of wideband MMs operating in the hard X-ray to Extreme UV spectral regions were described in literature (see, e.g., [4,[9][10][11] and references therein). While the reflectivity bandpass typically corresponds to the desired one, the reflectivity curve is usually deformed as compared to the prescribed one.…”

“…However for many applications it is desirable that either the spectral or angular bandwidth is essentially increased as compared to conventional MMs. The typical examples include X-ray astronomy [1], synchrotron radiation beam steering [2], Göbel optics for increasing the flux from X-ray tubes [3], and EUV lithography [4].…”