Thermal treatment of a Mo/Si multilayer stack enhances its reflectivity in the soft x-ray region. The multilayer x-ray mirrors are fabricated by electron beam evaporation in ultrahigh vacuum. In situ measurement of the reflectivity during the deposition allows thickness control and an observation of changes in quality of the boundaries. By heating the substrates during deposition we obtain a smoothing of the interfaces. This leads to x-ray mirrors with peak reflectivity around 50% for normal incident radiation of wavelengths between 130 and 140 Å.

Abstract. Mo/Si multilayers are fabricated by electronbeam evaporation in UHV at different temperatures (30 ° C, 150 ° C, 200 ° C) during deposition. After completion their thermal stability is tested by baking them at temperatures (Tbak) between 200°C and 800°C in steps of 50°C or 100 ° C. After each baking step the multilayers are characterized by small angle CuK -X-ray diffraction. Additionally, the normal incidence soft-X-ray reflectivity for wavelengths between 11 nm and 19 nm is determined after baking at 500 ° C. Furthermore, the layer structure of the multilayers is investigated by means of Rutherford Backscattering Spectroscopy (RBS) and sputter/Auger Electron Spectroscopy (AES) technique. While the reflectivity turns out to be highest for a deposition temperature of 150 ° C, the thermal stability of the multilayer increases with deposition temperature. The multilayer deposited at 200 ° C stands even a 20 min 500 ° C baking without considerable changes in the reflectivity behaviour. 68.55, 68.65, 78.65 In the wavelength range between 13 nm and 30 nm the combination of Mo and Si is most widely used for normalincidence multilayer mirrors. Reflectivities around 60% have been achieved with both sputtering [1] and e -beam evaporation in combination with thermal treatments during deposition [2,3]. Besides a high value for the reflectivity, the stability of the multilayer stack is also an important property, since applications include for example synchrotron radiation optics, where the multilayer can be heated to a few hundred degrees Celsius [4]. The long-term stability is also important for a number of applications. PACS:The thermal stability of Mo/Si multilayers has been studied earlier in several works [5][6][7][8][9][10][11] but in all of them the multilayers are fabricated by sputtering. In [12] it is shown that thermal treatment during e--beam deposition can considerably enhance the reflectivity of Mo/Si multilayer * Present address: European Synchrotron Radiation Facility, F-38043 Grenoble, France mirrors with a double layer spacing of about 7.5 nm. For a deposition temperature (Tdep) of 150°C the reflectivity is about a factor of two larger than the reflectivity of the 30 ° C and 200 ° C sample [12,13]. In other previous works [14,15] the influence of the deposition temperature on the microstructure of Mo/Si-multilayer systems fabricated by e--beam evaporation was studied. They have shown that Mo/Si muttilayers have interlayers of a mixture of Mo and Si at the Mo-Si interfaces and that the thickness of Mo-on-Si interlayers increases with increasing deposition temperature, while the thickness of the Si-on-Mo interlayers keeps constant. In our work the influence of the different microstructure for the multilayers which were deposited at different temperatures on the thermal stability is also investigated.

Mo/Si multilayers with a bilayer thickness of 2.6 nm are produced by electron beam evaporation in ultrahigh vacuum for soft X-ray optical applications. High reflectivities resulting from constructive interference in the stack are limited by the optical constants of the materials and by the quality of the interfaces. Smoothing of the boundaries is obtained by bombardment of the deposited layers with Ar + ions. The smoothness of the interfaces is controlled during the deposition by in situ measurement of the reflectivity for the C K radiation of the stack and after completion of the stack by means of a grazing X-ray reflection set-up with Cu Ks radiation. The soft X-ray reflectivity is measured with a laser-induced plasma light source.

Multilayer thin films consisting of alternatingpure molybdenum and silicon layers with layer thicknesses of a few nanometers are of increasing interest for soft X-ray optical applications in the wavelength region above the Si-L edge (i = 12.4 nm). In order to enhance the thermal and long term stability, which is of great importance for applications with high power soft X-ray sources, interdiffusion of molybdenum and silicon as a mechanism of thermal destruction of the multilayer system has to be reduced. For this purpose multilayers with absorber layers of two different Mo,Sil --x mixtures, Mo,,sSi,,,/Si and Moo,33Sio,6,/Si, and a double-layer thickness of about 7 nm are prepared by electron beam evaporation in a UHV system. The thermal stability for both systems is studied by post deposition annealing at different temperatures. For each temperature interdiffusion and interfacial roughness of the multilayers are examined by small angle X-ray diffraction (SAXD) at I = 0.154 nm, while the formation of nanocrystallites with lattice plane orientation parallel to the layer system is investigated by large angle X-ray diffraction (LAXD). In the case of the Mo,,;Si,,,/Si multilayer system these studies are completed by high resolution transmission electron microscopy (HRTEM) at multilayer cross sections and optical diffraction measurements (ODM).

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