B. Heidemann

Thermal stability of Mo/Si multilayer soft-X-ray mirrors fabricated by electron-beam evaporation

Mo/Si multilayers are fabricated by electron-beam 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 CuKα-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.

Electron-beam-deposited Mo/Si and MoxSiy/Si multilayer x-ray mirrors and gratings

For the wavelength region above the Si- L edge normal incidence, soft x-ray mirrors are produced with peak reflectivities close to 60%. The multilayer systems consist of molybdenum and silicon and are fabricated by electron beam evaporation in ultrahigh vacuum. A smoothing of the boundaries, and thereby a drastic enhancement of the reflectivity, is obtained by thermal treatment of the multilayer systems during growth. The thermal stability of the multilayer stacks could be improved considerably up to 850° C by mixing Mo and Si in the absorber layers and producing thus Mo x Si y /Si multilayers with x and y denoting the amounts of Mo and Si in the absorber layer, respectively. First attempts are reported to produce mirrors with a bilayer thickness of 2.6 nm. An improvement in the quality of these interfaces can be obtained by bombardment with Ar + ions. We report on normal incidence reflectivity measurements of the mirrors with synchrotron radiation and finally on the normal incidence diffraction efficiencies of a Mo/Si multilayer coated grating, for which values of 5.5% are achieved for the + 1st and - 1st diffraction orders.

High resolution Rutherford backscattering spectroscopy studies on Mo/Si multilayers

High resolution Rutherford backscattering spectroscopy with an electrostatic analysis of the ion energy is applied to Mo/Si multilayers with a period of 7 nm. The multilayers have been produced for X-ray optical purposes by electron beam evaporation in ultrahigh vacuum at three different temperatures during deposition: 30, 150 and 200 °C. In the Rutherford backscattering spectra the layer structure is resolved in all three cases. The multilayers deposited at 150 and 200 °C show interlayers of mixed Mo and Si of different thicknesses on the two sides of a Mo layer. The most distinct layer structure is found for the 150 °C sample, whereas the spectra for the 30 °C sample indicate a larger interfacial roughness and those for the 200 °C sample larger interfacial layers of mixed mo and Si than for the 150 °C sample. On baking the multilayers to temperatures higher than 400 °C, interdiffusion of Mo and Si is observed. The multilayers deposited at 150 and 200 °C are destroyed after baking to 600 °C, whereas the multilayer deposited at 30 °C has already been destroyed after baking to 500 °C. Up to a baking temperature of 600 °C neither losses of material from the stack nor accumulation of Mo or Si at the surface or the interface between the stack and substrate are observed.

Interlayer composition and interface stability in Mo/Si multilayers studied with high-resolution RBS

Abstract Mo/Si multilayers with a period thickness of ∼ 7.5 nm have been fabricated by e--beam evaporation in UHV at a deposition temperature of 150°C [1]. At their interfaces interlayers of mixed Mo and Si are obtained which are thicker at the Mo-on-Si than at the Si-on-Mo interface. The composition of as-deposited Mo/Si multilayers and bilayers and changes in the composition after baking the samples have been studied with high-resolution RBS. Differences in the behaviour of the two interfaces with baking are observed. The interdiffusion of Mo and Si starts mainly at the Mo-on-Si interface. The thickness of the interlayer at the Mo-on-Si interface increases with baking temperature. With the increasing thickness the Si/Mo mixture in the interlayer changes from Mo-rich to Si-rich. After baking at temperatures higher than 600°C strong Si diffusion into Mo is observed for both interfaces.

Fabrication and characterization of Si-based soft x-ray mirrors

The fabrication, by electron beam evaporation, of Mo/Si and Ta/Si mutilayers designed as soft-X-ray mirrors is described. The mirrors were characterized using surface analytical methods (RBS and sputtering in combination with AES), Cu-k(alpha) reflection, and soft-X-ray optical methods, and their soft-X-ray optical properties were correlated with microstructural characteristics. A comparison of in situ C-k reflectivity curves with calculations disclosed the existence of roughnesses at the interfaces, which can not be completely described by multiplying the reflected amplitude at each interface by a Debye-Waller factor. It was found that heating of Ta/Si samples induces a considerable change (up to and above 10 percent) in the d-spacing of multilayers, while the reflected amplitude is only reduced to two thirds of its original value.

Fabrication, thermal stability, and reflectivity measurements of Mo/Si multilayers as x-ray mirrors and other optical components

For the wavelength region above the Si-L edge normal incidence soft X-ray mirrors are produced with peak reflectivities around 55 percent. The Mo/Si multilayer systems are fabricated by electron beam evaporation in ultrahigh vacuum. Analysis of the quality of the stack is made by using an in situ monitoring system measuring the reflection of the C-K line and ex situ grazing X-ray reflection of the Cu-K-alpha line. A smoothing of the boundaries and thereby a drastic enhancement of the reflectivity can be obtained by thermal treatment of the multilayer system during growth. The microstructure of the multilayer systems is investigated by means of Rutherford Backscattering spectroscopy and Sputter/AES technique. Baking the final stack after deposition up to 900 C is applied to study the thermal stability of the soft X-ray mirror. Near normal incidence mirrors even for short wavelengths, e.g., the water window (2.4 - 4.4 nm), are produced with a Mo/Si bilayer thickness of 2.6 nm. An improvement in the quality of the interfaces for such ultrathin multilayer systems can be obtained by bombardment of the deposited layers with Ar(+) ions as well as by thermal treatment of the multilayer system and mixing of Mo and Si in the absorber layer during the deposition run. We report on reflectivity measurements of the mirrors and their behavior as polarizers and analyzers and on the diffraction efficiencies of laterally structured multilayer systems as gratings.