The authors report determination of interlayer composition with subnanometer sensitivity at the buried interfaces using soft x-ray resonant reflectivity technique. Near the absorption edge, fine structure features of energy-dependent atomic scattering factor are sensitive to the composition, and can be exploited for determination of composition at the buried interfaces. This technique is demonstrated for a Mo–Si multilayer system using soft x-ray resonant reflectivity measurement.

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A multilayer structure consisting of alternate layers of W and B4C has been deposited using a magnetron sputtering system. The structure of the as-deposited and vacuum-annealed W/B4C multilayer sample has been characterized using grazing incidence x-ray reflectivity, grazing incidence diffraction, and the normal incidence reflectivity has been measured using synchrotron radiation. A two-layer model consisting of tungsten and boron carbide is presented. The multilayer structure was found to be stable after 800°C annealing. Grazing incidence x-ray diffraction measurements suggested that W is polycrystalline with small grain size. No signature of tungsten carbide or tungsten boride formation could be observed during the annealing treatments. A near normal incidence soft x-ray reflectivity (SXRR) of ~8.3% was obtained at 6.8 nm wavelength. A little drop (~1%) in SXRR after 800°C annealing suggested that there were no compositional changes within the layers during the annealing treatments.

The microstructure and residual stress are investigated in W/B4C x-ray multilayer (ML) mirrors as a function of the number of layer pairs (N) varying from 20 to 400 at a fixed period, d ≈ 1.9 nm. The microstructure is analyzed using the x-ray reflectivity (XRR) and rocking scan methods. The total residual stress in the ML film is derived using the substrate curvature measurement method, whereas the stress in W layers of MLs is separately determined by grazing incidence x-ray diffraction measurements based on the sin2 χ method using synchrotron. The successive order Bragg peaks in XRR measured curves indicate good quality of the ML structure in terms of interface roughness and thickness errors. As N increases, the interface width of B4C and W varies in the range of 0.15–0.22 nm and 0.26–0.44 nm, respectively. The contribution of physical roughness to the interface width is significantly lower (∼sub-angstrom) compared to interfacial diffuseness (angstrom level) along with a small (few nanometers) correlation length in the ML structures as observed by rocking scan measurements. The residual stresses both in the W layers and in the ML film are compressive in nature. The total stress in the ML film decreases from −1.444 GPa to −0.389 GPa with increasing N. Measured residual stress in the ML film and W layers is correlated considering a net combined tensile stress arising from B4C layers and interfaces. The ML film with N = 400 shows the least residual stress and is suitable for large layer pair ML optics. Microstructure and stress are correlated considering the mechanism of film growth at the early stage and is discussed.

A systematic study of structure and thermal stability of a [(Mo/Y/) x 5] soft x-ray multilayer (ML) mirror prepared by electron beam evaporation system has been performed by means of x-ray reflectivity (XRR) and grazing incidence x-ray diffraction (GIXRD). The GIXRD patterns show that the Mo and Y layers are polycrystalline in nature. The obtained XRR patterns confirm the very good quality of the ML stack. Isochronal thermal annealing up to the temperature of 400 °C leads to a decrease in interface roughness which in turn results in sharpening of the ML interfaces. Further elevating the annealing temperature above 400 °C oxidizes the Y layers and the interface roughness increases. Thus increase in interface roughness beyond this temperature can be understood in terms of evolution of yttrium oxide, which significantly affects the ML structural properties. The modulation structure is well maintained even after annealing at 600 °C, which confirms very good thermal stability of the Mo/Y ML mirror. Our results demonstrate that low-temperature annealing (up to 400 °C) is an effective method to improve the quality of an as deposited Mo/Y soft x-ray ML mirror. Diffuse scattering measurements show very good correlation with the XRR fitting parameters.

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Fine structure features of energy-dependent atomic scattering factor near the atomic absorption edge, are used for structural analysis of low-Z containing thin film structures. The scattering contrast undergoes large and abrupt change as the incident photon energy approaches the natural frequency of the atom and is sensitive to variation in atomic composition and atomic density. Soft X-ray resonant reflectivity is utilized for determination of composition at the buried interfaces with subnanometer sensitivity. This is demonstrated through characterization of Mo/Si multilayers near Si L-edge. We also demonstrate the possibility of probing variation of atomic density in thin films, through the characterization of Fe/B 4 C structure, near B K-edge. Sensitivity of soft X-ray resonant reflectivity to native oxide is demonstrated through characterization of BN films near B K-edge.