Soft x-ray reflectometry, hard x-ray photoelectron spectroscopy and transmission electron microscopy investigations of the internal structure of TiO2(Ti)/SiO2/Si stacks

Depth-resolved compositional analysis of W/B4C multilayers using resonant soft X-ray reflectivity

W/B₄C multilayers (MLs) consisting of ten layer pairs with varying boron carbide layer thicknesses have been investigated. The ML structures were characterized using grazing-incidence hard X-ray reflectivity (GIXR), resonant soft X-ray reflectivity (RSXR), hard X-ray photoelectron spectroscopy (HAXPES) and X-ray absorption near-edge spectroscopy (XANES). Depth-resolved spectroscopic information on the boron carbide layer in W/B₄C MLs was extracted with sub-nanometre resolution using reflectivity performed in the vicinity of the B K-edge. Interestingly, these results show that the composition of boron carbide films is strongly dependent on layer thicknesses. HAXPES measurements suggest that most of the boron is in the chemical state of B₄C in the multilayer structures. XANES measurements suggest an increase in boron content and C-B-C bonding with increase in boron carbide layer thickness.

Angle resolved photoelectron spectroscopy as applied to X-ray mirrors: an in depth study of Mo/Si multilayer systems

The chemical composition and extension of interfaces of a multilayer mirror were investigated by means of ARXPS relying on the concept of the EAL.

Control of TiN oxidation upon atomic layer deposition of oxides

The interfaces of a physical-vapor deposited (PVD)-TiN electrode with atomic-layer deposited (ALD) HfO2 layers were studied using photoelectron spectroscopy with high kinetic energies of photoelectrons enabling nondestructive in-depth chemical profiling and phase analysis. Our results reveal the presence of only TiNxOy at the TiN/ALD-HfO2 interface with no measurable traces of the TiO2 phase. By contrast, the interface formed by ALD of HfO2 on top of PVD TiN contains both TiO2 and TiNxOy compounds and may be compared to an HfO2/TiN interface with intentional ALD TiO2 interlayer (IL) formation prior to HfO2 growth. Pre-growth of ALD Al2O3 IL drastically reduces the TiO2 and TiNxOy amounts present at the HfO2/TiN interface, which can be ascribed to oxygen scavenging from the initially oxidized TiN surface by energetically more favorable Al-O bonds. The present study demonstrates that the amount of TiO2 phase can be effectively controlled, i.e., increased or decreased, during the ALD process enabling engineering of vacancy-mediated processes.

Soft X-ray Reflection Spectroscopy for Nano-Scaled Layered Structure Materials

We introduce a novel approach that addresses the probing of interfacial structural phenomena in layered nano-structured films. The approach combines resonant soft x-ray reflection spectroscopy at grazing incidence near the "critical angle" with angular dependent reflection at energies around the respective absorption edges. Dynamic scattering is considered to determine the effective electron density and hence chemically resolved atomic profile across the structure based on simultaneous data analysis. We demonstrate application of the developed technique on the layered model structure C (20 Å)/B (40 Å)/Si (300 Å)/W (10 Å)/substrate. We precisely quantify atomic migration across the interfaces, a few percent of chemical changes of materials and the presence of impurities from top to the buried interfaces. The results obtained reveal the sensitivity of the approach towards resolving the compositional differences up to a few atomic percent. The developed approach enables the reconstruction of a highly spatio-chemically resolved interfacial map of complex nano-scaled interfaces with technical relevance to many emerging applied research fields.

Extended model for the reconstruction of periodic multilayers from extreme ultraviolet and X-ray reflectivity data

An extended model for the reconstruction of multilayer nanostructures from reflectometry data in the X-ray and extreme ultraviolet ranges is proposed. In contrast to the standard model approach, where the transitional region is defined in advance as a specific function, the transition layer is sought as a linear combination of several functions at once in the extended model. This allows one to describe a much wider class of multilayer structures with different dominant physical mechanisms for the formation of transition regions. The extended model occupies an intermediate position between the classical model approach and the so-called model-free methods. The efficiency of the described method is illustrated in detail in numerical simulations and in a real experiment on the annealing of a multilayer Mo/Be mirror.

Re-distribution of oxygen at the interface between γ-Al2O3 and TiN

Interface of TiN electrode with γ-Al2O3 layers was studied using near edge X-ray absorption fine structure, conventional X-ray photoelectron spectroscopy and photoelectron spectroscopy with high energies. Despite the atomic-layer deposited Al2O3 being converted into thermodynamically-stable polycrystalline cubic γ-phase by high-temperature (1000 or 1100 °C) anneal, our results reveal formation of a thin TiNxOy (≈1-nm thick) interlayer at the interface between γ-Al2O3 film and TiN electrode due to oxygen scavenging from γ-Al2O3 film. Formation of the TiO2 was not observed at this interface. As environmental effect, a strong oxidation resulting in formation of a TiO2(1.4 nm)/TiNxOy(0.9 nm) overlayers on the top of the TiN electrode is traced. Development of O-deficiency of γ-Al2O3 is observed and related to the polarization anisotropy due to the preferential orientation of spin states involved in the X-ray absorption in the plane parallel to the surface. Investigation of the TiN electrode reveals the predominantly "stretched" octahedra in its structure with the preferential orientation relative the interface with γ-Al2O3. This anisotropy can be correlated with ≈200 meV electron barrier height increase at the O-deficient TiN/γ-Al2O3 interface as compared to the TiN/γ-Al2O3 barrier formed under abundant oxidant supply condition as revealed by internal photoemission of electrons from TiN into the oxide.

Formation of He-Rich Layers Observed by Neutron Reflectometry in the He-Ion-Irradiated Cr/W Multilayers: Effects of Cr/W Interfaces on the He-Trapping Behavior

Cr/W multilayer nanocomposites were presented in the paper as potential candidate materials for the plasma facing components in fusion reactors. We used neutron reflectometry to measure the depth profile of helium in the multienergy He ions irradiated [Cr/W (50 nm)]3 multilayers. Results showed that He-rich layers with low neutron scattering potential energy form at the Cr/W interfaces, which is in great agreement with previous modeling results of other multilayers. This phenomenon provided a strong evidence for the He trapping effects of Cr/W interfaces and implied the possibility of using the Cr/W multilayer nanocomposites as great He-tolerant plasma facing materials.

Growth of nano-dots on the grazing-incidence mirror surface under FEL irradiation

A new phenomenon on X-ray optics surfaces has been observed: the growth of nano-dots (40-55 nm diameter, 8-13 nm height, 9.4 dots µm(-2) surface density) on the grazing-incidence mirror surface under irradiation by the free-electron laser (FEL) FLASH (5-45 nm wavelength, 3° grazing-incidence angle). With a model calculation it is shown that these nano-dots may occur during the growth of a contamination layer due to polymerization of incoming hydrocarbon molecules. The crucial factors responsible for the growth of nano-dots in the model are the incident peak intensity and the reflection angle of the beam. A reduction of the peak intensity (e.g. replacement of the FEL beam by synchrotron radiation) as well as a decrease of the incident angle by just 1° (from 3° to 2°) may result in the total disappearance of the nano-dots. The model calculations are compared with surface analysis of two FLASH mirrors.

The at-wavelength metrology facility for UV- and XUV-reflection and diffraction optics at BESSY-II

A technology center for the production of high-precision reflection gratings has been established. Within this project a new optics beamline and a versatile reflectometer for at-wavelength characterization of UV- and XUV-reflection gratings and other (nano-) optical elements has been set up at BESSY-II. The Plane Grating Monochromator beamline operated in collimated light (c-PGM) is equipped with an SX700 monochromator, of which the blazed gratings (600 and 1200 lines mm(-1)) have been recently exchanged for new ones of improved performance produced in-house. Over the operating range from 10 to 2000 eV this beamline has very high spectral purity achieved by (i) a four-mirror arrangement of different coatings which can be inserted into the beam at different angles and (ii) by absorber filters for high-order suppression. Stray light and scattered radiation is removed efficiently by double sets of in situ exchangeable apertures and slits. By use of in- and off-plane bending-magnet radiation the beamline can be adjusted to either linear or elliptical polarization. One of the main features of a novel 11-axes reflectometer is the possibility to incorporate real life-sized gratings. The samples are adjustable within six degrees of freedom by a newly developed UHV-tripod system carrying a load up to 4 kg, and the reflectivity can be measured between 0 and 90° incidence angle for both s- and p-polarization geometry. This novel powerful metrology facility has gone into operation recently and is now open for external users. First results on optical performance and measurements on multilayer gratings will be presented here.

Element-specific structural analysis of Si/B4C using resonant X-ray reflectivity

Element-specific structural analysis at the buried interface of a low electron density contrast system is important in many applied fields. The analysis of nanoscaled Si/B4C buried interfaces is demonstrated using resonant X-ray reflectivity. This technique combines information about spatial modulations of charges provided by scattering, which is further enhanced near the resonance, with the sensitivity to electronic structure provided by spectroscopy. Si/B4C thin-film structures are studied by varying the position of B4C in Si layers. Measured values of near-edge optical properties are correlated with the resonant reflectivity profile to quantify the element-specific composition. It is observed that, although Si/B4C forms a smooth interface, there are chemical changes in the sputtered B4C layer. Nondestructive quantification of the chemical changes and the spatial distribution of the constituents is reported.

Comparative study of the X-ray reflectivity and in-depth profile of a-C, B₄C and Ni coatings at 0.1-2 keV

The use of soft X-rays near the carbon edge of absorption (270-300 eV) greatly enhances studies in various branches of science. However, the choice of reflecting coatings for mirrors operating in free-electron and X-ray free-electron laser (FEL and XFEL) beamlines in this spectral range is not so evident and experimental justifications of the mirror efficiency are rather limited. In the present paper it is demonstrated experimentally that the reflectivity of B4C- and Ni-coated grazing-incidence mirrors is high enough for their operation in FEL or XFEL beamlines near the carbon K-edge of absorption. The minimal reflectivity of both mirrors proves to exceed 80% near the carbon absorption edge at a grazing angle of 0.6°. An in-depth profile of the chemical elements composing the reflecting coatings is reconstructed based on analysis of a set of reflectivity curves measured versus the grazing angle at different photon energies in the soft X-ray spectral region. This allows us to predict correctly the mirror reflectivity at any X-ray energy and any grazing angle.