Soft X-ray Reflectivity Research Articles

Time-Resolved Soft X-Ray Imaging of Femtosecond Laser Ablation Processes on Metals

Femtosecond laser ablation processes on platinum, gold, and tungsten were observed by the single shot pump and probe reflective imaging using a soft x-ray laser probe. To avoid the timing error due to the jitter, we adopted a posteriori correction technique by simultaneous measurement of timing between the pump and probe pulses for every single shot, using a soft x-ray streak camera. A clear difference was found in the temporal behavior of the dynamical response of the soft x-ray reflectivity depending on the irradiated laser fluence in these three materials. On the other hand, the narrow dark rings were found in Pt and W, while an additional bright ring was found outside the dark disk in Au. Our result gives the experimental data comparable with various numerical simulations.

Understanding molecular interactions in light-emitting polymer bilayers: The role of solvents and molecular structure on the interface quality

We explore the donor/acceptor interaction between distinct polymers at sequential bilayer interfaces to understand if their emission is influenced by the solvent. Resonant soft X-ray reflectivity and near-edge X-ray absorption fine structure measurements were used to investigate optical and morphological properties of polymer stacks, which were made using polymer chain structures diluted with different or equal solvents. We identified coupled and uncoupled bilayer systems using as a probe the exciton energy transfer effect inferred from the donor/acceptor interdiffusion region at the heterojunction interfaces. We also show that the overall emission is dominated by the affinity of the chemical structures at the interface, regardless of the solvent used for the deposition of each layer.

Study of growth kinetics and depth resolved composition of a-SiNx:H thin films by resonant soft X-ray reflectivity at the Si L2,3-edge

Abstract Angle dependent resonant soft X-ray reflectivity (R-SoXR) measurements in the energy range (82.67–206.7 eV) were performed on PECVD grown amorphous hydrogenated silicon nitride (a-SiN x :H) thin films of different compositions near the Si– L 2,3 edge (∼100 eV). The compositional difference is reflected in the optical density ( δ ) of the two films. It is demonstrated that R-SoXR can non-destructively distinguish between the compositional variations through the depth of a given thin film, whereby it becomes possible to differentiate between the growth kinetics of the films prepared under different conditions. The compositions determined from R-SoXR, are in qualitative agreement with those determined from Rutherford back scattering (RBS) and elastic recoil detection analysis (ERDA).

Application of soft X-ray reflectometry for analysis of underlayer influence on structure of atomic-layer deposited SrTixOy films

We explored the possibility to quantify the atomic in-depth distributions by using the energy-dependent soft X-ray reflectivity (SXRR) measurements, in particular, the possibility to obtain the profiles of low-Z elements [C, N, O, Si] in heterostructures containing high concentration of higher-Z atoms [Ti, Sr, Hf]. We have shown that the SXRR technique allows one not only to quantify the atomic composition of the Sr-rich SrTixOy insulators grown on (100)Si by the Atomic Layer Deposition method but also to obtain atomic profiles across a few-nm thick underlayer (UL) inserted between SrTixOy film and the Si substrate. The accuracy of atomic concentrations and densities estimated is already sufficient to trace even small variations in composition of the SrTixOy grown by ALD on the chemically different underlayers and most it is important the composition and extension of an interfaces.

Structural and electronic properties of anisotropic ultrathin organic films from dichroic resonant soft x-ray reflectivity

We developed a quantitative approach for the determination of molecular arrangement and electronic structure in anisotropic organic ultrathin films based on the measurement of polarized reflectivity at the carbon K-edge. The reflectivity spectra were fitted to a parameterized model calculation. The method was applied to a self-assembled monolayer of 1,4-benzenedimethanethiol on gold. To simulate reflectivity, the organic anisotropic film was described by a dielectric tensor, obtained by ab initio calculations for the single molecule and suitable rotations to describe the molecular organization in film domains. Film structure was obtained though the best fit of the simulation to the experiment. Results were consistent with a monolayer-thick film composed of domains of molecules with in-plane isotropic distribution of orientations. In each domain, molecules adopted a standing configuration, with a tilt of 28\ifmmode^\circ\else\textdegree\fi{} relative to the substrate normal. Information on the modification of the molecular electronic states due to chemical bonding was derived.

Co/Mo2 C multilayer as X-ray mirror: Optical and thermal performances

We combined soft X-ray reflectivity (SXR) and X-ray standing-wave-enhanced fluorescence to study the optical performance and the thermally-induced interface behavior of Co/Mo2C nanometric multilayers, designed to work in the soft X-ray range close to the Co L3 absorption edge. Grazing X-ray reflectivity measurements were performed at 0.154 nm (Cu Kα line) and at two wavelengths in the soft X-ray range, 1.59 and 1.33 nm, close to and far from the Co L3 edge. In the fluorescence experiment the intensity of the Co Lα emission was measured by the standing-wave method within a Co/Mo2C stack. We found that the Co/Mo2C multilayers present good optical performance in the soft X-ray range up to 600 °C annealing temperature. Indeed, the reflectivity value in the soft X-ray range at a grazing angle of 11° is 25% for the as-deposited sample, then slightly varies with annealing and is still 20% after annealing at 600 °C. This demonstrates the good thermal stability of the Co/Mo2C multilayer. We used a simplified two-layer model and modified optical indices of the Co nanofilms to fit the experimental X-ray reflectivity curves. The same modified values of the optical indices are used to fit the X-ray standing-wave curves, confirming the usefulness of the model of the stack.

Stability and normal incidence reflectivity of W/B4C multilayer mirror near the boron K absorption edge

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.

Cleaning of carbon layer from the gold films using a pulsed Nd:YAG laser

Hydrocarbon cracking and carbon contamination of optical elements in soft X-ray spectrometers and synchrotron radiation beamlines is a severe problem. Carbon contamination seriously affects the optics performance. In the present work, an Nd:YAG laser providing 2mJ of pulse energy and 100ns of pulse duration has been used for carbon cleaning experiments. The laser cleaning is a non-contact, accurate, efficient and safe process. A surface area of 48cm2 having ∼20nm thick carbon layer on gold surface has been removed with six number of laser passes and with 80% laser spot overlapping without any change in surface roughness of the underneath gold film. Effect of laser beam on gold film after carbon removal has been analyzed using X-ray photoelectron spectroscopy, soft X-ray reflectivity techniques. Atomic force microscopy was used to analyze surface morphology before and after laser cleaning process. Power spectral density function was calculated over large frequency range of 10−1 to 10−4nm−1 to understand topographic data.

Extended theory of soft x-ray reflection for realistic lamellar multilayer gratings

An extended set of coupled wave equations were derived to describe non-idealized lamellar multilayer grating structures with properties as obtained with state-of-the-art fabrication techniques. These generalized equations can include all relevant effects describing the influence of passivation and contamination layers, non-rectangular lamel profiles and sidewall scalloping. The calculations showed that passivation and contamination plays an important role in that it may significantly reduce peak reflectivity. However, we also derived a condition for layer thicknesses having negligible effects. Slightly positive tapered lamel profiles are shown to further reduce the bandwidth as compared to a rectangular lamel profile, whereas negative tapers significantly increased the bandwidth. The influence of intriguing effects, such as the sidewall scalloping caused by Bosch Deep Reactive Ion Etching, are also modeled. We identified the signature of such scalloping as additional side peaks in the reflectivity spectrum and present parameters with which these can be effectively suppressed.

Study on effective laser cleaning method to remove carbon layer from a gold surface

Hydrocarbon cracking and carbon contamination is a common problem in soft x-ray Synchrotron Radiation (SR) beamlines. Carbon contamination on optics is known to absorb and scatter radiation close to the C K-edge (284 eV) spectral region. The purpose of this work is to study and develop a laser cleaning method that can effectively remove the carbon contaminations without damaging the underneath gold-coated optics. The laser cleaning process is a non-contact, accurate, efficient and safe. Nd:YAG laser of 100 ns pulse duration is used for carbon cleaning. The effect of laser pulse duration, laser fluence, number of laser passes, angle of incidence and spot overlapping on the cleaning performance is studied. Cleaning effect and subsequent film quality after laser irradiation is analyzed using x-ray photoelectron spectroscopy (XPS) and soft x-ray reflectivity (SXR) techniques.

Study of Al2O3 nanolayers synthesized onto porous SiO2 using X-ray reflection spectroscopy

The structure of alumina (Al2O3) films with different thickness grown by the atomic layer deposition method on porous silica substrates has been studied using soft X-ray reflection spectroscopy. It was established that synthesized films were amorphous and the proportion of Al coordination (tetrahedral: octahedral) depends on the film thickness. The film growth starts from excess of tetrahedral (AlO4) coordination and thickening of the film leads to increasing of number of octahedral (AlO6) coordination in the structure. A critical thickness of amorphous Al2O3 film exists (in the range of studied films, this is a thickness of 13nm). For thicker films, the structure of amorphous Al2O3 film corresponds to massive film with the typical proportion of tetrahedrally and octahedrally coordinated sites in the structure.

Surface layering and melting in an ionic liquid studied by resonant soft X-ray reflectivity

The molecular-scale structure of the ionic liquid [C18mim](+)[FAP](-) near its free surface was studied by complementary methods. X-ray absorption spectroscopy and resonant soft X-ray reflectivity revealed a depth-decaying near-surface layering. Element-specific interfacial profiles were extracted with submolecular resolution from energy-dependent soft X-ray reflectivity data. Temperature-dependent hard X-ray reflectivity, small- and wide-angle X-ray scattering, and infrared spectroscopy uncovered an intriguing melting mechanism for the layered region, where alkyl chain melting drove a negative thermal expansion of the surface layer spacing.

Precise structural investigation of symmetric diblock copolymer thin films with resonant soft X-ray reflectivity

Symmetric diblock copolymers are known to form lamellar structures in the bulk of an organic thin film. Polymer/polymer and polymer/substrate interfaces play a critical role in this application. Here, we report the investigation of multiple buried interfaces by using a novel technique resonant soft X-ray reflectivity which benefits from enhanced contrast between different polymers near the carbon K-edge. This allows us to obtain a precise interface structure. We also present an alternative method to determine optical constants of polymers by fitting X-ray reflectivity of polymers with known structural parameters at specific soft X-ray energies. This approach is compared with the way of obtaining β by NEXAFS and calculating δ via the Kramers–Kronig relationship. Finally, by using the determined index of refraction, the precise structure of a multilayer formed by a diblock copolymer is obtained by successfully fitting the resonant soft X-ray reflectivity profile.

Thermal stability studies of ion beam sputter deposited C/B4C X-ray multilayer mirror

We report the results of thermal stability study carried out on C/B4C multilayer structure. We have analyzed the structure of as-deposited and vacuum annealed C/B4C multilayer film by soft X-ray reflectivity measurements. We observed that multilayer period expansion continues till 600°C and slight contraction at higher annealing temperature. The results show that the multilayer structure is stable even after 700°C annealing. Raman spectroscopy indicates graphitization of carbon layer with increasing annealing temperature. Graphitization of carbon results in increases of layer thickness and decreases in density as also observed by soft X-ray reflectivity. We observed reduction in measured soft X-ray reflectivity at 6.56 and 4.39nm wavelengths after 800°C annealing. C/B4C multilayer structure has been tested over a period of one year to investigate its temporal stability.

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Influence of dielectric-dependent interfacial widths on device performance in top-gate P(NDI2OD-T2) field-effect transistors

Resonant soft x-ray reflectivity (R-SoXR) is employed to determine the interfacial widths of the semiconductor/dielectric interface in P(NDI2OD-T2)-based top-gate organic field-effect transistors (OFETs). It is shown that the deposition of a polymer dielectric on top of a semiconducting polymer layer can affect the interface structure, even when cast from an orthogonal solvent. The observed differences in the interfacial widths for different dielectrics explain the insensitivity of OFET performance to dielectric choice for OFETs fabricated using an identical fabrication protocol. The R-SoXR results demonstrate that differences in the physical interface structure should be taken into account when considering the influence of polymer dielectrics on the performance of all solution-processed OFETs. Specifically, the importance of the choice of solvent for the deposition is highlighted.

Fabrication and Evaluation of Large Area Mo/Si Soft X-Ray Multilayer Mirrors at Indus SR Facilities

Large area Mo/Si multilayer (ML) mirrors with high reflectivity are fabricated using magnetron sputtering deposition system. Thin film growth is optimized for film roughness, density, and interface quality by changing process parameters through fabrication of thin films. Mo/Si MLs are fabricated with varying thickness ratio, number of layer pairs, and periodicity from 0.3 to 0.45, 5 to 65, and 40 to 100 Å, respectively. The samples are characterized using hard X-ray reflectivity and transmission electron microscopy. Soft X-ray performance tests of MLs are done by soft X-ray reflectivity using Indus-1 synchrotron radiation. ML coating with thickness errors of ~0.03% per layer and interface roughness in the range of 2 to 5 Å has been realized. The lateral variation of the periodicity is controlled within 0.5 Å over the mm2 area of the plane substrate by using substrate motion and appropriate masking arrangement. Maximum variation of periodicity from run to run is less than 0.5 Å. Peak reflectivity of ~63% at wavelength of ~127 Å is achieved for incident angle of 71 degree.

Kramers-Kronig constrained modeling of soft x-ray reflectivity spectra: Obtaining depth resolution of electronic and chemical structure

Resonant x-ray scattering is a powerful technique for the study of electronic structure at the nanoscale. In common practice, the optical properties of the constituent components of a material must be known prior to modeling of the scattered intensity. We present a means of refining electronic structure, in the form of optical properties, simultaneously with physical structure, in a Kramers-Kronig consistent manner. This approach constitutes a sensitive and powerful extension of resonant x-ray scattering to materials where the optical properties are not sufficiently well known. The application of this approach to specular reflectivity from a single crystal of SrTiO${}_{3}$ is presented as an example case, wherein we find evidence for both a nonresonant surface contaminant layer and a modified SrTiO${}_{3}$ surface region. Extrapolating from this study we comment on the potential utility of this approach to resonant scattering studies in general.

NbC/Si multilayer mirror for next generation EUV light sources

In the present study we report a new multilayer combination comprised of refracting layers of niobium carbide and spacer layers of silicon as a more stable and high reflecting combination for the 10 - 20 nm wavelength region. The reflectivity of the new combination is comparable to Mo/Si conventional mirrors. Annealing experiments carried out with NbC/Si multilayer at 600°C temperature showed a ~2.5% drop in the soft x-ray reflectivity along with a marginal contraction in the multilayer period length. The multilayer structure is found stable after the heat treatment. Crystallization of the niobium carbide and silicon layers is responsible for the compaction in the period length as revealed by the grazing incidence x-ray diffraction measurements. No signature of silicide formation or any other chemical species could be detected. The multilayer structures were grown by ion beam sputtering technique using a compound target of niobium carbide. Soft x-ray reflectivity measurements performed at the Indus-1 and BESSY-II synchrotron radiation sources are found in good agreement with the simulations.

The influence of porous silica substrate on the properties of alumina films studied by X-ray reflection spectroscopy

The structure of alumina (Al2O3) films of various thicknesses grown by the atomic layer deposition method on porous silica (por-SiO2) substrates has been studied using soft X-ray reflection spectroscopy. It is established that the synthesized films are amorphous and that the ratio of Al atoms with tetrahedral and octahedral coordinations in a film depends on its thickness. It can be suggested that thicker Al2O3 films contain a greater proportion of Al atoms with tetrahedral coordination.