Periodic Multilayers Research Articles

Reflectance and color purity obtained for novel Ti[sbnd]C[sbnd]N/Ti[sbnd]Nb[sbnd]C[sbnd]N multilayers as function of niobium (Nb) modulation

Carbon-nitride multilayer systems were synthetized on silicon substrates by r.f. magnetron sputtering with alternatively changing the sputtering plasma composition between pure Ti+C and Nb elements under a reactive mixture of Ar/N2. Bilayer numbers were increased from nanometric range (15 nm) to higher micrometric range (1.5 μm). The structural and chemical properties were analyzed by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), cross-sectional transmission electron microscopy (TEM), Electron energy loss spectroscopy (EELS) and optical properties via color purity. The multilayer period effect on the optical properties such as color purity was studied and the color dependence as function of bilayer number with niobium (Nb) modulation in the TiCN/TiNbCN system determined. The spectra for all layers showed high reflectances at large wavelengths, e.g. 92% for the TiNbCN layer, 89% for the TiCN layer and about 75% for the TiCN/TiNbCN multilayer deposited with n = 200, Λ = 15 nm, So the changes in the color purity in relation to (Nb) fluctuation can be applied in optical and industrial applications.

Evolution of interfacial structure of Co-based periodic multilayers upon annealing

The evolution upon annealing from room temperature to 600 °C of the microstructure of Co/Mo2C/Y and Co/Y/Mo2C multilayer stacks has been studied by transmission electron microscopy combining HAADF-STEM, EDX and HRTEM. Both kinds of samples show significant amount of intermixing even in the as-deposited state. The mixing further increases upon annealing. Most of the mixing occurs between the Co and the Mo elements, the Y layers staying comparatively preserved. After annealing at 600 °C a surprising sharpening of the interfaces is observed. It is ascribed to the formation of Co-Mo compounds. Combining these analyses with the results we have already published, we can explain in a consistent way the evolution of the x-ray reflectivity measurements of the samples upon annealing. Understanding the evolution the optical properties of such stacks designed to work in the soft x-ray range is essential for improving the optical performance of multilayer structures.

Kossel diffraction observed with X-ray color camera during PIXE of nano-scale periodic multilayer

By combining Kossel diffraction with particle induced X-ray emission, we have developed a new methodology to analyze nano-scale thin films. We report the Kossel diffraction generated by irradiating Pd/Y based nano-scale periodic multilayers with 2 MeV protons. The intensity of characteristic Pd Lα X-ray emission is measured as a function of the detection angle (grazing exit). An oscillation of its intensity is observed when the detection angle varies around the Bragg angle, which corresponds to the energy of the emission and the period of the multilayer. Use of the X-ray color camera enables the whole setup to be fixed so that no angular scan is required, greatly simplifying the experimental condition. From the features of the Kossel curves, we are able to deduce that nitrided Pd/Y multilayers exhibit much less layer intermixing than the non-nitrided multilayers. The experimental results show that it is possible to distinguish by the shape of Kossel curves of multilayers with B4C barrier layers located in different interfaces. This demonstrates that Kossel diffraction is structural sensitive.

Characterization of Pd/Y multilayers with B4C barrier layers using GIXR and X-ray standing wave enhanced HAXPES.

Pd/Y multilayers are high-reflectance mirrors designed to work in the 7.5-11 nm wavelength range. Samples, prepared by magnetron sputtering, are deposited with or without B4C barrier layers located at the interfaces of the Pd and Y layers to reduce interdiffusion, which is expected from calculating the mixing enthalpy of Pd and Y. Grazing-incident X-ray reflectometry is used to characterize these multilayers. B4C barrier layers are found to be effective in reducing Pd-Y interdiffusion. Details of the composition of the multilayers are revealed by hard X-ray photoemission spectroscopy with X-ray standing wave effects. This consists of measuring the photoemission intensity from the samples by performing an angular scan in the region corresponding to the multilayer period and an incident photon energy according to Bragg's law. The experimental results indicate that Pd does not chemically react with B nor C at the Pd-B4C interface while Y does react at the Y-B4C interface. The formation of Y-B or Y-C chemical compounds could be the reason why the interfaces are stabilized. By comparing the experimentally obtained angular variation of the characteristic photoemission with theoretical calculations, the depth distribution of each component element can be interpreted.

磁控溅射法制备极紫外6.8~11.0 nm波段Mo/B4C横向梯度多层膜

Laterally graded periodic [Mo/B4C]60 multilayer mirrors for an EUV interval of 6.8-11.0 nm were deposited by direct current magnetron sputtering on silicon substrate. The structure properties and performance of laterally graded [Mo/B4C]60 multilayers were investigated by grazing X-ray reflectivity measurements and synchrotron radiation reflectance measurements. The results show that the multilayer period thickness D-spacing varies linearly from 4.39 nm to 7.82 nm in the long direction of the sample, indicating the average D-spacing gradient of 0.054 nm/mm. Reflectance of all measured point on the mirror is about 10% at the incident angle of 45. Spectral width (FWHM) of the reflectance peaks varies from 0.13 nm to 0.31 nm with the increase of multilayer period thickness.

Bragg supermirror with polarization-dependent amplification of reflected light

Reflection and transmission of light by a periodic multilayer, with a unit cell consisting of two different columnar thin films (CTFs), was numerically investigated. The nanocolumns of the CTFs were taken to be made from dissipative materials with the intercolumnar void regions filled with an active material. A combination of inverse and forward homogenization formalisms was employed to estimate the constitutive parameters of the two CTFs. The periodic multilayer exhibits the Bragg phenomenon in two generally distinct polarization-dependent spectral regimes for incident linearly polarized light. The presence of both dissipative and active materials allows the high reflectance to generally exceed unity for incident light of one linear polarization state but not for incident light of the other polarization state, in their respective Bragg regimes, but transmittances are low in both Bragg regimes. Thus, the chosen periodic multilayer is at best a good Bragg mirror for one linear polarization state, but it is a Bragg supermirror for the other linear polarization state.

Generation of X-rays by electrons recycling through thin internal targets of cyclic accelerators

The use of thin (< 10−3 radiation length) internal targets in cyclic accelerators leads to multiple passes (recycling effect) of electrons through them. The multiplicity of electron passes (M) is determined by the electron energy, accelerator parameters, the thickness, structure and material of a target and leads to an increase in the effective target thickness and the efficiency of radiation generation. The increase of M leads to the increase in the emittance of electron beams which can change the characteristics of radiation processes. The experimental results obtained using the Tomsk synchrotron and betatron showed the possibility of increasing the yield and brightness of coherent X-rays generated by the electrons passing (recycling) through thin crystals and periodic multilayers placed into the chambers of accelerators, when the recycling effect did not influence on the spectral and angular characteristics of generated X-rays.

On the Properties of WC/SiC Multilayers

A study of the materials properties of WC/SiC multilayer coatings is presented. We investigated the dependence of interface and surface roughness, intrinsic stress, microstructure, chemical composition, and stoichiometry as a function of multilayer period and in some cases compared these to W/SiC multilayer systems. The WC/SiC material pair forms multilayers with extremely smooth and sharp interfaces and both materials remain amorphous over a wide range of thicknesses. These properties are desirable for multilayer-based high-resolution diffractive x-ray optics, such as multilayer Laue lenses (MLLs), which require very thick films in which the layer spacing varies considerably. Thermal and structural stability studies show that WC/SiC multilayers have exceptional thermal stability, making this an extremely robust and favorable material pair for MLLs and other multilayer-based X-ray optical elements.

Characterization of periodic extreme ultraviolet multilayer based on multi-objective evolutionary algorithm

In order to refine the layered structure of extreme ultraviolet multilayers, a multi-objective evolutionary algorithm which is post-hybridized with the standard Levenberg-Marquardt algorithm is applied to analyze the grazing incidence X-ray reflectivity (GIXR) and the normal incidence extreme ultraviolet reflectance (EUVR). In this procedure, the GIXR data and EUVR data are simultaneously fitted as two objectives, and the high sensitivities of these two sets of data to layer thicknesses and densities are combined. This set of mathematical procedures is conducive to obtain a more correct model of periodic multilayers which can simultaneously describe both GIXR and EUVR measurements. As a result, the layered structure of Mo/Si multilayers with a period of about 7.0 nm is obtained.

Nonlocal effect on the transmission of a transverse electric electromagnetic wave through periodic dielectric-metal multilayers

The problem of electromagnetic response of a periodic dielectric-metal structure to transverse electric electromagnetic excitation is solved analytically on the basis of the Boltzmann kinetic equation formalism. We analyze the dispersion relation, transmission coefficient, and absorption for different angles of incidence. We show that, due to nonlocal dependence of the electron current density on the electric field in metal layers, the THz and near-infrared photonic properties of dielectric-metal structures are strikingly different from those obtained with the use of the simplified description based on the Drude–Lorentz model. In the limit of weak spatial dispersion, the developed theory reproduces the known results for transverse electric electromagnetic waves scattered by layered structures consisting of spatially local layers.

Magnetic Properties of Fibonacci-Modulated Fe-Au Multilayer Metamaterials.

Herein we experimentally study magnetic multilayer metamaterials with broken translational symmetry. Epitaxially-grown iron-gold (Fe-Au) multilayers modulated using Fibonacci sequence—referred to as magnetic inverse Fibonacci-modulated multilayers (IFMs)—are prepared using ultra-high-vacuum vapor deposition. Experimental results of in-situ reflection high-energy electron diffraction, magnetization curves, and ferromagnetic resonance demonstrate that the epitaxially-grown Fe-Au IFMs have quasi-isotropic magnetization, in contrast to the in-plane magnetization easy axis in the periodic multilayers.

Ta2N3 nanocrystals grown in Al2O3 thin layers.

Tantalum nitride nanoparticles (NPs) and cubic bixbyite-type Ta2N3 nanocrystals (NCs) were grown in (Ta–N+Al2O3)/Al2O3 periodic multilayers (MLs) after thermal treatment. The MLs were prepared by magnetron deposition at room temperature and characterized using grazing incidence small-angle X-ray scattering (GISAXS), X-ray reflectivity (XRR), grazing incidence X-ray diffraction (GIXRD), secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS). We found amorphous tantalum nitride NPs at 600–800 °C, with a high degree of ordering along the surface normal and short-range ordering within the layers containing tantalum (metallic layers). At an even higher annealing temperature of 900 °C the NPs crystallize in the rare and relatively unexplored Ta2N3 phase. However, the environment, morphology and spatial ordering of the NCs depend on the thickness of the metallic layers. For 12 nm thick metallic layers, the Ta2N3 NCs have an average diameter of 6 nm and they are confined and short-range ordered within the metallic layers. When the metallic layers are thinner, the NCs grow over 20 nm in diameter, show no spatial ordering, while the periodic structure of the ML was completely destroyed. The results presented here demonstrate a self-assembly process of tantalum nitride NPs, the morphological properties of which depend on the preparation conditions. This can be used as a generic procedure to realize highly tunable and designable optical properties of thin films containing transition-metal nitride nanocrystals.

Prediction of perovskite and other ternary oxide multilayers as mirrors for soft X-rays

We propose multilayers of perovskite and related ternary oxides with a general formula of ABO3 as high reflectivity mirrors, especially for use in water window region (2.3–4.4nm). The high reflectivity combinations of oxide multilayers were deduced using evolutionary search algorithms such as genetic algorithms, and the calculation speed was accelerated via parallel computing methods We also propose a figure of merit for X-ray reflectivity in periodic multilayers systems, which can simplify the future efforts on identifying material combinations, and the search through this multi-dimensional parameter space. The highest reflectivity value was found to be over 33% at 3.1nm in the water window region. The effect of interface roughness was simulated and the decrease in reflectivity was found to be modest for practically achievable roughness values. This work establishes the foundation for future experimental and theoretical studies towards achieving high reflectivity x-ray mirrors of complex oxide multilayers.

CrVN/TiN nanoscale multilayer coatings deposited by DC unbalanced magnetron sputtering

The growing interest in development coatings with enhanced properties opened the door to the research and development of nanoscale multilayer coatings. Some of the most interesting properties of these multilayer coatings when compared to monolayers are hardness higher than 40GPa, friction coefficients <0.3, improved adhesion and increased heat and corrosion resistance.In this work, CrVN/TiN nanoscale multilayer coatings were deposited onto H13 steel substrates by DC unbalanced magnetron sputtering. For the deposition of the multilayer coatings, two targets of Cr-V (70–30wt%) and Ti (99.5wt%) confronted opposite each other were used. In order to vary the bilayer period, a microcontroller was used to control the rotation and residence time of the substrates in front each target, thereby obtaining multilayer periods of 15nm, 30nm and 43nm. CrVN and TiN monolayer coatings were also deposited.X-ray diffraction (XRD) showed a FCC crystal structure for all coatings. SEM images revealed a columnar structure for CrVN and a cauliflower-like structure for TiN, similar to that obtained for the multilayer coating. All coatings showed a Hardness and Young's modulus around 19GPa and 270GPa, respectively. The CrVN/TiN multilayer with a period of 15nm showed the lowest friction coefficient (0.31) and the lowest wear rate (2.23×10−9kg/Nm) of all coatings, representing a reduction of 36% compared to the friction coefficient of H13 steel and a reduction in the wear rate of three orders of magnitude compared to the TiN. Finally, it was possible to identify three stages in the wear mechanism in the multilayer coatings, the lowest period, molecule-to-molecule adhesive wear occurred, followed by a rupture in the TiN and CrN bonds when the period increased to 30nm, while with the highest period of 43nm, oxides of titanium, chromium and vanadium were formed.

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.

Perpendicular magnetic anisotropy and microstructure properties of nanoscale Co/Au multilayers

We investigated the role of microstructure and Co layer thickness on the perpendicular magnetic anisotropy of as-deposited and annealed Ta (5 nm)/[Co (tCo)/Au (2 nm)] × N = 20 multilayers with 1 ⩽ tCo ⩽ 2 nm prepared using dc-magnetron sputtering. These multilayers were characterized using a vibrating sample magnetometer, a p-MOKE magnetometer and a microscopy magnetometer, small angle x-ray reflection (XRR), and wide angle x-ray diffraction (XRD) analysis. These multilayers demonstrated strong perpendicular magnetic anisotropy with their saturation magnetization close to the bulk magnetization of Co. Magnetization and magnetic anisotropy increased with annealing and this increase is directly linked to the strain relaxation and sharpening of the interfaces after annealing. Using XRR analysis before and after annealing, and fitting these XRR data, the multilayer periodicities are extracted and the refined layer thickness and surface roughness are determined. Using XRD analysis and fitting these XRD spectra, information regarding both the average lattice spacing of atoms and the strain developed on an individual layer were determined.

Structural coloured feathers of mallards act by simple multilayer photonics.

The blue colours of the speculum of the mallard (Anas platyrhynchos), both male and female, and the green head feathers of the male arise from light interacting with stacks of melanosomes residing in the feather barbules. Here, we show that the iridescent colours can be quantitatively explained with an optical multilayer model by using a position-dependent effective refractive index, which results from the varying ratio of melanin and keratin. Reflectance spectra obtained by multilayer modelling and three-dimensional finite-difference time-domain calculations were virtually identical. The spectral properties of the barbules' photonic structures are sensitive to variations in the multilayer period and the cortex thickness, but they are surprisingly robust to variations in the spatial parameters of the barbules' melanosome stacks. The blue and green reflectance spectra of the structural-coloured feathers correspond with the sensitivity spectra of the short- and middle-wavelength-sensitive photoreceptors, indicating their biological significance for intraspecific signalling.

Electromagnetic field enhancement in Bloch surface waves

We present a systematic comparison between guided modes supported by slab waveguides and Bloch Surface Waves (BSWs) propagating at the surface of truncated periodic multilayers. We show that, contrary to common belief, the best surface field enhancement achievable for guided modes in a slab waveguide is comparable to that observed for BSWs. At the same time, we demonstrate that, if one is interested in maximizing the electromagnetic energy density at a generic point of a dielectric planar structure, BSWs are often preferable to modes in which light is confined uniquely by total internal reflection. Since these results are wavelength independent and have been obtained by considering a very wide range of refractive indices of the structure constituent materials, we believe they can prove helpful in the design of future structures for the control and the enhancement of the light-matter interaction.

Impact of B4C co-sputtering on structure and optical performance of Cr/Sc multilayer X-ray mirrors.

The influence of B4C incorporation during magnetron sputter deposition of Cr/Sc multilayers intended for soft X-ray reflective optics is investigated. Chemical analysis suggests formation of metal: boride and carbide bonds which stabilize an amorphous layer structure, resulting in smoother interfaces and an increased reflectivity. A near-normal incidence reflectivity of 11.7%, corresponding to a 67% increase, is achieved at λ = 3.11 nm upon adding 23 at.% (B + C). The advantage is significant for the multilayer periods larger than 1.8 nm, where amorphization results in smaller interface widths, for example, giving 36% reflectance and 99.89% degree of polarization near Brewster angle for a multilayer polarizer. The modulated ion-energy-assistance during the growth is considered vital to avoid intermixing during the interface formation even when B + C are added.

The Yttrium Effect on Nanoscale Structure, Mechanical Properties, and High-Temperature Oxidation Resistance of (Ti0.6Al0.4)1–x Y x N Multilayer Coatings

As machine tool coating specifications become increasingly stringent, the fabrication of protective titanium aluminum nitride (Ti-Al-N) films by physical vapor deposition (PVD) is progressively more demanding. Nanostructural modification through the incorporation of metal dopants can enhance coating mechanical properties. However, dopant selection and their near-atomic-scale role in performance optimization is limited. Here, yttrium was alloyed in multilayered Ti-Al-N films to tune microstructures, microchemistries, and properties, including mechanical characteristics, adhesion, wear resistance, and resilience to oxidation. By regulating processing parameters, the multilayer period (Λ) and Y content could be adjusted, which, in turn, permitted tailoring of grain nucleation and secondary phase formation. With the composition fixed at x = 0.024 in (Ti0.6Al0.4)1–x Y x N and Λ increased from 5.5 to 24 nm, the microstructure transformed from acicular grains with 〈111〉 preferred orientation to equiaxed grains with 〈200〉 texture, while the hardness (40.8 ± 2.8 GPa to 29.7 ± 4.9 GPa) and Young’s modulus (490 ± 47 GPa to 424 ± 50 GPa) concomitantly deteriorated. Alternately, when Λ = 5.5 nm and x in (Ti0.6Al0.4)1–x Y x N was raised from 0 to 0.024, the hardness was enhanced (28.7 ± 7.3 GPa to 40.8 ± 2.8 GPa) while adhesion and wear resistance were not compromised. The Ti-Al-N adopted a rock-salt type structure with Y displacing either Ti or Al and stabilizing a secondary wurtzite phase. Moreover, Y effectively retarded coating oxidation at 1073 K (800 °C) in air by inhibiting grain boundary oxygen diffusion.