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.

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W/B4C x-ray multilayers (MLs) with 300 layer pairs and a period in the range of d = 2–1.6 nm are fabricated and investigated for the x-ray optical element in the soft x-ray regime. The structural analyses of the MLs are carried out by using hard x-ray reflectivity (HXR) measurements at 8.047 keV. Well-defined successive higher order Bragg peaks (up to 3rd order) in HXR data collected up to glancing incidence angles of ∼9° reveal a good quality of the periodic structure. The ML mirrors have an average interface width of ∼0.35 nm and have a compressive residual stress of ∼0.183 GPa and 0. 827 GPa for d = 1.62 nm and d = 1.98 nm, respectively. MLs maintain structural stability over a long time, with a slight increase in interface widths of the W layers by 0.1 nm due to self-diffusion. Soft x-ray reflectivity (SXR) performances are evaluated in the energy range of 650 to 1500 eV. At energy ∼ 1489 eV, measured reflectivities (energy resolution, ΔE) are ∼ 10% (19 eV) and 4.5% (13 eV) at glancing incident angles of 12.07° and 15° for MLs having periods of 1.98 nm and 1.62 nm, respectively. The optical performance from 1600 eV to 4500 eV is theoretically analysed by considering the measured structural parameters. The structure-stress-optical performance is correlated on the basis of the mechanism of film growth. The implications of W/B4C MLs are discussed, particularly with respect to the development of ML optics with high spectral selectivity and reflectance for soft x-ray instruments.

Interfacial atomic diffusion, reaction, and formation of microstructure in nanoscale level are investigated in W/B4C multilayer (ML) system as functions of thickness in ultrathin limit. Hard x-ray reflectivity (XRR) and x-ray diffuse scattering in conjunction with x-ray absorption near edge spectroscopy (XANES) in soft x-ray and hard x-ray regimes and depth profiling x-ray photoelectron spectroscopy (XPS) have been used to precisely evaluate detailed interfacial structure by systematically varying the individual layer thickness from continuous-to-discontinuous regime. It is observed that the interfacial morphology undergoes an unexpected significant modification as the layer thickness varies from continuous-to-discontinuous regime. The interfacial atomic diffusion increases, the physical density of W layer decreases and that of B4C layer increases, and further more interestingly the in-plane correlation length decreases substantially as the layer thickness varies from continuous-to-discontinuous regime. This is corroborated using combined XRR and x-ray diffused scattering analysis. XANES and XPS results show formation of more and more tungsten compounds at the interfaces as the layer thickness decreases below the percolation threshold due to increase in the contact area between the elements. The formation of compound enhances to minimize certain degree of disorder at the interfaces in the discontinuous region that enables to maintain the periodic structure in ML. The degree of interfacial atomic diffusion, interlayer interaction, and microstructure is correlated as a function of layer thickness during early stage of film growth.

The evolution of residual stress and its correlation with microstructure are investigated systematically in nano-scaled periodic W/B 4 C multilayers (MLs) as a function of individual layer thicknesses at the ultra-thin limit ($0.4-3 nm). Details of the microstructure are accessed through hard X-ray reflectivity and X-ray diffuse scattering (rocking scan) measurements. To understand the contributions of stresses in the layers of each type of material to the total stress in ML films, both the total stress in MLs and the stress in nanocrystalline W layers are analyzed and correlated. It is observed that the physical properties of the materials as well as their interfacial morphology undergo significant modification as the layer thickness varies from the continuous to the quasidiscontinuous regime. A non-monotonic variation of compressive total residual stress in the MLs is observed as a function of thicknesses of W and B 4 C and explained using a model of the mechanism of film growth. The observed value of in-plane total compressive residual stress of W/B 4 C MLs is less than the residual stress in W layers in the MLs, which indicates that the net combined stress from B 4 C layers and interfaces is tensile in nature. The observed compressive stress and the increase of lattice spacing with respect to the stress-free structure in W layers provide evidence of a peening effect. The observed higher surface density of grains with smaller average size and phase formation also provide high compressive stress in W layers. research papers J. Appl. Cryst. (2019). 52, 332-343 A. Majhi et al. Stress and microstructure in nano-scaled multilayers 333

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.

The present finding illuminates the physics of the formation of interfaces of metal based hetero-structures near layer continuous limit as an approach to develop high-efficiency W/B4C multilayer optics with varying periods at a fixed large layer pairs.