Evaluation of microstructure and residual stress in W/B4C multilayer optics

Abstract

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.

Sub-nanograin metal based high efficiency multilayer reflective optics for high energies

“…45 It is noted that the contribution of measured stress in W-layers in MLs to the broadening of diffraction peak is negligible. 46 Also, the measured GIXRD spectra at 15.6 keV determine the approximate average crystallite sizes in out-of-plane direction for W (211) as ∼1 nm range (see the ESI Fig. S2 and S3 † ).…”

Understanding of stress and its correlation with microstructure near the layer continuous limit in nano-scaled multilayers

“…In the MLs, the thicknesses of the individual layers were changed by changing the deposition time at a fixed deposition rate. The details of the process parameters for the fabrication of the MLs are described elsewhere (Majhi, Dilliwar et al, 2018). In sample set A, the W layer thickness was varied in the range 2.03-1.06 nm at a fixed B 4 C layer thickness of $2.90 nm.…”

“…1(a) shows a schematic of the experimental arrangement for the determination of total residual stress using the Fizeau interferometer. Details of the experimental setup for XRR, diffuse scattering and the Fizeau interferometer and the procedure for experimental data fittings are described elsewhere (Majhi, Dilliwar et al, 2018). To correlate the total residual stress in the ML stack with the residual stress present in the W layers of the stack, the residual stresses in the W layers were estimated using grazingincidence X-ray diffraction (GIXRD) measurements with varying angles.…”

“…The incorporation of nitrogen in MLs may decrease the contrast at the interface and also increases absorption in the spectral region near the nitrogen edge. Similarly, Majhi, Dilliwar et al (2018) analyzed stressmicrostructure correlation in W/B 4 C MLs with varying N = 20-400 at a fixed period of 1.9 nm for soft gamma-ray optics. Furthermore, an attempt was made previously to understand the effect of microstructure alone as the individual layer thickness varies near the continuous-to-discontinuous transition region in W/B 4 C MLs .…”

Studies on the stress and thermal properties of Mo/B4C and MoxC1-x/B4C multilayers

“…Residual stress has proved to be related to microstructure in a multilayer system [25,26]. In [21], the reduction of stress value is mainly caused by the crystalline-to-amorphous of W layer after the introduction of reactive sputtering.…”