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.