# Sc/SiC/Al Multilayer Optimization for Li K Spectroscopy
## 3 Results and Discussion  
After deposition, the Sc/SiC/Al multilayer was characterized using GIXR in the hard X-ray range at 0.154 nm. The corresponding reflectivity curve and its fit as a function of the thickness and roughness of the various layers are shown in Figure 1. The quality of the multilayer is demonstrated by the large number of Bragg peaks observed up to the 10 th order of diffraction. Above the glancing angle of 4\({}^{\circ}\), the Bragg peaks are too weak and cannot escape the background noise: there is no more reflectivity of the multilayer. The fitted layer thicknesses are d\({}_{\text{Sc}}\) = 1.9 nm, d\({}_{\text{SiC}}\) = 1 nm, and d\({}_{\text{Al}}\) = 8.95 nm, with a capping SiC layer of 3 nm thickness. The interfacial roughnesses between the different layers, as well as between the stack and the substrate, comprising both physical and chemical aspects, are limited to between 0.3 and 0.5 nm. The fitted parameters are presented in Table 1.  
Figure 1: Experimental (red solid line) and fitted (black dashed line) X-ray reflectivity curves of the Al/SiC/Sc sample, measured at a photon energy of 8048 eV (0.154 nm wavelength). The reflectivity scan was performed as a function of energy in the EUV range of 45-65 eV (19-27.5 nm) at a fixed near-normal incidence angle of 7\({}^{\circ}\), as shown in Figure 2. The reflectivity curve presents a main peak corresponding to the first-order diffraction of the incident radiation by the periodic structure of the multilayer stack. Some small oscillations can be seen at both sides of this main peak. Figure 2 reveals that the measured reflectance of the multilayer (solid line) presents a strong reflectivity of about 38% at 54.1 eV, and a narrow bandwidth, measured at full width at half maximum, of 2.6 eV. The dashed line represents the simulated reflectivity for an ideal multilayer (with no roughness and no interlayers) using the thicknesses determined above. The maximum of the simulated reflectance obtained is 40.8% at 54.1 eV. The reflectivity difference between the simulated and experimental values is ascribed to the interface imperfections, such as geometrical roughness and interdiffusion, leading to a loss of optical contrast between the different layers and thus to the deterioration of the optical performance of the stack. The experimental data are very consistent with the simulations in terms of reflectance and bandwidth and have been a starting point to evaluate new structures. The possible upgrades to make the multilayer adapted for the analysis of Li K emissions for grazing angles of 30\({}^{\circ}\) can be achieved by changing the number of periods, the individual layer thicknesses, and the order of deposition. A previous study explored Al/Sc-based EUV multilayers as a function of the number of periods, individual layer thicknesses, and order of deposition [13]. We optimized the Sc/SiC/Al multilayer structure in order to maximize the reflectivity of the multilayer in the vicinity of the Li K-edge. For simulations, we used a sample with 20 periods, which leaves some room to improve the reflectivity by depositing multilayers with a bigger number \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{3}{c}{**Multilayer: Al/SiC/Sc**} & \multicolumn{3}{c}{**Period: 20**} \\ \hline \(\mathbf{d_{Sc}}\) (nm) & \(\mathbf{d_{SiC}}\) (nm) & \(\mathbf{d_{Al}}\) (nm) & \(\mathbf{\sigma[SiC-on-Sc]}\) (nm) & \(\mathbf{\sigma[Sc-on-Al]}\) (nm) & \(\mathbf{\sigma[SiC-on-Al]}\) (nm) \\ \hline
1.9 & 1.0 & 8.95 & 0.4 & 0.4 & 0.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Fitted parameters of Al/SiC/Sc multilayer, with d being the layer thicknesses and \(\mathbf{\sigma}\) being the interfacial roughness. The roughnesses on top of the Si substrate and SiC capping layer are 0.3 and 0.5 nm, respectively. The roughness between the last Sc layer of the multilayer and the SiC capping layer is 0.5 nm.