# On the Properties of WC/SiC Multilayers
## 4 Summary and Conclusions  
Layers of W and Si create structures of very high contrast in electron density, making them attractive for applications at photon energies above 10 keV. Previous studies showed that replacing tungsten with tungsten carbide and silicon with silicon carbide further improves the quality of layer interfaces, which in turn allows for fabrication of ultra-short period multilayer coatings pushing the \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline \multicolumn{6}{c}{**“RBS” Thickness (nm)**} & \multicolumn{3}{c}{**Atomic Concentrations (at\%)**} & \multicolumn{2}{c}{**Assumed Density (at/cm\({}^{3}\))**} \\ \hline  & **Si** & **C** & **W** & **Kr** & \\ \hline Layer 1 & 40 & 47.6 & 52.4 & & & \(8.25\times 10^{22}\) \\ Layer 2 & 12 & & 54.8 & 43.8 & 1.4 & \(8.93\times 10^{22}\) \\ Layer 3 & 13 & & 61.4 & 37.2 & 1.4 & \(9.25\times 10^{22}\) \\ Layer 4 & 14 & & 52.6 & 46.0 & 1.4 & \(8.82\times 10^{22}\) \\ Bulk & & 100 & & & & \(5.00\times 10^{22}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Thickness, atomic concentrations and assumed densities of SiC (layer 1) and WC (layers 2–4) and Si substrate (bulk) after annealing at 700 \({}^{\circ}\)C for 24 h as determined from RBS analysis and fits. Figure 14: The plot in the top right corner shows full RBS spectrum of as-deposited and annealed WC/SiC sample, consisting of only two layers, each 40 nm thick and deposited directly on Si substrate. These two spectra are basically indistinguishable. Peak between channel 50 and 280 comes from the Si substrate. The small peak at \(\sim\)310 is due to the Si in the SiC layer. The peak between 370 and 400 is due to Kr and between 390 and 420 is the W in the WC layer. Experimental data of W peak in as-deposited sample (black dots) with theoretical fit (black line) and of annealed sample (red dots) with theoretical fit (red line) are plotted again in the main plot to show details. \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline \multicolumn{2}{c}{**“RBS” Thickness (nm)**} & \multicolumn{3}{c}{**Atomic Concentrations (at\%)**} & \multicolumn{2}{c}{**Assumed Density (at/cm\({}^{3}\))**} \\ \hline  & **Si** & **C** & **W** & **Kr** & \\ \hline Layer 1 & 40 & 47.6 & 52.4 & & & \(8.25\times 10^{22}\) \\ Layer 2 & 13 & & 54.8 & 43.8 & 1.4 & \(8.93\times 10^{22}\) \\ Layer 3 & 13 & & 61.4 & 37.2 & 1.4 & \(8.82\times 10^{22}\) \\ Layer 4 & 13 & & 52.6 & 46.0 & 1.4 & \(8.82\times 10^{22}\) \\ Bulk & & 100 & & & & \(5.00\times 10^{22}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Thickness, atomic concentrations and assumed densities of SiC (layer 1) and WC (layers 2–4) and Si substrate (bulk) in as-deposited state as determined from RBS analysis and fits. operative working range to photon energies higher than 200 keV [2; 3; 15]. Building on this previous knowledge the WC/SiC multilayer pair was successfully used to prepare the highest numerical aperture and highest efficiency multilayer Laue lenses so far [8], which enabled focusing of 16.3 keV X-rays to a sub-10 nm beam spot and demonstrated potential for fast imaging. In order to assess their thermal stability and potential use with intense XFEL beams we carried out a study on multilayer structures of different periods. Thermal annealing usually leads to inter-diffusion and compound formation at interfaces. In WC/SiC multilayers, both layers are already deposited as compounds and based on the results presented here the multilayers are very stable in period and interface roughness up to 700 \({}^{\circ}\)C. However, stress changes are observed at temperatures of 100 \({}^{\circ}\)C and higher. Thus, for applications using high X-ray intensities that would cause the temperature of the structure to exceed 100 \({}^{\circ}\)C, one could \"cure\" the multilayer at high temperature to make it stable in stress as well. Above 700 \({}^{\circ}\)C substantial changes in interface roughness and/or interface width of WC/SiC multilayers start taking place.