# On the Properties of WC/SiC Multilayers
## 3 Results and Discussion
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**WC/SiC**} \\ \hline
**T (\({}^{\circ}\)C)** & **Period (nm)** & **Stress (MPa)** & **Roughness (nm)** \\ \hline As-deposited & 28.63 & \(-\)631 & 0.20 \\
100 & 28.67 & \(-\)604 & 0.20 \\
200 & 28.68 & \(-\)403 & 0.20 \\
300 & 28.66 & \(-\)117 & 0.20 \\
350 & 28.65 &  
Most noticeable is the region between the 1 st and 2 nd Bragg peaks. However, longer annealing time at this temperature (9 and 17 h) show visible decrease in Bragg peak intensities and an increase in surface and interface roughness (Figure 5). Going just slightly higher in temperature to 850 \({}^{\circ}\)C, similar change in SAXRD spectrum can be observed only after 1 h anneal (Figure 4). The sudden drop in Bragg peak intensities is correlated with period thickness and surface roughness increase (Table 3). However, it took 9 h at 825 \({}^{\circ}\)C before the surface roughness changed from 0.24 nm to 1.40 nm. It is obvious that we have to consider the whole spectrum of measurements, and not only period thicknesses, to evaluate thermal stability. The multilayer intrinsic stress also changes with temperature. Independent of their period thickness all multilayers studied here initially had a compressive stress (Table 1). We define negative stress as compressive and positive stress as tensile. The initial stress of a long period WC/SiC multilayer is \(-631\) MPa while the stress measured in the short period multilayer is \(-310\) MPa. The former was annealed sequentially in steps of 100 \({}^{\circ}\)C, 50 \({}^{\circ}\)C or 25 \({}^{\circ}\)C between 100 \({}^{\circ}\)C to 700 \({}^{\circ}\)C (Table 2). The stress crossed the zero point at around 350 \({}^{\circ}\)C and became tensile with further increase in temperature. At 700 \({}^{\circ}\)C (last point in Table 2) the stress was tensile, at 657 MPa (absolute stress). This is a difference of \(\sim\)1300 MPa as compared to the initial stress. The initial stress in short period multilayer was \(-300\) MPa. Up to about 300 \({}^{\circ}\)C stress development for short and long period multilayers are similar. However, at temperatures \(>\)300 \({}^{\circ}\)C short period multilayer shows smaller changes as compared to the long period multilayer. At \(\sim\)800 \({}^{\circ}\)C there is an inflection point where stress starts decreasing (Figure 6). The inflection point correlates well with the temperature where Bragg peak intensities drop and an increase in interface and surface roughness is observed. Figure 4: SAXRD spectra of a short (\(\sim\)3.8 nm) period WC/SiC after being annealed to 800 \({}^{\circ}\)C, 825 \({}^{\circ}\)C, 850 \({}^{\circ}\)C and 900 \({}^{\circ}\)C. This multilayer consists of 50 bilayers. Figure 5: SAXRD spectra of a short (\(\sim\)3.8 nm) period WC/SiC after being annealed at 825 \({}^{\circ}\)C for 1 h, 9 h and 17 h. This multilayer consists of 50 bilayers. To better understand observed changes we show fits of the diffraction spectra from W/SiC and WC/SiC multilayers in as-deposited states and after annealing them to 700 \({}^{\circ}\)C for 24 h. Figure 7 displays experimental and fitted data of a 25-bilayer W/SiC multilayer with a period of 3.96 nm. Based on this fitting, \(\Gamma\), defined as the ratio between the thickness of the W layer and the multilayer period is 0.4 (\(\Gamma=\mathrm{d_{W}}/(\mathrm{d_{W}}+\mathrm{d_{SiC}})\)). A slight shift in Bragg peaks towards larger angles in the annealed sample indicates period contraction from 3.96 nm to 3.75 nm. We also observed an increase in the peak intensity of the 5 th Bragg peak, which indicates a change in the ratio between the absorber and the spacer layer thickness. The fit result indicates a decrease in \(\Gamma\) from 0.4 to 0.3 after anneal. The change in the intensity of other peaks, especially 2 nd and 3 rd Bragg peaks, is also noticeable. This indicates slightly smoother or wider interfaces in the annealed sample (0.28 nm vs. 0.22 nm).