# On the Properties of WC/SiC Multilayers
## 3 Results and Discussion
Our model assumes period consisting of only two layers, SiC and W, and does not include any interface or inter-diffusion layers. Obviously a more sophisticated model is required to fully describe the measured spectra. W/SiC multilayers with similar periods were previously investigated for thermal stability [15] however, only up to 300 \({}^{\circ}\)C. In that paper it is reported that W/SiC revealed no significant changes  
Figure 6: Stress change as a function of annealing temperature for two WC/SiC multilayers of different multilayer periods. All data displayed here are for annealing times of 1 h.  
Figure 7: SAXRD experimental data and fits of the same W/SiC multilayer before and after anneal at 700 \({}^{\circ}\)C for 24 h.  
when annealed to 100 \({}^{\circ}\)C. A slight increase in the reflectivity of the 1 st Bragg peak was reported for multilayers heated to 300 \({}^{\circ}\)C for 1 h.  
In our study, we observed an increase in the 1 st Bragg peak intensity already when sample was annealed to 120 \({}^{\circ}\)C for 1 h. The intensity of the first Bragg peak continued to increase with temperature (data points were collected at 200 \({}^{\circ}\)C and 300 \({}^{\circ}\)C). However, at 400 \({}^{\circ}\)C the intensity dropped and was comparable to that in the initial, as-deposited state. At temperatures of 500 \({}^{\circ}\)C and above the intensity of the 1 st Bragg peak intensity decreased with increasing temperature. Figure 8 shows experimental data and fits of diffraction spectra from a 30-bilayer WC/SiC multilayer before and after it was annealed to 700 \({}^{\circ}\)C for 24 h. As already indicated in Figure 3 this multilayer remained very stable at this temperature even for extended annealing time. The measured period expansion was small, from 3.85 nm to 3.88 nm. The initial value of \(\Gamma\) of 0.56 changed to 0.63, opposite to what we observed in the W/SiC multilayer (Figure 7). Based on these results the observed changes seem to be diffusion driven. WC/SiC annealed to 700 \({}^{\circ}\)C shows a slight increase in period and surface roughness as a function of annealing time. Stress, which is initially compressive (\(-\)234 MPa) becomes tensile (676 MPa) and does not change with prolonged time at this temperature. Spectra measured in a sample annealed to 700 \({}^{\circ}\)C for 24 h look almost identical to the spectra obtain from a sample annealed to 825 \({}^{\circ}\)C for 1 h (Figures 3 and 4). Keeping the multilayer at 825 \({}^{\circ}\)C much longer causes a noticeable increase in the interface and surface roughness and a stress decrease. This is different to what we observed in W/SiC multilayers where the period consistently decreased with increasing temperature. Also, when annealed to 850 \({}^{\circ}\)C for only 1 h W/SiC multilayer delaminated from the substrate (Table 4). Further information on the microstructure can be obtained from HAXRD spectra. First we plot different period multilayers in as-deposited state (Figures 9 and 10). They all exhibit two sharp peaks at 33.1\({}^{\circ}\) and 69.2\({}^{\circ}\) identified as Si (100) coming from the Si single crystal substrates. W/SiC multilayer with 2 nm period consists of amorphous layers and shows no peaks between \(\Theta\) = 30\({}^{\circ}\) and 90\({}^{\circ}\) (Figure 9). With the period increase to 5.8 nm a broad peak centered at around \(\Theta\) = 38\({}^{\circ}\) becomes noticeable. With further increase in the period thickness to 9.1 nm this peak becomes narrower, shifts to \(\sim\)40\({}^{\circ}\) and can be identified as W(110) [18]. This particular multilayer shows two additional peaks, one centered at \(\Theta\) = 73\({}^{\circ}\) and the other at 87\({}^{\circ}\), corresponding to W (211) and W (220) [18], respectively. All aforementioned peaks become more intense and narrower with further increase in the period thickness (e.g., 21 nm period multilayer).