# On the Properties of WC/SiC Multilayers
## 3 Results and Discussion  
Typically, multilayer periods in a MLL range from few nanometers to several tens or even hundreds of nanometers. It is thus important to explore how their properties change as a function of layer thickness as well as a function of temperature. Here we investigate periodic multilayers of different period thickness, which can be considered sections of an MLL. Table 1 lists the initial (as-deposited) stress and surface roughness (as described in Experimental Section) of W/SiC and WC/SiC multilayers of various periods. Within the measurement error the initial surface roughness is the same as the surface roughness of the bare Si wafer. Slightly higher surface roughness was measured only on multilayers with periods above 14 nm. Figure 1 shows normalized periods of nine W/SiC multilayers with initial thicknesses between 2.15 nm and 26.66 nm and two WC/SiC multilayers with initial thickness of 3.81 nm and 28.63 nm, whose initial period, stress and surface roughness are also listed in Table 1. It exhibits the effect of the temperature on the multilayer period for multilayers of different period thicknesses. The multilayer period changed (contracted) by less than 0.5% relative to the multilayer period in most cases. Multilayers annealed above \(\sim\)400 \({}^{\circ}\)C show larger contraction (W/SiC) or expansion (short period WC/SiC) with the exception of large period WC/SiC multilayer. If not mentioned otherwise the annealing time was 1 h. Only one multilayer delaminated, indicated by a blue star in Figure 1. This happened to the W/SiC multilayer with an initial period of 3.8 nm when annealed to 850 \({}^{\circ}\)C. All other multilayers in this study, including the WC/SiC multilayer annealed to 900 \({}^{\circ}\)C, showed no cracking or delamination. Stress evolution in W/SiC multilayers as a function of period thickness (Table 1) is interesting as well. An increase in stress towards more negative (more compressive) stress is observed between 6 nm and 9 nm multilayer period. This region coincides with the region where the microstructure  
Figure 1: Normalized multilayer period of W/SiC (black) and WC/SiC (red) multilayers as a function of annealing temperature. The blue star indicates the point where the W/SiC multilayer delaminated from the silicon substrate. of W layers changes from amorphous to polycrystalline [8]. After this transition stress relaxes (gets less compressive) but with increasing period grows again towards more negative (more compressive) stress (Figure 2 and Table 1). Table 2 lists period, stress and AFM surface roughness associated with a long period WC/SiC (28.6 nm) multilayer as a function of annealing temperature. It demonstrates that both period and surface roughness remain unchanged up to 700 \({}^{\circ}\)C.  
Short period (3.8 nm) WC/SiC has not been measured as extensively (Table 3). However, it was annealed and investigated at higher temperatures and also for different lengths of time. We will first comment on the period and the surface roughness change. Annealing the sample to 700 \({}^{\circ}\)C resulted in a negligible change in period thickness (Table 3). SAXRD plots are remarkably similar when one compares samples annealed at 700 \({}^{\circ}\)C for 1 h, 6 h or 24 h (Figure 3). Surface roughness, as measured with AFM, shows a small increase from 0.16 nm in as-deposited sample to 0.20 nm in a sample annealed to 700 \({}^{\circ}\)C for 24 h (Table 3). However, this is hardly seen in SAXRD spectra (Figure 3). At 800 \({}^{\circ}\)C we still do not see any noticeable change in SAXRD spectra (Figure 4) except a slight increase in the period thickness (Table 3). The sample annealed at 825 \({}^{\circ}\)C starts showing signs of modifications.