# Low-Temperature Direct Bonding of SiC to Si via Plasma Activation
## 3 Results and Discussion
### Surface and Interface Characterizations  
Surface roughness is an important factor for direct bonding. Surface roughness directly influences the efficiency and strength of wafer bonding. Generally, a smoother and cleaner surface will bring better bonding results [17]. Figure 3 shows the root mean square (RMS) of the Si and SiC surfaces before and after O\({}_{2}\) plasma activation. After 30 s of O\({}_{2}\) plasma activation, the surface roughness of the Si wafer decreased from 0.596 nm to 0.514 nm and the surface roughness of the SiC wafer decreased from 0.147 nm to 0.134 nm. According to the decrease in surface roughness, the surface of the Si and SiC wafers was cleaned by RIE plasma activation. The organic contaminants on the primary surface were diminished by the O\({}_{2}\) plasma treatment. In addition, the decreased roughness might also come from the  
Figure 2: (**a**) Bonding efficiency of SiC/Si samples as a function of O\({}_{2}\) plasma activation time. Optical images of SiC/Si samples under the annealing temperature of (**b**) 150 \({}^{\circ}\)C and (**c**) 300 \({}^{\circ}\)C.  
oxidation and etching of the asperity peaks. Furthermore, the three-dimensional surface morphology demonstrated that 30 s of plasma activation did not produce significant damage to the wafer surface. After plasma activation, the characterization of the surface chemical states was necessary to study the bonding mechanism. To investigate the surface chemical states including surface elementary composition and functional groups, X-ray photoelectron spectroscopy (XPS) was carried out. Figure 4a,b shows the Si 2p spectra of the Si surface. The main peaks were Si 2p\({}^{3/2}\), Si 2p\({}^{1/2}\), and Si-O, respectively. After O\({}_{2}\) plasma activation, the intensity of the Si-O covalent bond peak increased. This variation indicated that the O\({}_{2}\) plasma broke the Si-Si bonds, and the oxidation of the Si surface was formed, which might produce more Si-O dangling bonds. With higher energy, the Si-O bonds could connect free hydroxyl groups in the atmosphere that formed Si-OH groups. These increased Si-O bonds were vital to the reformation of covalent bonds in the bonding interface during the annealing process. For the SiC surface, the main peaks of Si 2p were Si-C and SiO\({}_{x}\)C\({}_{y}\), as is shown in Figure 4c,d. Similar to the Si surface, the SiO\({}_{x}\)C\({}_{y}\) peak increased greatly after O\({}_{2}\) plasma activation, which means the SiC surface was also oxided after O\({}_{2}\) plasma activation. In the process of bonding and annealing, these unsaturated oxygen-containing groups can react with each other to form stable covalent bonds. In the meantime, they can also bring external oxygen into the system, which will affect the interface properties in the long run. Figure 3: Three-dimensional surface morphology with the roughness (Rq) of the Si surface before (**a**) and after (**b**) and the SiC surface before (**c**) and after (**d**) O\({}_{2}\) plasma activation for 30 s.  
TEM was used in our experiment to characterize the bonding interface after bonding and annealing. TEM samples were fabricated by the focused ion beam (FIB) technique. The TEM images of SiC/Si bonding samples annealed at 150 \({}^{\circ}\)C and 300 \({}^{\circ}\)C were obtained and are shown in Figure 5.  
For bonding samples annealed at 150 \({}^{\circ}\)C, TEM images of the selected region are shown in Figure 5a,b. The lattice diffraction pattern indicated that the Si and SiC layers remained monocrystalline after bonding and annealing. From the cross-section TEM images of the SiC/Si interface, there was an obvious amorphous interlayer of 48 nm between the SiC and Si layers, which was much thicker than what was expected. In addition, the amorphous layer near the Si side was distorted, which might be caused by the oxidation of the Si layer. In order to ascertain the component of the interlayer, the element analysis of Si, O, and C was adopted by EDS. The result of EDS line analysis across the interface is shown in Figure 5c.