# Low-Temperature Direct Bonding of SiC to Si via Plasma Activation
## 3 Results and Discussion
### Bonding Mechanism for SiC/Si PAB  
Based on our measurements and analysis, the bonding mechanism of SiC/Si PAB is discussed in this section, in which we refer to the classical PAB model for conventional Si-based bonding [17, 18]. Figure 6 shows the schematic diagram of the interface during the bonding process. Figure 5: Cross-sectional TEM images of the SiC/Si bonding interfaces under annealing temperatures of (**a,b**) 150 \({}^{\circ}\)C and (**d,e**) 300 \({}^{\circ}\)C. Element analysis across the interface with EDS line scanning of annealing at (**c**) 150 \({}^{\circ}\)C and (**f**) 300 \({}^{\circ}\)C.  
As a natural hydrophilic material, SiC wafers can easily absorb water molecules from the air and form silanol (Si-OH) groups after standard cleaning. For Si wafers, after being cleaned by the diluted HF solution, a hydrophobic surface was obtained. After our specific hydrophilic treatment, the Si surface regained the ability to absorb water molecules. After RIE plasma activation, the Si-Si bonds of the Si surface and the Si-C bonds of the SiC surface were broken by O\({}_{2}\) plasma. The O\({}_{2}\) plasma bombardment can deform the wafer surfaces and produce many dangling bonds, increasing the actual contact area. After prebonding at room temperature, the surfaces of SiC and Si contacted with each other. As the hydrophilic surfaces have already absorbed many water molecules from the air, during this period, the two wafers were mostly bonded by hydrogen bonds between the chemisorbed water molecules located on the opposing wafer surfaces. The reaction between dangling bonds was not strong enough to dominate the formation of the bonding interface. Therefore, the bonding strength was still quite weak. As shown in Figure 6c, under low-temperature annealing at 150 \({}^{\circ}\)C, the applied pressure could make full contact between the prebonded wafers. The dangling bonds and unsaturated oxygen-containing groups between the Si and SiC surfaces were connected by polymerization reactions (1) and (2). \[\mathrm{C_{x}O_{y}Si-OH+Si-OH\to C_{x}O_{y}Si-O-Si+H_{2}O} \tag{1}\] \[\mathrm{Si_{x}O_{y}C-OH+Si-OH\to Si_{x}O_{y}C-O-Si+H_{2}O} \tag{2}\]  
Similar to the Si-based PAB model proposed by [PERSON] et al. [18], the original adsorbed and reaction-generated water molecules could not be eliminated completely through the nanogaps at low temperatures. The residual water might have diffused towards the Si bulk below the native oxide, where additional oxide containing crystal water was formed. In addition, oxidation at the interface mainly occurred near the void area. Similar to the Si/SiO\({}_{2}\) bonding system [19], water stress corrosion can soften the roughness asperities, which will increase the bonding area. The remaining water in the surface can also help the subsurface layer to become smoother and the roughness asperity to deform. Furthermore, the oxidation of the interface asperity will induce an increase in the asperity volume, which will help to close the interface and increase the bonding energy. Figure 6: Schematic diagram of the SiC/Si plasma activation bonding mechanism. Interface (**a**) after hydrophilic treatment, (**b**) after O\({}_{2}\) plasma activation, (**c**) after annealing at 150 \({}^{\circ}\)C, and (**d**) after annealing at 300 \({}^{\circ}\)C.  
When the annealing temperature rose from 150 to 300 \({}^{\circ}\)C, the water molecules may have either diffused out along the bonded interface or reacted with Si according to reaction (3). \[\text{Si}+\text{H}_{2}\text{O}\rightarrow\text{SiO}_{2}+\text{H}_{2}\uparrow \tag{3}\]  
Due to the high vacuum pressure and the increase in temperature, these vaporized water molecules along with hydrogen were expelled along the interface. Therefore, the thickness of the amorphous oxide layer decreased, which agreed with our TEM results in Figure 5. From the bonding mechanism, we can confirm that the oxide-free interface for SiC/Si cannot be obtained by low-temperature wafer bonding via O\({}_{2}\) plasma activation.