# Low-Temperature Diffusion of Au and Ag Nanolayers for Cu Bonding
## 3 Results and Discussion  
An SEM analysis was conducted to observe the state of the Au and Ag layers deposited by e-beam evaporation. As shown in Figure 3, Au and Ag similarly exhibited porous structures and non-film-like layers, characteristic of metal deposited using an e-beam evaporator. Given that the materials were deposited in a porous form, the deposition thickness was not perfectly uniform. Here, the Au and Ag layers were deposited at average thicknesses of about 12 nm and 15 nm, respectively. The chemical state and amount of each element (Au, Ag, Cu, and O) were determined by XPS, as shown in Figure 4, to discern the diffusion characteristics between Ag and Cu, and between Au and Cu. First, for Au and Cu, it was found that the atomic percentage of Au gradually decreases on the surface and the atomic percentage of Cu gradually increases as the temperature increases. In addition, O increases as Cu increases on the surface, likely a phenomenon caused by oxidation due to the increased Cu on the surface as Cu diffuses more actively to Au at higher temperatures. At 25 \({}^{\circ}\)C, there is a very small amount of O at the surface compared to the Ag-Cu case. For Ag and Cu, as the temperature increases, the atomic percentage of Ag on the surface gradually decreases, with Ag completely diffused into Cu at 200 \({}^{\circ}\)C. The Au and the Ag surfaces both have oxygen because they both have porous structures; in particular, Ag tends to be easily oxidized compared to Au. In Figure 4e, The O1s peak profile shows that CuO becomes more prominent than Cu2O as the temperature increases, as Cu is more oxidized and forms a more stable state from Cu2O to CuO [19]. Also, under high-temperature heating, the chemisorbed OH- peak appeared to be distinctly high. On the other hand, for Ag, it was observed that Cu2O changes to CuO as the temperature increases, but unlike the Au case, there is almost no chemisorbed OH- peak, and a strong satellite peak appears around 942 eV, as shown in Figure 4d. The presence of this satellite peak indicates a similar coexistence of Cu2+ and Cu+, although Cu2+ may be more abundant [20]. As shown in Figure 4e,f, oxygen content was observed on the surface before the bonding process in this study due to a porous form of metal nanolayer. To control oxygen permittivity on the bonding surface, a more densely structured metal layer may be required, especially given the use of a nanolayer. Next, the diffusion activation energy required for diffusion between Au and Cu, Ag, and Cu was estimated using the data obtained through the XPS analysis and with the Arrhenius equation below. In Equation (1) below, K is the amount of each element obtained through the XPS analysis, A is the diffusion constant, E\({}_{\text{a}}\) is the diffusion activation energy, R is the gas constant (8.314 J \(\cdot\) K\({}^{-1}\cdot\)mol\({}^{-1}\)), and T is the absolute temperature value. \[\text{K}=\text{A}\exp\left(\frac{-\text{E}_{\text{a}}}{\text{RT}}\right) \tag{1}\]  
If natural logarithms are taken on both sides of the above equation, it can be transformed in the following way. \[\text{ln}(\text{K})=\text{ln}(\text{A})-\left(\frac{\text{E}_{\text{a}}}{ \text{R}}\right)\left(\frac{1}{\text{T}}\right) \tag{2}\]  
Figure 3: SEM image of Au and Ag deposited using an e-beam evaporator. The diffusion activation energy obtained through Equation (2) for Au and Cu was 6369.52 J/mol when Au diffuses to Cu, and 9816.50 J/mol when Cu diffuses to Au, as shown correspondingly in Figure 5a,b. The lower the diffusion activation energy, the easier the diffusion; hence, according to this result, it is believed that the diffusion of Cu into Au was more difficult than the diffusion of Au into Cu. As shown in Figure 5c,d, the diffusion activation energy for Ag and Cu was 17,933.21 J/mol when Ag diffuses into Cu and 15,626.82 J/mol when Cu diffuses into Ag, unlike the Au case, showing that the diffusion of Cu into Ag is easier than the diffusion of Ag into Cu.