# Low-Temperature Diffusion of Au and Ag Nanolayers for Cu Bonding
## 3 Results and Discussion
Ag has similar electronegativity toward Cu, while Au has stronger electronegativity toward Cu. Additionally, Ag may be more difficult to observe the diffusion of Cu into Cu. Figure 4: Evaluation of surface XPS peak profiles: (**a**) Au4f peak profile in Au-Cu diffusion, (**b**) Ag3d peak profile in Ag-Cu diffusion, (**c**) Cu2p peak profile in Au-Cu diffusion, (**d**) Cu2p peak profile in Ag-Cu diffusion, (**e**) O1s peak profile in Au-Cu diffusion, (**f**) O1s peak profile in Ag-Cu diffusion. oxidized at the surface, whereas Au does not oxidize at all. Therefore, diffusion from Cu to Ag requires less energy than diffusion from Ag to Cu, while diffusion from Cu to Au requires greater activation energy than diffusion from Au to Cu. Based on the phase diagrams of Au-Cu [21] and Ag-Cu [22], AuCu\({}_{3}\) may exist at 200 \({}^{\circ}\)C if an alloy is formed between Au and Cu, but Figure 4a,c show no evidence of the formation of the AuCu\({}_{3}\) alloy. For Ag-Cu, mostly the \(\alpha\) phase, which is the Cu phase, may exist at 200 \({}^{\circ}\)C with a minuscule amount of the \(\alpha\) phase. In this study, IMC (intermetallic compound) formation was not observed for either Au or Ag due to non-bulk diffusion at high temperatures. The diffusion activation energy value estimated in this study was compared with the diffusion activation energy value of Au and Cu at bulk levels at high temperatures. The activation energy of Au-Cu bulk diffusion at above 700 \({}^{\circ}\)C was approximately 178-197 kJ/mol when Au diffuses to Cu and about 169-172 kJ/mol when Cu diffuses to Au [23; 24]. In addition, the activation energy of Ag-Cu bulk diffusion at temperatures exceeding 700 \({}^{\circ}\)C was about 184-198 kJ/mol when Ag diffuses to Cu and about 192 kJ/mol when Cu diffuses to Ag [23; 24]. Given that diffusion at low temperatures primarily occurs in the form of grain boundary diffusion rather than lattice diffusion, it is unsurprising that the diffusion activation energy observed in this study is lower compared to diffusion at elevated temperatures. Within the scope of this study, distinct diffusion behaviors of Cu on the surface were observed when comparing porous Au and Ag nanolayers with bulk Cu films. In the context of Au-Cu interactions, it was evident that Au exhibited rapid diffusion into Cu, whereas in the Ag-Cu scenario, Cu demonstrated rapid diffusion into Ag. Figure 5: Estimation of diffusion activation energy between Au-Cu and Ag-Cu layers: (**a**) diffusion from Au to Cu diffusion, (**b**) diffusion from Cu to Au, (**c**) diffusion from Ag to Cu, (**d**) diffusion from Cu to Ag. In general, a high temperature of over 400 \({}^{\circ}\)C is required to facilitate Cu bonding, though this can deteriorate the performance of semiconductor devices [25]. In addition, the bonding temperature of semiconductors varies depending on the product, but low temperatures below 250 \({}^{\circ}\)C are increasingly required. Cu bonding using a metal passivation layer can be applied to products either ultra-fine pitch Cu pad structures or larger Cu pad structures with a low Cu volume that does not sufficiently expand at low bonding temperatures. In this study, Cu bonding was evaluated at a temperature of 180 \({}^{\circ}\)C, and annealing was carried out at a temperature of 200 \({}^{\circ}\)C. Figure 6 shows the ion beam milling preprocessing conducted for SEM and TEM measurements of the bonding interface. Figure 7 shows the SAT images of the Cu bonding interface with Au and Ag nanolayers. Figure 8 shows the FE-SEM and TEM results of the Cu bonding interface using an Au nanolayer. In the FE-SEM image shown in Figure 8a,b, the black area is Cu and oxygen introduced before bonding, and the white line next to the black area is Au. According to the EDS (energy dispersive spectroscopy) line mapping chart in Figure 8c, a large amount of oxygen was observed at the interface. It is believed that oxygen contamination was already present at the Au and Cu interface before bonding.