# The Effect of an Ag Nanofilm on Low-Temperature Cu/Ag-Ag/Cu Chip Bonding in Air
## 3 Results and Discussion
### Cu/Ag-Ag/Cu Bonding  
The two Cu samples with an Ag nanofilm were bonded at the temperatures of 150 \({}^{\circ}\)C and 180 \({}^{\circ}\)C under 30 MPa pressure in air using a heating press. After bonding, cross-sectional observation was undertaken by FIB-SEM, and elemental analysis was performed with EDS. For both the 150 \({}^{\circ}\)C and 180 \({}^{\circ}\)C bonding cases, a seam was observed at the bonded interface without voids, as shown in the FIB-SEM cross-sectional images of Figure 4. According to the inset images in Figure 4, this seam at the bonded interface was not a physically separated space. Rather, it appears that the Ag nanofilm was slightly etched off at the interface during the sample preparation by the FIB process. This was confirmed by the EDS measurements, shown in Figure 5, where Ag diffused into Cu but still remained at the bonded interface. In addition, Cu was observed at the bonded interface.  
Figure 4: FIB-SEM cross-sectional images of Cu/Ag-Ag/Cu bonded samples.  
Figure 3: XPS depth profiles measured after heating for 1 h at 25 \({}^{\circ}\)C, 100 \({}^{\circ}\)C, and 150 \({}^{\circ}\)C: (a) Ag3d, (b) Cu 2p, and (c) O1s.  
For quantitative analysis of bonding quality, the bonded samples were cut into 2 \(\times\) 2 mm, and a shear test was performed with a 600 \(\upmu\)m/sec tool tip speed. The shear test was taken as the average value after ten or more measurements and is shown in Figure 6. Some of the samples broke before Cu bonding interface separation, and some were SiO\({}_{2}\)/Ti-interface separated. In both cases, the other weak interfaces were separated prior to the Cu bonding interface. For the estimation of shear strength, the samples that were broken at the Cu bonding interface were considered. The average shear strength of Cu/Ag-Ag/Cu bonding was 13.4 MPa for 150 \({}^{\circ}\)C bonding and 14.0 MPa for 180 \({}^{\circ}\)C bonding. In both cases, there was little difference in average shear strengths, but the bonding at 150 \({}^{\circ}\)C showed a larger shear strength deviation than that of the bonding at 180\({}^{\circ}\), as shown in Figure 6. It can be seen that the bonding at 180 \({}^{\circ}\)C has better reproducibility and uniform bonding than those of the bonding at 150\({}^{\circ}\). In addition, the shear strength of broken or separated samples at other interfaces was as high as 35.9 MPa, which was often higher than the estimated average shear strength of 14.0 MPa. Therefore, although there was no annealing process in this study, it is expected that the bonding quality can be uniformly improved if a sufficient annealing process is performed after the Cu/Ag-Ag/Cu bonding of 30 min in order to activate further Ag diffusion.  
Figure 5: EDS measurements of Cu/Ag-Ag/Cu bonding interface (sample bonded at 150 \({}^{\circ}\)C).  
Figure 6: Shear strength measurements of Cu/Ag-Ag/Cu bonded samples.  
The bonded interface was further analyzed by the TEM measurements. Figure 7 shows the TEM images and EDS profile across the interface of the Cu bonding at 180 \({}^{\circ}\)C. As shown in Figure 7, the bonding interface was well bonded, and the tiny voids visible at the interface were caused by the FIB process during the TEM specimen preparation. After 30 min of bonding at 180 \({}^{\circ}\)C, most Ag remained at the bonding interface, and some Ag diffusion into Cu was observed in the area in contact with the copper grain boundary.