## 3 Results and Discussion
### Thermal Stability of COOH-APTMS SAM  
Figure 6 depicts the XRD patterns of the samples after annealing at different temperatures. According to JCPDS no. 96-900-8469, the XRD profiles of the specimens annealed at temperatures below 500 \({}^{\circ}\)C revealed only the reflexes of copper, which indicated that the COOH-APTMS diffusion barrier could prevent the copper diffusion up to 500 \({}^{\circ}\)C. As the annealing temperature reached 550 \({}^{\circ}\)C, a (300) diffraction peak was detected at the position 20 of 45.00\({}^{\circ}\), which was attributed to a Cu\({}_{3}\)Si phase (JCPDS no. 00-051-0916). This indicated that the COOH-APTMS diffusion barrier began to fail, losing its diffusion barrier performance at 550 \({}^{\circ}\)C. Before and after annealing the Cu/COOH-APTMS/SiO\({}_{2}\)/Si samples below 500 \({}^{\circ}\)C, the Cu (111) diffraction peak was stronger than the Cu (200) one, which meant that the Cu film possessed a (111) preferred orientation. According to the report, [40] a (111) textured Cu film could ensure a higher electromigration resistance. Figure 5: Interfacial toughness of samples: (**a**) As-deposited Cu/APTMS/SiO\({}_{2}\)/Si; (**b**) As-deposited Cu/COOH-APTMS/SiO\({}_{2}\)/Si; (**c**) Cu/APTMS/SiO\({}_{2}\)/Si annealed at 400 \({}^{\circ}\)C; (**d**) Cu/COOH-APTMS/SiO\({}_{2}\)/Si annealed at 400 \({}^{\circ}\)C.  
Figure 6: XRD patterns of samples after annealing at different temperatures. Figure 7 displays the SEM images of the annealed Cu/COOH-APTMS/SiO\({}_{2}\)/Si samples. It was evident that the morphology of the copper film was dependent on the annealing temperature. When the temperature was lower than 400 \({}^{\circ}\)C, the copper film remained flat (Figure 7a). In turn, at the temperatures above 400 \({}^{\circ}\)C, there were some protrusions on the film surface due to the agglomeration of copper atoms (Figure 7b,c). As the annealing temperature increased to 500 \({}^{\circ}\)C (Figure 7d), some microholes were observed on the surface of the Cu film, whose emergence was attributed to the thermal stress in the film. At the annealing temperature of 550 \({}^{\circ}\)C, the copper film was destroyed and lost its continuity (Figure 7e), indicating the failure of the COOH-APTMS diffusion barrier. Figure 7f depicts the EDS map with white particles, according to which the copper-to-silicon atomic ratio of these particles was 3:1. This evidenced the origin of the Cu\({}_{3}\)Si phase and the failure of the COOH-APTMS diffusion barrier, which was consistent with the XRD (Figure 6) results [41]. Figure 8 depicts the AFM images of the Cu/COOH-APTMS/SiO\({}_{2}\)/Si samples after annealing at different temperatures. From these images, the RMS for each Cu/COOH-APTMS/SiO\({}_{2}\)/Si sample was found to be 5.74 nm, 7.02 nm, 13.51 nm, 27.36 nm, and 114.34 nm, corresponding to the annealing temperatures of 350 \({}^{\circ}\)C, 400 \({}^{\circ}\)C, 450 \({}^{\circ}\)C, 500 \({}^{\circ}\)C, and 550 \({}^{\circ}\)C, respectively. It was obvious that the roughness of the Cu film was increasing with the increase in the annealing temperature. The RMS of the Cu film rose sharply to 114.34 nm when the annealing temperature reached 550 \({}^{\circ}\)C. At this time, the Cu film was destroyed, and its continuity was lost, which proved the failure of the COOH-APTMS  
diffusion barrier. The conclusions obtained from the AFM images of the annealed samples were consistent with the SEM (Figure 7) results. Figure 9 displays the change in sheet resistance (\(R_{c}\)) of the samples after annealing at different temperatures. It was found that the sheet resistance of the Cu/APTMS/SiO\({}_{2}\)/Si sample slightly decreased as the annealing temperature rose to 400 \({}^{\circ}\)C, which could be ascribed to the grain growth and defect annihilation in the copper film [42]. However, once the temperature increased to 450 \({}^{\circ}\)C, there was a dramatic increase in the sheet resistance.