The morphology of the as-fabricated Cu/Sn and Cu/Ni/Sn microbumps is presented in Fig. 1, which is uniform in shape, neatly arranged, smooth in surface and highly flush, meeting the experimental requirements. The cross-section of Cu/Sn bump as shown in Fig. 1 (b) indicates the thickness of Cu layer is ~ 5 µm, and that of Sn layer is ~ 4 µm. Figure 1 (d) exhibits a Ni layer in interface between Cu and Sn. However, it is difficult to distinguish the Ni layer and Cu pillar, since the atomic number of Ni is close to Cu. So EDS line scanning was applied to obtain the thickness of Ni layer as shown in the inset in Fig. 1 (d). In order to obtain results more precisely, 10 to 15 random sites of each cross-section were scanned and the average thickness of Ni is ~ 0.6 µm.
3.1 IMC growth during isothermal aging of Cu/ Sn microbumps
As shown in Fig. 2, BSE micrographs of Ф10 µm Cu/Sn microbumps aged at 170 °C and 200 °C for different time duration were investigated. At 170 °C, scallop-type Cu6Sn5 was generated at the interface during the early aging stage. (Fig. 2a) Proceeded with aging, the Cu/Cu6Sn5 interface became a rich Cu region, which introduced layer-type Cu3Sn and the reaction rate was controlled by the diffusion of Cu atoms. With the diffusion of Cu atoms, Cu6Sn5 continued to grow and transformed into Cu3Sn at the same time, which caused the thickening of the IMC layer and the depletion of Sn after aging for 81 h, as shown in Fig. 2 (b). More significantly, after aging for 169 h, Cu6Sn5 decomposed and transformed into porous Cu3Sn as shown in Fig. 2 (c). When the aging temperature reached 200 °C, the Cu/Sn microbumps interface had completely changed into IMC after aging only 16 h (Fig. 2d), indicating that high temperature greatly promotes the formation of IMC. Figure 2 (e) shows that all the Cu6Sn5 has transformed into porous Cu3Sn after aging 36 h and the sidewall of Cu pillar has become a sink for Sn atoms decomposed by Cu6Sn5, which formed the sidewall Cu3Sn. Compared with the microstructure at 170 °C, the time node of porous Cu3Sn formation also advances. In general, as the aging temperature increases to over 170 ℃ and the aging time goes on, the Cu6Sn5 will decompose into porous Cu3Sn after the depletion of Sn, and high temperature will accelerate the progress.
3.2 IMC growth during isothermal aging of Cu/Ni/Sn microbumps
To observe the growth and evolution progress of IMC in Cu/Ni/Sn microbumps, the isothermal aging processes at 170 °C and 200 °C were recorded by BSE as shown in Fig. 3. At 170 °C, during the early stage of aging, the BSE results of Fig. 3 (a-b) indicates that Cu6Sn5 didn’t form on the interlayer but only Ni3Sn4 grew on the Ni layer, which is attributed to the resistance of Ni layer to Cu atoms.  As the reaction between Ni and Sn went on, Ni had been largely consumed and Cu atoms began to diffuse. Figure 3 (c) shows that Ni3Sn4 gradually transformed into the upper thicker (Cu, Ni)6Sn5 layer and lower (Ni, Cu)3Sn4 layer with a large volume of Sn solder remained after aging for 64 h. It can’t be ignored that the growth of Ni3Sn4 didn’t introduce voids caused by the volume shrinkage, which demonstrates the superiority of ~ 0.6 um Ni layer to traditional Ni barrier layer. Further aging to 121 h, Fig. 3 (d) exhibits that IMC evolved into a typical (Cu, Ni)6Sn5 / (Cu, Ni)3Sn layered structure with a small quantity of Sn remaining. When aging temperature is raised to 200 °C, the evolution of IMC was similar to that at 170 °C as shown in Fig. 3 (e-h). However, IMC formed more rapidly to make Sn completely deplete. Nevertheless, it’s worth noting that the (Cu, Ni)6Sn5 still maintained stable and didn’t decompose to form porous Cu3Sn even at high aging temperature as shown in Fig. 3 (e), which demonstrates that the thin Ni barrier layer possesses an excellent inhibition effect on the decomposition of Cu6Sn5. Compared with the Cu/Sn bumps, the time node in Cu/Ni/Sn bumps of the Sn depletion is postponed and porous Cu3Sn disappear, which demonstrates that the Ni layer tremendously reduces the growth rate of IMC, and the Ni layer effectively retards the porous Cu3Sn formation.
3.3 The impact mechanism of thin Ni layer to the porous IMC growth
According to the BSE analysis, the most intuitive impact of the Ni layer on the porous Cu3Sn formation is retarding the IMC growth. In order to confirm the inhibition effect of the thin Ni layer on atoms diffusion, EDS mapping was applied to Cu/Sn and Cu/Ni/Sn microbumps aged at 200 °C as exhibited in Fig. 4. For Cu/Sn bump (Fig. 4a-b), abundant red Cu atoms diffused into Sn solder (green) during aging process. On the contrary, by inserting the thin Ni layer, few Cu atoms were observed in Sn solder area as shown in Fig. 4 (c-d), which indicates that the Ni layer can effectively inhibit the Cu atoms diffusion. Moreover, the reaction rate of Ni and Sn is slower than that of Cu and Sn[23, 24], demonstrating that the Ni barrier layer can significantly retard the growth of IMC, which controls one of the necessary condition for the porous Cu3Sn formation.
In addition to obstruct the diffusion of metal atoms, the Ni barrier layer plays an important role in stabilizing η-Cu6Sn5 and limiting its decomposition to inhibit the growth of porous IMC. XRD analysis was applied to observe the phase transitions of the interface IMC aging at 200 ℃ for Cu/Sn and Cu/Ni/Sn plane samples as shown in Fig. 5. For Cu/Sn system, the diffraction peaks of IMC show the characteristics of monoclinic η’-Cu6Sn5, whose feature 2θ angle is 30–80°, indicating the phase transition of Cu/Sn structure during the solid aging stage. Nonetheless, for Cu/Ni/Sn, the characteristic peaks only contain hexagonal η-Cu6Sn5, demonstrating that the thin Ni layer hinders the phase transition. The effect contributes to the growth of ternary IMC such as (Cu, Ni)6Sn5, (Ni, Cu)3Sn4 and so on. After the Cu atoms in the Cu6Sn5 cell being replaced by Ni, the volume of the cell contracts and the distance between the atoms is also reduced, which enhances the bonding force between atoms.[21, 25] Besides, (Cu, Ni)6Sn5 possesses greater formation energy than Cu6Sn5 and thus the possibility of phase transition is relatively remote.[21, 25] Consequently, the stable (Cu, Ni)6Sn5 without phase transition effectively reduces the interface stress, weakening the tendency of the porous Cu3Sn formation. In addition, a ~ 0.6 µm Ni layer with short diffusion distance ensures the earlier formation of stable ternary IMCs, which helps to maintain the stability of the structure in the long run.