Bonding Behavior and Quality of Pressureless Ag Sintering on (111)-Oriented Nanotwinned Cu Substrate in Ambient Air

Abstract

(111)-oriented nanotwinned Cu ((111)nt-Cu) has shown its high surface diffusion rate and better oxidation resistance over common polycrystalline Cu (C-Cu). The application of (111)nt-Cu as an interface metallization layer in Ag-sintered technology under the role of oxygen was investigated in this work, and its connecting behavior was further clarified by comparing it with C-Cu. As the sintering temperature decreasing from 300 to 200 °C, the shear strength on the (111)nt-Cu substrate was still greater than 55 MPa after sintering for 10 min. The fracture surface correspondingly changed from the interface of Ag/die to mixed fracture mode, involving the interface of the Ag/Cu substrate and Ag/die. The existence of copper oxide provided a tight connection between Ag and the (111)nt-Cu substrate at all of the studied temperatures. Although lots of small dispersed voids were seen at the interface between copper oxide and (111)nt-Cu at 300 °C, these impurity-induced voids would not necessarily be a failure position and could be improved by adjusting the sintering temperature and time; for example, 200 °C/10 min or heating to 300 °C, and then start cooling at the same time. The microstructure of Ag-Cu joint on (111)nt-Cu behaved better than that on C-Cu. The thinner copper oxide layer and the higher connection ratio of the interface between copper oxide and Ag were still found on the (111)nt-Cu connection's structure. The poor connection between copper oxide and Ag on C-Cu easily became the failure interface. By controlling the thickness of copper oxide and the content of impurity-induced voids, the use of (111)nt-Cu in advanced-packaging could be improved to a new level.

Keywords: bonding quality; interfacial oxide; microstructure; nanotwinned Cu; pressureless Ag sintering.

Figure 1

Cross-sectional FIB images and XRD patterns of (a,c) (111)nt-Cu and (b,d) C-Cu substrates.

Figure 2

(a) Die shear strength and (b,c) fracture mode of (111)nt-Cu and C-Cu substrates sintered at 300 °C for 10 min.

Figure 3

Die fracture surface with corresponding element mapping of (a) (111)nt-Cu and (b) C-Cu substrates sintered at 300 °C for 10 min. (a1,a2) and (b1,b2) are magnified views of local areas in (a) and (b), respectively.

Figure 4

BSE images and EDS mapping of Ag-Cu joint on (111)nt-Cu substrate sintered at 300 °C for 10 min.

Figure 5

Compositional curves of Ag-Cu joint sintered at 300 °C for 10 min along the diffusion direction, (a) (111)nt-Cu and (b) C-Cu substrates. The enrichment phenomenon of Ag in (a) was highlighted with a circle indicating by black arrow.

Figure 6

BSE images and EDS mapping of Ag-Cu joint on C-Cu substrate sintered at 300 °C for 10 min.

Figure 7

(a) Fracture mode and (b,c) fracture surface with corresponding element mapping of (111)nt-Cu and C-Cu substrates sintered at 250 °C for 10 min. (b1,b2) are magnified views of local areas in (b).

Figure 8

Morphology of Ag-Cu joint on (a) (111)nt-Cu and (b) C-Cu substrates sintered at 250 °C for 10 min.

Figure 9

(a) Fracture mode and (b) fracture surface with corresponding element mapping of (111)nt-Cu substrate sintered at 200 °C for 10 min. (b1,b2) are magnified views of local areas in (b).

Figure 10

Morphology and EDS mapping of Ag-Cu joint on (a) (111)nt-Cu and (b) C-Cu substrates sintered at 200 °C for 10 min. Ave: Average thickness of copper oxide. (a1,b1) are magnified views of local areas in (a,b), respectively.

Figure 11

Morphology of Ag-Cu joint on (111)nt-Cu substrate sintered at 300 °C for (a) 0 and (b) 30 min. Ave: Average thickness of copper oxide.

Figure 12

(a) Thickness of copper oxide and (b) connection ratio of the interface between Ag and copper oxide on (111)nt-Cu and C-Cu substrates.

Figure 13

Schematic illustration of diffusion bonding of the Ag-Cu joint on (111)nt-Cu and C-Cu substrates, (a,b) the initial sintering stage, (c) the intermediate sintering stage, (d) the final sintering stage. J_{Cu-s((111)nt-Cu)}: surface diffusion flux of Cu on (111)nt-Cu, J_Cu-s(C-Cu): surface diffusion flux of Cu on C-Cu, J_Cu-i: interior diffusion flux of Cu, J_p and J_O: diffusion flux of impurity elements and O. W₍₁₁₁₎ or W_poly: the energy of Cu atoms escaping from (111) crystal lattice on (111)nt-Cu or other crystal lattices on C-Cu.