Crystal Lattice Rearrangement Occurred at Au/Ag Bonded Interface in Atomic Diffusion Bonding in Vacuum

Abstract

Atomic diffusion bonding (ADB) processing of wafers using thin metal films is a promising route to achieving room-temperature wafer bonding [1,2]. When two film surfaces mutually contact at room temperature during ADB, crystal lattice rearrangement occurs, thereby enhancing the bonding performance. Generally, films of the same material used for bonding are fabricated on two flat wafer surfaces. Therefore, methods for examining the crystal lattice rearrangement mechanism are few. Two representative material films used for bonding, those of the metals Au and Ag, have fcc crystal structure and show a solid solution in the whole Au–Ag composition range. Moreover, lattice mismatch of these materials is very slight: less than 0.2%. For this study, Au film and Ag film were bonded in vacuum using ADB. The crystallographic structure of Au/Ag bonded films and depth profiles of Au and Ag atoms were examined. Au and Ag films were fabricated respectively on two flat wafer surfaces using sputter deposition, followed by bonding of the two films on the wafers in vacuum. Figure 1 portrays cross-section STEM images of Au(20 nm)/Ag(20 nm) bonded films: (A) a bright field image, and (B) an HAADF image corresponding to (A). Films were deposited on Si wafers, with 5-nm-thick Ta films underneath. Figure 1(A) shows that no interface corresponding to the original Au/Ag interface is visible; no vacancy is observed. Grains were formed continuously across the original interface over the entire thickness of the bonded films, indicating that crystal lattice rearrangement occurred at room temperature when the films mutually contacted in vacuum. Although the crystallographic structures of Au and Ag layers are identical because of the crystal lattice rearrangement, the contrast between the upper and lower half areas in Figure 1(B) is expected to be attributable mainly to the different atomic weights of Au and Ag. Figure 2 presents the depth profiles of Au, Ag, and Ta evaluated using EELS analysis along line (a) in Figure 1(B). Results supported that interdiffusion at the Au/Ag bonded interface is not remarkable. This report presents discussion of the relation between crystal lattice rearrangement and interdiffusion of atoms based on results of analyses of Au/Ag bonded films with various layer structures. T. Shimatsu and M. Uomoto, J. Vac. Sci. Technol., B 28, 706 (2010).T. Shimatsu and M. Uomoto, ECS Trans., 33 (4), 61 (2010). Figure 1