(Second Best Paper Award) Preferred Grain Orientation to Enhance Interdiffusion at Room Temperature in Atomic Diffusion Bonding: A Fundamental Study Using Ni and Cu Films

Abstract

Atomic diffusion bonding (ADB) of wafers using thin metal films is a promising process to achieve room-temperature wafer bonding [1,2]. During ADB processing, the bonding performance is enhanced by crystal lattice rearrangement, which occurs when two film surfaces mutually contact at room temperature. It is reasonable to infer that the degree of the interdiffusion of the two films at the bonded interface, including crystal lattice rearrangement, depends on the surface roughness and crystallographic grain orientation of the metal films used for bonding. For this study, after Ni film and Cu film were bonded using ADB, the interdiffusion at the bonded Ni/Cu film interface was examined quantitatively using values of saturation magnetization M s. We assessed the effects of crystallographic preferred grain orientation on the interdiffusion thickness at the bonded interface.The Ni film thickness was fixed at 20 nm, but the Cu film thickness was varied from 5 nm to 300 nm. For both films, Ta underlayer films were used. Surface roughness S a, as estimated using AFM of Ni(20 nm) film, was 0.25 nm. The value of S a for Cu films increased from 0.21 nm to 0.85 nm as the Cu film thickness was increased from 5 nm to 300 nm. XRD findings revealed that both Ni and Cu films showed the (111) preferred grain orientation, but the orientation was degraded at Cu thicknesses of less than 50 nm. Figure 1 presents a STEM cross-section image of bonded Ni(20 nm)/Cu(50 nm) films as an example: no vacancy was observed at the bonded interface. Figure 2 presents values of the interdiffusion thickness T ID for bonded films as a function of the Cu film thickness. T IDshowed a maximum of 6.5 nm for a 50-nm-thick Cu film. S a decreased as the Cu thickness decreased from 50 nm. However, T ID decreased remarkably. Results suggest that deterioration of the (111) preferred grain orientation of Cu films with reduction of film thickness degraded the interdiffusion at the Ni/Cu bonded interface.To support this assumption, Ti underlayers were used underneath Ni(20 nm) and Cu(10, 20 nm) films to enhance the degree of the (111) preferred grain orientation of these films. Results indicate that T ID at the Ni(20 nm)/Cu(10 nm) bonded interface was enhanced to 5.4 nm, which was one order greater than that of the bonded films with Ta underlayers, indicating that enhanced bonding performance of ADB can be achieved with low surface roughness and a high degree of (111) preferred grain orientation. This enhancement is expected to be obtained also via ADB processing using other metal films having an fcc crystal structure, such as Au, Ag, and Al. 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