Low Temperature Metal-to-Metal Direct Bonding in Atmosphere using highly (111) Oriented Nanotwinned Silver Interconnects

As the technique node of semiconductor is going to encounter its physical limits, the concept of "More than Moore" is promoted by using 2.5D/3D IC to accomplish heterogenous integration. With the lower contact resistance and the ability to increase joint density, metal-to-metal direct bonding has drawn lots of attention to replace traditional solder joints. In this study, we propose to introduce highly (111) oriented nanotwinned Ag as an interconnect material for direct bonding. The nanotwinned Ag interconnects developed in this study possess 99.4% (111) preferred orientation, high hardness of 1.9 GPa, low roughness of 11.6 nm, and low resistivity of 2.03 μΩ-cm. During the bonding process, thermal-compression process was conducted under air atmosphere at 200 °C to understand the bonding mechanism and performance. The results show more than 90% interface can be successfully bonded within 5 minutes, and the strength of die shear can be higher than 70.0 MPa, which is 2-3 times higher than the traditional solder joint. Associated with the dissociation of Ag oxide at elevated temperatures and the highly (111) oriented interface, the mechanism of surface creep helps Ag atoms fill up voids at the interface by the stress-induced surface diffusion on the (111) plane and make nanotwinned Ag films be bonded together. It is noteworthy that the Ag interconnects could remain nanotwinned structure after bonding and provide good specific contact resistance, nearing ~10
^-8
Ω-cm
²
. This approach provides a promising method to achieve high quality direct bonding by using highly (111)-oriented nanotwinned Ag for heterogeneous integration in microelectronic industry.