Surface passivation of cu for low temperature 3D wafer bonding

Surface oxidation is the main barrier to achieve copper (Cu) wafer bonding at low temperature (<350?C). Self-assembled monolayer (SAM) is employed to temporarily passivate the Cu surface. Another possible method is by permanent deposition of a thin layer of gold (Au) on Cu. After bonding at 250?C, SAM passivated sample show grain growth across the bonding interface. For Au capped Cu surface, out-diffusion of Cu across the bonding interface due to the Kirkendall effect is observed. Mechanical shear test revealed that Au passivation presents the lowest bond strength possibly due to the formation of inter-metallic compound and severe Au/Cu inter-diffusion.