Ambient copper-copper thermocompression bonding using self-assembled monolayers

A typical copper-copper thermocompression bonding process is carried out in an ultrahigh vacuum (UHV) or inert environment at a bonding temperature >300°C. The ultraclean environment serves a single purpose - to maintain oxide-free copper surfaces, allowing intimate physical contact between copper atoms. This study investigates the temperature dependence of direct copper bonding from room temperature to 300°C under ambient condition. An anomalous thermal dependence of bond strength occurs between 80°C to 140°C where an increase in bonding temperature within this regime is in fact, detrimental to joint strength. This is interpreted as a thermal competition between oxidation and bond formation. This study also demonstrates that by simply coating the copper surface with a self assembled monolayer of 1-undecanethiol prior to bonding, Cu joints can be successfully formed at close to ambient temperature without a vacuum, yielding joint shear strengths on the order of 70MPa. The densely packed monolayer serves to passivate the copper surface against oxidation under ambient conditions. The ultrathin organic monolayer structure, as compared to a bulk oxide layer, could be easily displaced during the mechanical deformation at the bonding interface which accompanies thermocompression. This method could be an effective simple bonding solution for three-dimensional integrated chips.