Investigation of bonding mechanism for low-temperature Cu Cu bonding with passivation layer

The bonding mechanism of low-temperature CuCu thermal compression bonding (TCB) with passivation layer has been investigated in this research. After the structures of Cu with Au passivation and Cu with Pd passivation were bonded at 180 °C, the analyses of transmission electron microscope (TEM) were performed for the bonded structures to explore the bonding behavior of passivated-Cu during different stages of the TCB process. The results indicate that grain boundary diffusion of Cu leads to formation of amorphous Cu at the bonding interface in the passivated-Cu structure. Furthermore, diffusion of Cu atoms and recrystallization of amorphous Cu into polycrystalline Cu can eliminate voids at the bonding interface during the TCB process. Comparing to conventional Cu direct bonding with high crystallinity Cu at the bonding interface, passivated-Cu bonding can be achieved by much lower energy, leading to high quality bonding results with low thermal budget.