Observation of interfacial void formation in bonded copper layers

Silicon wafers are bonded using copper (Cu) as the bonding medium and annealed to enhance the bonding strength at temperatures ranging from 300to400°C. The original bonding interface disappears and the Cu layers merge to form a single homogeneous layer. Cross-section transmission electron microscopy reveals a number of voids at the original bonding interface. It is observed that the size of these interfacial voids increases with bonding temperature. Thermal stress relaxation is proposed as the cause for interfacial void formation in the bonded Cu layers.