Atom diffusion mechanism of thermo-sonic flip chip bonding interface

The TSFC (thermosonic flip chip) bonding was realized in a self-structured TSFC bonder. The atom inter-diffusion on Au-Ag bonding interface was characterized by TEM, and high-resolution TEM pictures reveales that the dislocation density in the bump increases after the acting of ultrasonic. And the interlaced dislocation slip lines were observed in the SEM pictures of bumps surface, which indicates the dislocations motion in the interior of bumps. A FEM model was used to simulate the stress on the bonding interface. It is noticed that the ultrasonic vibration causes high stress in the contact interface of bump and pad, which increases the dislocation density and provides short-circuit diffusion channel for Au and Ag atom inter-diffusion. Finally, a preliminary discussion about the atom diffusion, based on the atom diffusion theory, is also presented. Studies show that the stress is a significant component of atom diffusion driving force. The Gibbs free energy, chemical potential and acting force of Au and Ag atom on bonding interface are increased by the stress gradient. With the driving force caused by stress, the probability for atom to overcome the energy barrier increased, and the diffusion speed increased. And the atom diffusion depth on bonding interface is about 200-500 nm in several hundred milliseconds, which forms good bonding strength.