Observations on HRTEM features of thermosonic flip chip bonding interface

The bonding interface features in thermosonic flip chip (FC) bonding are of interest to researchers in microelectronics packaging. In this study, a die with Al pads and eight gold bumps was bonded to a silver-coated pad on our lab test bench. The interface of the sample was analysed by using a high-resolution transmission electron microscope (HRTEM). For FC bonding parameters (e.g. ultrasonic power 2 W, bonding time 350 ms, heating temperature 150 °C, and bonding force 3.2 N), the thickness of atom diffusion at the Au–Ag interface is about 200 nm and that at the Au–Al interface is about 500 nm. In addition, ultrasonic vibration of FC bonding leads to the growth of the dislocation density in bonded materials and the formation of a cluster of dislocations at the interface. Therefore, short circuit diffusion plays a major rule during ultrasonic bonding when the temperature rise is relatively low. These observations will be helpful for further analysis.