Abstract
In this study, we evaluated the ultrasonic vibration energy required for Cu-Cu bonding using flip-chip bonding technology in air atmosphere. The transmissibility of the ultrasonic vibration was assumed to be different in bump structures with high or low stiffness values. Therefore, we investigated the bonding strengths of Cu bumps with different aspect ratios (bump heights of 5 μm, 20 μm, and 40 μm). As a result, we found that the 20 μm-high Cu bumps were properly bonded with sufficient bonding strength by comparison with the other bumps. No significant voids that decrease the bonding reliability were present at the Cu-Cu interface of well-bonded Cu bumps. In addition, coplanarity between the bonding head and the stage surface was found to be an important factor for ultrasonic bonding, because it enabled all bumps to start to bond simultaneously.
