Surface Modification for Ultrasonic Cu-to-Cu Direct Bonding

Due to the advantages of low electrical resistance and size miniaturization, Cu-to-Cu direct bonding has become one of the important trends in microelectronic interconnect fabrication. Achieving robust Cu-to-Cu bonding at low temperature, ambient pressure, and with short processing time is crucial in practical applications. To fulfill these requirements, ultrasonic bonding techniques have been recently adopted. This study introduces a plasma-surface modification technique to ultrasonic Cu-to-Cu direct bonding, significantly enhancing bonding performance. Experimental results demonstrate that proper plasma bombardment can effectively increase joint strength. Remarkably, if the plasma treatment is applied exclusively to the upper bumps, the bonding performance can be further optimized, making the process more suitable for practical applications.