Effect of (111) Surface Ratio on the Bonding Quality of Cu-Cu Joints

In this study, we deposited nano-twinned Cu with various (111) ratios onto Si substrates by adjusting the electroplating parameters. Cu films with 38.7, 75.5, 94.1% of (111) surface ratio were electrodeposited. The grain sizes and surface roughness for the different specimens were controlled. To investigate the effect of the (111) ratio on Cu-to-Cu bonding, the samples were bonded at a temperature of 200 °C using a thermal compression bonding machine. The results indicate the surface (111) ratio affects the bonding strength significantly. The shear strength was 10.22 MPa, 19.08 MPa, and 23.63 MPa for the specimen with 38.7%, 75.5%, and 94.1% (111) surface ratio, respectively. Additionally, continuous voids were observed in the bonded specimen with a low (111)-oriented ratio of 38.7%. The continuous voids can be eliminated by increasing the (111)-oriented surface ratio. This research provides crucial fundamental knowledge for the effect of (111) surface percentage on the Cu-Cu bonding for 3D IC technology.