Reduction of Underpad Stress in Thermosonic Copper Ball Bonding

Ball bonding processes on test chips with Al metallized bonding pads are optimized with one Au and two Cu wire types, all 25 $\mu {\rm m}$ diameter, obtaining average shear strengths of more than 120 MPa. The process temperature is about $11 0^{\circ}{\rm C}$. The results demonstrate that ball bonds made with Cu wire show at least 15% higher shear strength than those made with Au wire. The estimated maximum shear strength $c_{pk}$ value determined for Cu ball bonding $(c_{pk}=3.7\pm 1.2)$ is almost 1.5 times as large than that of the Au ball bonding process $(c_{pk}=2.3\pm 0.9)$, where LSL is 65.2 MPa. However, the ultrasound level required for Cu is approximately 1.3 times that required for Au. Consequently, about 30% higher ultrasonic forces induced to the bonding pad are measured using integrated real-time microsensors. The higher stresses increases the risk of underpad damage. One way to reduce ultrasonic bonding stresses is by choosing the softer of the two Cu wire types, resulting in a measured ultrasonic force reduction of about 5%. A second way is to reduce the ultrasound level. While this causes the average shear strength to fall by 15%, the ultrasonic force falls by 9%. The $c_{pk}$ value does not change significantly, suggesting that a successful Cu ball bonding operation can be run with about 0.9 times the conventionally optimized ultrasound level. The process adjusted in this way reduces the extra stress observed with Cu wire compared to that observed with Au wire by 39%. Hence, significantly lower than optimized ultrasound levels can be used in a Cu wire bonding process to obtain $c_{pk}$ values higher than that of a comparable Au wire bonding process while reducing the risk for underpad damage.