3D stacking using Cu-Cu direct bonding for 40um pitch and beyond

Most of the challenges for Cu-Cu bonding are bump co-planarity, surface roughness, thermal budget, and interface bonding quality. Normally the process needs high bonding temperature, pressure, and long bonding time. Regarding bump co-planarity, several options are considered, including plating optimization, CMP planarization, fly cut, and bump reflow. In this work we have demonstrated that bump co-planarity and surface roughness are greatly improved by fly cut. The dies with flatten bumps are used to perform Cu-Cu direct bonding via thermal compression bonding (TCB) process. We identify the role and influence of UF (underfill) materials during TCB process. Detailed correlations for TCB parameters and electrical yield are investigated. Cu oxidation layer is found at the interconnect interface and affects stacking yielding. With suitable UF, electrical yield shows improvement up to 100% by optimization of TCB bonding conditions. Cross section SEM (Scanning Electron Microscope) shows the Cu bumps are nicely connected and there is no obvious layer at the interface. Furthermore, fly cut effect is also examined by doing fly cut on top & bottom bumps and fly cut on only top bumps. The result shows that fly cut on both side has higher yielding and lower daisy chain resistance than fly cut only on one side. Finally, we successfully demonstrated that TCB temperature and time could be reduced below 300C and less than 900sec, respectively, with 80% electrical yielding.