Impacts of Pressure on the Stability of Chip Stack Structures in the Presence of Noncoplanar Cu Pillars

Cu-Cu bonding is widely used in 3-D stacking for chip interconnection. However, noncoplanarity of Cu pillars can significantly influence the reliability of the stacked chips. In this work, the impacts of bonding pressures on chip stacking with noncoplanar Cu pillars were investigated through finite element modeling and molecular dynamics (MDs) simulations. From finite element modeling, the maximum residual stress in the TSV was located in the bottom Cu pillar furthest from the center of the chip and increased with bonding pressure. It was also found that the overall warpage increased linearly with the number of chip layers initially and then stabilized when Cu pillars were noncoplanar. The linear increase rate raised with higher bonding pressure. Based on MDs simulations, the thickness of bonding layer in the shorter pillar increased continuously with bonding time and pressure. In contrast, the thickness in standard pillars grew over time and eventually stabilized. During this, the bonding pressure posed negligible effects on the bonding layer thickness in standard pillars, and dislocations were generally concentrated near the bonding layer, serving as channels for Cu atom diffusion into the bonding layer.