Numerical Simulations to Assist Chip-to-Wafer Hybrid Bonding Process Development

Development of chip to wafer hybrid bonding (C2W-HB) process is essential to achieve direct bonding of fine pitch (≤ 10 μm) Cu interconnects for heterogeneous integration. As the polymer mechanical behavior is more complex, C2W-HB with direct bonding of Cu/polymer-dielectrics requires significant experimental efforts, resources, and time to design the test vehicles and formulate the process recipe for high quality and yield. In this work, numerical simulations are performed to evaluate the bonding progression for a special case where SiO2 dielectric is used for the bottom wafer and polymer dielectric is used for the top dies. As these two materials are quite different in their thermomechanical behaviour, bonding of the Cu/Cu interfaces depends on the relative expansion/contraction of the surrounding SiO2 (bottom wafer) and dielectric polymer (top die) interface. Our simulations help to elucidate the mechanics at the bonding interface and explain the reasons for the bonding failures observed in the experimental runs. The simulation results show that Cu pad dishing is not suitable for this configuration and protrusion of Cu pads is required to get a successful Cu/Cu direct bonding.