Simulation of Cu pad expansion in wafer-to-wafer Cu/SiCN hybrid bonding

Wafer-to-wafer Cu hybrid bonding relies on non-elastic Cu-pad expansion to achieve permanent Cu-Cu pad bonding between the bonded wafers. Understanding the mechanism and being able to predict the amount of Cu expansion and the mechanism of this Cu expansion is the key for defining the chemical-mechanical polishing specifications and process window, as well as lowering the required annealing temperature for achieving proper Cu-Cu bonding. Cu primary creep is investigated as the potential mechanism for Cu expansion through finite element simulations. Pad expansion results for different processing conditions are reported. Comparison with measurements indicates that primary creep is however not the dominant mechanism for Cu expansion in wafer -to-wafer hybrid bonding.