Atomic-scale theoretical investigation into the role of metal passivation layers in lowering Cu-Cu bonding temperature

Context

Low-temperature Cu-Cu bonding is critical for 3D integration. Metal passivation layers promote this bonding, but their atomic mechanisms are not fully understood. Here, molecular dynamics simulations investigate how these layers enhance atomic diffusion and reduce bonding temperature. Results show that increasing temperature and time accelerate diffusion, temperature dominates diffusion via its exponential effect on the diffusion coefficient, while time linearly influences only the diffusion distance. The choice of metal passivation layer significantly influences the process. Titanium (Ti) most effectively facilitates diffusion. Initially, diffusion in the passivation layer resembles bulk metal interdiffusion, but over time, the formation of numerous and deeper grain boundaries within the passivation layer occurs, marking a shift toward low-energy grain boundary diffusion. It promotes low-temperature Cu-Cu bonding. Polycrystalline gold, with its initial grain boundaries, further accelerates diffusion and lowers the activation energy for bonding. Therefore, grain refinement is identified as an effective means of enhancing atomic diffusion and bonding quality. This work provides theoretical insights into lowering the temperature of Cu-Cu bonding and offers scientific foundations for optimizing bonding processes.

Methods

Molecular dynamics (MD) simulations using the LAMMPS package were conducted to study Cu-Cu bonding with various metal passivation layers (Ag, Au, Ti, polycrystalline Au). The Cu-passivation layer-Cu model was constructed, with atomic interactions described by the MEAM potential. Diffusion behavior was simulated under the NVT ensemble at target temperatures for 300 ps. The bonding interface evolution was visualized using OVITO, and diffusion coefficients and activation energies were derived from the mean square displacement (MSD), enabling inference of the passivation layer’s role in Cu-Cu bonding.