Simulation of Bulge-Out Mechanism Enabling Sub-0.5 μm Scaling of Hybrid Wafer-to-Wafer Bonding

For wafer-to-wafer Cu/SiCN hybrid bonding with unequal sized pads it was previously reported that a diffusion based "bulge-out" mechanism mitigates the drastic decrease in elastic expansion that accompanies pad size reduction, allowing further scaling thanks to stress-driven Cu surface diffusion along the Cu/dielectric interface. In this work this observation is extended to curvature-driven surface diffusion in well-aligned equal sized pads where a Cu/dielectric interface is absent, and a model is developed. The mechanism is mainly determined by the contact angle between Cu and pad barrier/liner material. The time required to complete diffusion increases with a power 3.7 of pad size, such that the dominant pad extension mechanism shifts from elasto-plastic at pitches above 2 μm, to surface diffusion for pitches below 0.5 μm. For 50 nm pads with a Ta barrier, ~4 nm bulge-out is predicted, independent of anneal temperature.