Abstract
<title>Abstract</title> <p>Copper-to-copper (Cu–Cu) direct bonding is a key enabler for fine-pitch three-dimensional (3D) integration, yet how atomic-scale interfacial evolution governs wafer-scale bonding integrity remains unclear. Metal passivation layers are widely used to preserve bondable Cu surfaces, but are typically designed as static protective coatings rather than as transformable interfacial precursors. Here, we demonstrate bonding-driven self-optimization of the Cu–Cu interface using an Ag/Ti heterobilayer passivation stack. Atom probe tomography reveals that the Ag-only joint retains a highly Ag-rich interlayer, whereas the Ag/Ti bilayer develops a localized AgCu-rich transition adjacent to a Ti-containing interfacial zone. First-principles calculations indicate that the reconfigured AgCu/Ti interface is energetically more favorable and more adhesive than the initial Ag/Ti interface. This reconfiguration is correlated with wafer-scale bonding integrity and robust die-scale reliability, maintaining average shear strengths of 30.17 MPa after non-biased HAST and 26.83 MPa after 500 temperature cycles. These results establish transformable passivation as a cross-scale strategy for robust Cu–Cu bonding in 3D integration.</p>