Enhancement of Low-Temperature Al-Al Bonding for Cost-Effective 3D Integration

Al-Al bonding is considered as a promising alternative for direct metal bonding applications providing low-cost fine-pitch interconnections with minimum additional process effort and complexity. Conventional Al-Al thermocompression bonding requires high temperature ( $\gt 300~^{\circ }$ C) and elevated pressure to overcome surface oxide layers, which limits its process compatibility and potential applications in advanced packaging. This work proposes a low-temperature Al-Al bonding method utilizing an Au passivation layer, achieving highly reliable interconnections at $160\sim 250~^{\circ }$ C with 2 MPa bonding force. The Ti/Au metallic stack passivation effectively suppresses Al surface oxidation. During bonding process, Al atoms diffuse through Ti grain boundaries to the bonding interface, forming a high-strength joint with low porosity. Mechanical tests demonstrate an average shear strength of 40.85 MPa at the bonded interface. Electrical characterization reveals a contact resistivity of $0.93\sim 2.0\times 10^{-7} ~\Omega \cdot $ cm2, comparable to conventional high-temperature Al-Al bonding. Further microscopic analysis elucidates the mechanisms of Al-Au interdiffusion and interfacial intermetallic compound formation during bonding. This technique offers a low-temperature and cost-effective solution for 3D integration.