Thermal stability enhancement of low temperature Cu-Cu bonding using metal passivation technology for advanced electronic packaging

This work investigates the thermal stability of Cu-Cu bonding using a thin Ag passivation layer in applications targeting advanced packaging. Conventional Cu-Cu bonding often requires elevated temperatures (≥250 °C) that can exacerbate thermal stress and limit process flexibility, making multi-chip stacking more challenging. By introducing a 3 nm Ag passivation layer, we demonstrate reliable bonding at lower temperatures with improved durability against high-humidity and high-temperature environments, as confirmed by both Highly Accelerated Stress Tests (HAST) and burn-in measurements. In-situ transmission electron microscopy (TEM) and 4D-STEM strain mapping reveal that Ag diffusion along Cu grain boundaries not only retards abnormal grain growth but also reduces interfacial void formation at elevated temperatures. These enhancements collectively maintain a stable interface and superior mechanical strength relative to that for non-passivated Cu-Cu bonding. The results highlight the importance of metal passivation in enabling low-temperature Cu-Cu bonding technologies with robust thermal stability, providing the feasibility for next-generation advanced packaging platforms.