Effect of Palladium Passivation via Electroplating and Electroless Plating on Low-Temperature Cu-to-Cu Bonding

As advanced packaging moves toward fine-pitch interconnects, low-temperature Cu-to-Cu bonding becomes increasingly important. However, Cu is highly susceptible to oxidation, which can degrade bonding quality. In this study, we investigated the effect of Pd passivation layers formed by electroplating (EP) and electroless plating (ELP) on Cu surfaces to improve bonding performance at low temperature. A ~6nm-thick Pd layer was deposited on Cu/Ti/ SiO/Si samples, and the surface analysis was done via atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Bonding was carried out at 180°C and 200°C, followed by annealing.The EP-Pd sample formed a uniform Pd thin film, while the ELP-Pd sample exhibited a non-uniform and less dense morphology. In the EP-Pd sample, Pd slowly diffused into the Cu layer at room temperature, reaching a depth of approximately 15nm and forming a Pd-rich Pd-Cu alloy. After bonding at 200°C, a non-uniform copper oxide layer (100-150nm thick) appeared at the bonding interface. In contrast, the ELP-Pd sample enabled faster Pd diffusion, up to ~40nm, and bonding resulted in a thinner copper oxide layer (50-100nm) at the interface. Shear tests confirmed that the EP-Pd sample achieved higher bonding strength. These findings highlight the potential of EP and ELP Pd coatings as nano passivation layers for Cu, offering valuable insights for optimizing passivation strategies in Cu-Cu bonding.