Novel Ga Assisted Low-temperature Bonding Technology for Fine-pitch Interconnects

Thermal compression bonding (TCB) of Cu pillars at high temperature often induces undesirable warpage occurrence due to the mismatch in coefficient of thermal expansion (CTE) among heterogeneous components. Reducing the bonding temperature to avoid warpage is desirable for the development of Cu-to-Cu bonding in three-dimensional integrated circuit (3D IC) packaging.One of the approaches for lowering bonding temperature is to implement low melting temperature materials between Cu pillars. We presented in this article a novel low-temperature bonding technology for fine-pitch, less than 20 μm, Cu-to-Cu interconnects with Cu substrates. The TCB was conducted at 150°C. The low-temperature bonding was assisted by an electroplated intermediate Ga/X-alloy bilayer. The surface of the Ga layer was pre-treated with dilute sulfuric acid for better wetting behavior. The intermediate Ga layer melted and gave rise to liquid/solid interdiffusion with the X-alloy layer during the bonding according to the binary Ga-X-alloy phase diagram. The Ga component further diffused through the X-alloy layer and preferentially reacted with the Cu substrate to form thermodynamically stable CuGa2 intermetallic compound (IMC) at the Cu/X-alloy interface. The crosssectional scanning electron microscope (SEM) and focus ion beam (FIB) analyses indicated that the uniform IMC layer has around 2 μm in thickness. The energy dispersive X-ray spectroscopy (EDS) analysis showed that the electroplated Ga layer was completed consumed and mostly converted to interfacial IMC and partially dissolved in the X-alloy layer after the bonding. The microstructure characterization of the joint revealed an indistinct bonding interface with few impurities or defects, showing pronounced effect of interdiffusion during the bonding. The produced joint structure exhibited a bonding strength greater than 5 MPa as measured by a chip-scale universal testing machine. The low-temperature liquid/solid interdiffusion bonding process could be operated without the need of chemical mechanical polish (CMP). It is believed, basing on the bonding performance, that the Ga assisted low-temperature Cu-to-Cu bonding approach could be more feasible for new applications in fine-pitch 3D IC packaging.