Low Temperature Cu Nanorod/Sn/Cu Nanorod Bonding Technology for 3D Integration

Three-dimensional integration is an emerging and promising architecture for system-level integration towards higher performance, smaller form factor, lower cost and more functionality. In this paper, a novel low temperature Cu nanorod/Sn/Cu nanorod solid-state-diffusion bonding approach for 3D integration was proposed and investigated. Cu nanorods were deposited on the electroplated Cu surface by an oblique angle deposition technique, and a Sn film was deposited on the Cu nanorod layer in sequence. Samples with electroplated Cu/Sn films were also prepared for comparison. The morphology evolutions of the Cu nanorod/Sn and Cu/Sn structures after reflowed from room temperature to 200°C were studied. The Cu nanorod/Sn structure exhibited a higher coalescent rate compared with the Cu/Sn structure because of the high surface energy of Cu nanorod, which may promote the solid state diffusion and reaction of Cu and Sn atoms during the thermo-compression process. The samples with Cu nanorod/Sn structures were then bonded face-to-face at temperatures below the melting point of Sn with an external pressure. The bonding duration was 20 minutes. The SEM results showed that the Cu nanorod/Sn layers on both samples fused together successfully and intermetallic compounds were formed at a bonding temperature as low as 100°C. When increasing the bonding temperature to 200°C, we obtained a denser bonding interface accompanying with further growth of intermetallic compounds. Our study demonstrated the feasibility of using Cu nanorod/Sn/Cu nanorod solid-state-diffusion for low temperature wafer bonding.