Mechanical characterizations of η′-Cu6(Sn, In)5 intermetallic compound solder joint: Getting prepared for future nanobumps

As advanced silicon manufacturing processes slow down in scaling, three-dimensional stacking of chips has become a future trend in microelectronic industry. High-density bonding technology is one of the core technologies used to achieve the stacking. In this work, we explored a new bonding technology that is possible to manufacture future high-density interconnects. We fabricated a series of sandwich structure solder joints with different thicknesses using SnBiIn-based nanoparticles under a low bonding temperature of 100 °C. The intermetallic compound (IMC) at the interface is proven to be η′-Cu6(Sn, In)5 nanocrystals with random grain orientations through X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), and atomic probe tomography (APT). The hardness and elastic modulus of η′-Cu6(Sn, In)5 were measured to be 4.093 GPa and 79.586 GPa by micropillar compression tests, respectively, decreasing by 35% and 29.1% compared to Cu6Sn5. Therefore, doping In atoms reduces the brittleness of IMCs, providing a new approach for mitigating the mechanical failures of full IMC bump. First-principles calculation also verified the change in mechanical properties. Furthermore, a full-Cux(Sn, In)y IMCs solder joint and an array of 3 μm solder joints were fabricated experimentally using the bonding technology. This technology successfully demonstrates a pioneer interconnection bonding methodology for future ultra-high-density chip stacking using low melting point solder nanoparticles.