Advanced 3D Integrated Circuit Packaging: Metallurgical Improvement and FLOLA (Flux-Less Oxide-removal Laser Assembly) Process at a 9㎛-Pitch Micro-Bump

The study aimed to explore the thermal stability of the new bump metallurgy and evaluate the feasibility of flux-less bonding in 9㎛ pitch. A mechanical 3D-IC package structure was implemented to achieve this, connecting two chips vertically with a 9-micron bump pitch. The study used a diffusion barrier to suppress intermetallic compounds (IMCs) between the metal pillar “M” and the solder. A newly adopted Flux-Less oxide-removal Laser Assembly (FLOLA) was used to interconnect the top die to a 12-inch bottom wafer vertically at the wafer level process using neither fluxing nor cleaning. To verify the heat resistance property of diffusion barrier embedded micro-bumps, a high-temperature storage test (HTST) was conducted at 150℃ for 1000 hours and 180℃ for 500 hours, respectively. That samples were precisely grinded and polished to inspect cross-sections of solder joint, field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), and plasma focused ion beam (PFIB) were used to characterize the behavior of IMC growth for iso-thermal aging time conditions. The reliability test result showed that diffusion barrier layer could suppress IMC growth effectively, and eliminate the void risk by volume shrinkage, shown like as a conventional nickel-solder(Ni-Sn) under bump metallurgy (UBM), completely.