A Low-Temperature Ag-Ag Direct Bonding Technology Towards Ten-Thousand-Level Microbumps Arrays Utilizing Plasma-Induced Grown Nanoparticles of Silver Oxide

With the development of microelectronics technology, the semiconductor industry has entered the postMoore's Law era. Many semiconductor manufacturing companies have showcased their 3D integration solutions, such as Foveros of Intel and AMD's 3D V-Cache. The current mainstream approach to 3D stacking involves utilizing through silicon vias (TSV), Cu micro-bumps, and lead-free solder to achieve 3D-IC packaging or heterogeneous integration. However, the formation of brittle intermetallic compounds (IMCs) and the manifestation of size effects can lead to higher electrical resistance and severe reliability issues in solder-based micro-bumps for 3D-IC packaging. Recently, our group has invented a novel interconnection technology through Ag-Ag direct bonding, utilizing plasma-induced grown nanoparticles of silver oxide, whose process was designed for 3D-ICs interconnects. Through controlling the surface morphology of plasma-induced grown silver oxide, this technology can effectively overcome the initial asperities on the bonding surface, thus accommodating larger surface roughness (> 100 nm). In this work, for the first time, we have demonstrated the application of this technology to achieve 'point-to-point' bonding with a high-density micro-bump array at tenthousand-level, whereby finding the optimal bonding parametric settings. The wafer-level copper micro-bump arrays (diameter < 30 μm) are prepared by using photolithography, electroplating, and chemical mechanical polishing (CMP) processes, and Ag is deposited as the all-metal bonding layer using magnetron sputtering technology. Subsequently, plasma-induced nanoparticles of silver oxide (Plasma-induced Ag2O NPs) have grown on the Ag layer with our patented technology. More importantly, we design a new bonding scheme with a 'two-steps' process to achieve lowtemperature solid-state bonding with high-density micro-bump array at the ten-thousand-level, which comprised with two stages: the pre-bonding stage and the final bonding stage. Due to the excellent plastic deformation ability of Ag, the prebonding stage allows for pre-deformation of the Ag layer, ensuring sufficient contact between micro-bumps, thereby providing more engineering tolerance for the flatness of the chip substrate. The final bonding stage provides adequate temperature and time, which allows the silver oxide nanoparticles to fully decompose, thereby promoting the diffusion and rearrangement of Ag atom at the bonding interface. Additionally, the scanning electron microscopy (SEM) and focused ion beam microscopy (FIB) are used to observe the morphology of the micro-bump array and the bonding interface. Shear force testing is used to assess the shear strength of the bonding interface. This study has provided a low-cost alternative technology for the advanced 3D-ICs packaging interconnection method and paved the way for the future development towards high-density interconnection applications.