Evaluation and Benchmarking of Cu Pillar Micro-bumps with Printed Polymer Core

Cu pillar micro-bumps are the enabling technology for high-density fine-pitch interconnection in flip-chip die stacking and 3D IC integration. Due to the significant mismatch in coefficient of thermal expansion (CTE) between the silicon chip and organic substrate or the printed circuit board (PCB), the rigid joints made of pure Cu pillars may suffer rather high thermomechanical stress during reflow bonding and operations. Cu pillar micro-bumps with printed polymer core are proposed in this study to reduce thermomechanical stress and improve joint reliability. Micro-scale cylindrical polymer cores with diameter of ~20μm and height of ~25μm were fabricated by synchronized UV-cured polymer jetting and real-time in situ UV LED curing. Height uniformity of printed polymer cores in a piece of wafer (14700 polymer cores) was characterized and it was concluded that the height variation between two adjacent polymer cores was only 1.4 × 10^-3%. For one chip with 588 polymer cores, the height variation was within 1%. It was concluded that the printed polymer cores were uniform in height and the fabrication process for printed polymer cores was acceptable. After surface metallization, Cu pillars micro-bumps with printed polymer core with diameter of ~30μm and height of ~30μm were achieved. Shear test showed that shear strength of Cu pillar micro-bumps with printed polymer core was 20% higher than that of conventional Cu pillars because adhesion between UV-cured polymer and SiO₂ surface was better than fracture toughness of TiW layer. It was concluded that Cu pillars micro-bumps with printed polymer core were capable to be applied in high-density fine-pitch interconnection based on height uniformity and shear force characterization.