Challenges of high-robustness self-assembly with Cu/Sn-Ag microbump bonding for die-to-wafer 3D integration

We demonstrated surface tension-driven self-assembly of chips with Cu/Sn-Ag microbumps in order to satisfy requirements for both high throughput and high alignment accuracy toward 3D system integration. The chips were singulated with different dicing methods: standard single-cut, precise single-cut, and modified step-cut. The alignment accuracies were compared among the three methods. The chips obtained by modified step-cut were precisely aligned within approximately 2 μm and comparable to that obtained by precise single-cut. By optimizing liquid volumes, the step-cut chips having Cu/Sn-Ag microbumps were accurately self-assembled irrespective of microbump densities. The self-assembled chips were successfully bonded at 280°C by thermal compression. The Cu/Sn-Ag daisy chains indicated good electrical characteristics with a resistance of 35 mΩ/joint.