Advanced Face-to-Back Cow $2.0- \mu \mathrm{m}$ Pitch Cu-Cu Hybrid Bonding Process for Three Layer-Stacked 3D Heterogeneous Integration

We present an advanced process for fine-pitch face-to-back chip-on-wafer integration in three-dimensional heterogeneous integrated systems. We introduce a novel chemical mechanical polishing (CMP) process that incorporates oxidation inhibitors to optimize $\mathbf{C u}$ recess control to enhance fine-pitch $\mathbf{C u}$- Cu hybrid bonding. The optimized process is successfully implemented on a single chip for various pad sizes, ranging from $3.0 \mu \mathrm{m}$ to $1.0 \mu \mathrm{m}$, achieving void-free bonding. Implementation of the advanced CMP process resulted in significant improvement in Kelvin connection yields, with approximately 99 % and 95 % success rates for pad sizes of 1.4 and $1.0 \mu \mathrm{m}$, respectively. This represents a substantial improvement over conventional processing yields of 90 % and 50 % for pad sizes of 1.4 and $1.0 \mu \mathrm{m}$, respectively. Moreover, excellent electromigration resistance was observed, with projected device lifetimes exceeding 10 years. These results validate the effectiveness of the proposed fabrication process for three-layer stacked chip-on-wafer-on-wafer structures, establishing a robust foundation for high-density three-dimensional heterogeneous integration in next-generation semiconductor devices.