Die to Wafer Hybrid Cu Bonding for Fine Pitch 3D-IC Applications

Advanced packaging continues to attract a lot of attention as the demands for higher performance, higher bandwidth, and lower power consumption increase. To satisfy aforementioned demands, researchers have tuned to 3D IC technology such as X-Cube using micro-bumps announced by Samsung in 2020. As much higher I/O bandwidth with smaller form factor is required, hybrid Cu bonding (HCB) is considered as one of the most promising technologies for the next-generation 3D-based chip products. Unlike the conventional solder based chip bonding, HCB can achieve fine pitch bonding by connecting copper-to-copper and oxide-to-oxide directly. HCB technology enables sub 10 um bonding pitch as well as improves electrical performance with no power and signal penalty. However, there are many challenges affecting HCB interface quality. Detailed understanding and analyses are necessary to obtain reliable fine pitch interconnects. In this paper, key factors, which impact the electrical connectivity and reliability of HCB, are investigated using die-to-wafer HCB chip with 4-μm pitch and 2-μm pad. The dishing and bonding accuracy are analyzed and precisely controlled to achieve robust electrical connection. As an effort to achieve void free interconnects, the characteristics of voids according to the type and size of particles are analyzed quantitatively. The electrical connectivity and reliability were demonstrated through the investigations of key factors of HCB. It was verified over 98% electrical connectivity for all 210 chains on the chip. All the chips both large top and small top structures, that were assigned to each category of reliability tests, including TC, u-HAST, and HTS, passed the package level reliability tests.