Regulating intermetallic compound growth and bridging in SnAg solder under electromigration stress through Ni addition and sn crystallographic orientation-grain size

As semiconductor devices scale down, electromigration (EM) failures in interconnects become more severe, requiring effective under-bump metallization (UBM) strategies. Herein, we investigated EM failures correlated with the development of intermetallic compounds (IMCs) in two UBM structures (Cu/SnAg/Cu and Cu/SnAg/Ni/Cu). Results showed that the Ni layer resulted in thinner IMCs. It acted as a diffusion barrier, which effectively suppressed IMC growth. We also found that the IMC formation in both solder structures was significantly influenced by the Sn grain orientation. A lower c-axis angle of beta-Sn to EM flow associated with faster IMC formation. Sn grain size also impacted IMC growth, with larger grains resulting in slower IMC formation as a result of the reduced grain boundary density. In addition, the IMC bridging phenomenon was observed in the joints. It was found that IMC bridging occurred less frequently in Ni UBM solder joints compared to Cu/SnAg/Cu counterparts. Such a difference could be attributed to the lower solubility of Ni in Sn compared to Cu. The Ni served as a barrier, which limited the Ni dissolution into the Sn solder. It suppressed the IMC formation/growth, thereby reducing the IMC bridging probability in the Cu/SnAg/Ni/Cu joints.