Diffusion barrier performance of electrodeposited Co–Mo layer in 3D interconnect Cu/Sn bonding

Co–Mo barrier layers with wheat grain, nodular and flocculent structures were prepared by electrodeposition through adjusting the concentration of complexing agent in the electrolyte (0.1 M∼0.3 M). The work was to determine which kind of microstructure was most favorable to block the mutual diffusion of copper and tin, thereby solving the failure problem caused by Cu–Sn mutual diffusion in the 3D package. The barrier performance of the Co–Mo layers in Cu/Sn bonding was systematically investigated under different severe conditions. It was found that the Co–Mo layer could effectively control the mutual diffusion of Cu/Sn and exhibited excellent barrier performance. The diffusion coefficients of copper were 4.65 × 10 −15 cm 2 s −1 , 1.42 × 10 −15 cm 2 s −1 , and 2.49 × 10 −16 cm 2 s −1 when using the Co–Mo barrier with wheat grain, nodular, and flocculent morphologies, respectively. It was found through the analysis of microstructure combined with resistivity that the Co–Mo layer with the flocculent structure exhibited the best barrier performance for the mutual diffusion of copper and tin. The results also showed the Co–Mo layers exhibited good barrier performance, preventing the diffusion of Cu and Sn at 150 °C and 150 °C/85% RH. The failure of the barrier layer at 220 °C/VH resulted in cracks in the Co–Mo layer due to the release of compressive stress. The cracks in the Co–Mo layer provided a pathway for the mutual diffusion of Cu and Sn, leading to the formation of the high-resistance compound Cu 3 Sn, which indicated the failure of the Co–Mo layer.