Study of Thermal Stability of Nanotwinned and Electromigration Failure Mode for Microbumps with Nanotwinned Copper Metallization
|關鍵字:||奈米雙晶銅;熱穩定性;電遷移;微凸塊;nanotwinned copper;thermal stability;electromigration;microbump|
Nanotwinned copper has some outstanding properties that could be used in electronic device. In this study, the thermal stability of nanotwinned copper was investigated and used it as under-bump-metallization in microbump to analyze the failure mode with current stressing. First, nanotwinned copper samples were produced with different current density. After annealing at 300oC for 1h, nanotwins were stable in most of samples. After annealing at 350oC for 1h, nanotwin in most of samples disappeared and copper grains exhibited abnormal grain growth. Second, the relationship between the bath temperature and the twin spacing was investigated. When the bath temperature decreases, twin spacing decreases. When the bath temperature increases, twin spacing increases. The thermodynamic model was proposed to explain this phenomenon. Third, the thermal stability of nanotwinned copper produced with different electroplating method was discussed by annealing samples at lower temperature for a long time. The results showed that the microstructure of pulsed electroplating copper and direct current electroplating copper were stable after annealing at 100oC for 2000 h and 150oC for 1500 h. However, the microstructures were different after annealing at 200oC. Nanotwins in DC electroplating samples disappeared and copper grains had abnormal grain growth after annealing for 500 h. The pulsed electroplating sample began to have the same phenomenon after annealing at 200oC for 1500 h. Finally, nanotwinned copper was used as UBM in microbump and performed electromigration test. The nanotwinned copper can reduce the Kirkendall voids caused by Cu3Sn formation. Because of the small size of microbumps, there was a few of tin grains in a microbump. And the relationship between electron direction and tin grain c-axis direction was more important and would determine the failure mode.