Reliability study of fine pitch Cu-Sn micro-bump structure electromigration test by Finite Element Simulation

As 3D stacking of memory products leads the trend, dimension of micro-bump scales down. General micro-bump pitch has been reduced to 20 similar to 40 mu m, which is almost one order of magnitude smaller than flip chip solder ball. Current density flowing through the micro-bump will increase dramatically under the same supply current when chips are stacked together. It can be seen that electromigration failure of micro-bump will become a severe issue. Investigation of electromigration reliability for fine pitch micro-bump is urgently needed. However, due to the extremely narrow gap between micro-bumps, reliability study through experimental methods is usually resource-consuming. Therefore, numerical simulation method is particularly important to study electromigration reliability. In this paper, a 3D stacked test vehicle model with Cu-Sn micro-bump as interlayer connection is established. Atomic Flux Divergence (AFD) method is used to investigate the dominate migration principle in Cu-Sn micronstructure under different working conditions. Results reveal the behavior of electronic migration, thermal migration, and stress migration under different electromigration test conditions. Stress migration is observed larger than electronic migration and thermal migration when current density=1x10(9)A/m(2), temperature=150 degrees C, which is in accordance with the experiment under the same condition. Besides, stress migration is observed always having great influence on electromigration process with different current density when test temperature is high (150 degrees C). Low test temperature is suggested if the effect of electronic migration needs to be manifested in final experiment result.