Generic rules to achieve bump electromigration immortality for 3D IC integration

An immortal solder micro-bump (μbump) electromigration (EM) lifetime has been demonstrated for 3D IC integration. This ultimate goal was achieved under strictly controlled conditions, including the optimal design of bump metallurgy, geometry, optimized processes, along with well-defined stressing conditions and manufacturing window. The current carrying capability and EM lifetime of μbump have been investigated as functions of stressing conditions which are correlated with the degradation mechanisms. When stressed under the appropriate g conditions, all μbump test samples survived prolonged stressing, some over 13,000 hours, without a failure. Cross-sectional analyses revealed that the entire solder joint almost all transformed into intermetallic compounds (IMCs) with very minor or no voids. The resistance plots showed an initial fast rise in resistance due to IMC formation, then gradually leveled off and eventually reached a steady state. The observed degradation mechanism is dominated by IMC formation, which is the same as that of the user conditions. On the other hand, void formation that eventually led to open failure was the dominant degradation mechanism when samples were aggressively stressed. In other words, when all other conditions were the same, the stressing conditions make a huge difference in determining between an almost immortal EM lifetime vs. a short lifetime using the same high quality μbumps. The boundary that separates the stressing conditions is roughly defined and will be discussed. In addition, since full IMC μbump will become inevitable in the future miniaturized solder interconnect structure, the EM behavior of IMC dominated μbump has also been evaluated in this study. Under highly accelerated stressing conditions of 174°C, at 1.6×10⁵ A/cm² current density, the IMC dominated μbumps were able to survive more than 600 hours and are still going strong. In comparison, solder μbump failed quickly after just 107 hours when stressed under the same condition. This comparison study clearly demonstrated that IMC dominated joint has significantly higher current carrying capability than that of the solder joint. After reviewing all the data, we have concluded that the failure criteria for solder μbump should be raised significantly higher than the 20% criteria traditionally used for the much larger C4 bumps.