Electromigration failure induced by interdiffusion between Al trace and Cu seed layer in the immortal microbump in three-dimensional integrated circuit

Owing to ultrahigh current density and severe Joule heating, electromigration (EM) in the microbump is a crucial reliability issue in three-dimensional integrated circuit. In this study, we investigated a novel EM weak link in our 13 μm microbump after all of the solder transformed into intermetallic compound (IMC). Under the test condition of 132 °C and 1.57 × 10 5 A/cm 2 , there was not any sign of failure of the microbump sample until 4100 h, and we proved that the microbump had already transformed into a full-IMC structure much earlier. The current density was then doubled at 4100 h and the EM condition turned into 164 °C and 3.14 × 10 5 A/cm 2 . In this case, we detected a semi-failure at 5000 h from the resistance curve of the sample and observed the interdiffusion between Al trace and Cu seed layer in the microbump, accompanied by Al 4 Cu 9 IMC formation. A crack also formed at the Cu/Al interface due to the tensile stress induced by the volume shrinkage of 4.43 % during the Al 4 Cu 9 IMC formation process. The interdiffusion was attributed to the breakdown of the Ti barrier layer between the Al trace and Cu seed layer. When the current stress was continuously applied, a burn-out open circuit and catastrophic failure occurred in the circuit. Finally, a schematic diagram describing the EM failure evolution in the microbump with a full-IMC structure was provided.