Void Migration Kinetics in Fine Line Cu RDL under Electric Current Stressing and the Improvement of Electromigration Reliability by Polyimide Passivation

The present study adopted in-situ SEM to investigate the electromigration behavior of fine line Cu RDL, 2 μm (width) x 3 μm (thickness) x 20 μm (length), especially focusing on the void migration and the effect of PI passivation on suppressing electromigration. The electromigration were conducted at high current density (10 6 A/cm 2 ~ 3.06 x 10 7 A/cm 2 ) and 77~100°C (heat plate). A critical current density of 1 x 10 7 A/cm 2 was identified below which no electromigration occurrence was observable on the bare Cu RDL. Hillocks accumulated at the anode side at above 1.5 x 10 7 A/cm 2 . Voids were generated and grew on the cathode side at the triple joint of grain boundary. The in-situ video revealed that the void started migrating towards the anode after growing up to around 0.3 μm. The void was seen moving rapidly within a large grain, while hindered by grain boundary when hit smaller grains. With the kinetic information captured from the in-situ video, the migration flux as well as the diffusion coefficient was calculated. The diffusion coefficient was estimated to be 3.95~7.85 x 10 -10 cm 2 /sec and 1.68 x 10 -9 ~ 1.04 x 10 -8 cm 2 /sec at the low (227°C) and high (333°C) circuit temperatures estimated with temperature coefficient resistance, respectively. Electromigration experiment of the PI passivated Cu RDL showed that no hillock was observable at the anode side after current stressing.