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.