Effect of Microstructure on the Electromigration Performance of 2- μm-Cu Redistribution Line under In-situ SEM Observation

A specialized experiment method for in-situ video investigation of the electromigration behavior in fine line Cu RDL has been established in this study. A Kelvin structure design features electroplated Cu RDL straight-line structure, with polyimide passivation removed via a plasma-decap process and surfaces polished using a FIB. The experiments were conducted at a high current density of 2 x 107A/cm2 and a temperature of 77°C to accelerate the electro migration process. During the investigation, voids were observed to form and grow on the cathode side at the triple joint of the grain boundary, while hillocks accumulated on the anode side. Two types of Cu RDLs were electroplated using different electroplating solutions, and their microstructures were analyzed using EBSD, while mechanical properties were assessed with a pico-indenter system. The lifetime of Cu RDLs with smaller copper grains was found to be approximately seven times longer than that of those with larger grains. The superior lifetime could be attributed to improved mechanical properties and a greater number of grain boundaries, which serve to impede the movement of voids (or atoms). These findings suggest that reducing grain size is an effective strategy for enhancing the electromigration performance of fine line RDLs in high current density environments.