Athermal and thermal coupling electromigration effects on the microstructure and failure mechanism in advanced fine-pitch Cu interconnects under extremely high current density

Abstract

The fine-pitch and small line width Cu redistribution lines (RDLs) serve as the key factor in achieving high-density advanced fan-out packaging products. However, the dimension scaling of the next generation Cu RDL will cause more severe electromigration damage to the narrow Cu interconnects. The present study reported the microstructure variations and the failure mechanisms involved in an advanced 2μm/2 μm line/spacing Cu RDL interconnect under the electromigration experiment at an extremely high current density, 6 × 106 A/cm2, to investigate the electrical-thermal coupling interactions. The athermal electromigration effect induced rapid mass transport of Cu atoms, giving rise to RDL width reduction (migrated Cu depletion) and width growth (migrated Cu accumulation) phenomena. Meanwhile, the large amount of Joule heat generation induced the high-temperature material degradation of the polyimide (PI) dielectric layer and the delamination across the RDL/dielectric interface. The thermal stress resulting from the coefficient of thermal expansion mismatch further induced RDL displacement within the degraded PI dielectric layer near the current-stressed Cu RDL. The results suggest the predominant thermal electromigration effects on the PI degradation and RDL displacement.