The Dynamic Behavior of Electromigration in a Novel Cu Tall Pillar/Cu Via Interconnect for Fan-out Packaging

We presented in this article an in situ electromigration behavior in a novel Cu tall pillar/Cu via interconnect. A Cu tall pillar connecting to a single Cu via for joining the fan-out Cu redistribution lines (RDLs) was introduced for high-power device applications. The novelty of this study was to design a special ladder-shaped Cu tall pillar for the purpose of the fan-out effect of high-density electric current. The in situ SEM (scanning electron microscope) microstructure investigation during the electromigration experiment provided interesting information for the reliability evaluation and in-depth mechanism exploration of the electromigration behavior in the novel Cu interconnect. A stepwise electromigration experiment was conducted at an initial 2.5 A electric current, approximately 10⁵ to 10⁶ A/cm² current density for the components in the Cu interconnect, followed by a larger current input of 3 A. Electrons flowed from the fan-out Cu RDL (cathode) through the Cu via to the Cu tall pillar and the connecting Cu line (anode). The investigations revealed the dynamic void nucleation and coalescence phenomena in the cathodic Cu via during the early stage electromigration. Cu consumption due to the surface diffusion mechanism also occurred on the sidewall of the Cu via. Grain rotation within the Cu interconnect as a result of the electromigration-induced toque was also observed. The considerable mass loss in the Cu via upon electromigration caused a near-collapse microstructure of the Cu interconnect, leading to a potential reliability concern. Electromigration also induced the formation of Cu hillocks on the anodic Cu line, revealing a typical polarity effect. The Cu hillocks were derived from the predominant consumption of the Cu tall pillar rather than the Cu via or the Cu RDL due to the critical mass transport. The voids formed within the Cu tall pillar mostly appeared with a triple junction grain boundary morphology for the Cu consumption, suggesting a grain-boundary-related Cu migration mechanism. The grain boundary diffusion phenomenon was proposed to be the predominant mechanism for explaining the electromigration-induced void and hillock formation in the novel Cu tall pillar/Cu via interconnect.