Electromigration Study of a Novel Cu Stack-via Interconnect for Advanced High-Density Fan-Out Packaging

Cu via technology has been developed as interconnects for connecting Cu redistribution lines (RDLs) for achieving high-density packaging requirements. We presented in this article a novel Cu stack-via interconnect composed of three vias for the advanced fan-out chip on substrate (FOCoS) packaging. It is of particular interest to evaluate the electromigration reliability of the designed high-density packaging configuration considering the potential damage resulting from the high-density electron wind impacts. In this study, the electromigration behavior of the novel Cu stack-via interconnect was investigated in terms of the electrical resistance change, and the mechanism was also explored by the microstructure evolutions. The electromigration experiment of the Si chip/Cu stack-via/Ni metallization/Sn-Ag solder/Ni(P) metallization/Cu trace structure was conducted at 0.9 A, approximately 5.1 × 10^5 A/cm² (estimated from the cross-sectional area of the 15-um-diameter via), for 591.2, 855.7, and 1450.2 h under a 180°C condition. The resistance of the two vias during the electromigration experiment, R1 via referred to the one near the Si chip and R3 via referred to the one near the solder, was investigated for comparison. The resistance changes after the electromigration experiment were expressed in a ratio form of R1/R0 compared with the initial resistance value (R1). The microstructure of the Cu stack-via interconnect was investigated using a field-emission scanning electron microscope (FESEM) accompanying energy-dispersive X-ray spectroscopy (EDS) for the phase characterization. The electromigration behavior in the Cu stack-via interconnect with respect to two electron flow directions, from Cu trace to Cu stack-via (denoted as cathodic interconnect hereafter) and from Cu stack-via to Cu trace (denoted as anodic interconnect hereafter), was also investigated. Electromigration in the R1 and R3 vias of the cathodic stack-via interconnect showed insignificant microstructure and resistance variations for up to 1450.2 h, suggesting great electromigration resistance of the novel stack-via interconnect. Similar behavior of the resistance fluctuation in a range from 0.98 to 1.02, corresponding to the intact Cu via microstructure, can be found in the R1 via of the anodic stack-via interconnect after the electromigration experiment for up to 1450.2 h. Whereas, we observed divergent electromigration behavior in the R3 via of the anodic stack-via interconnect for the micr