Thermo-mechanically and electrically induced interfacial incompatibility in 3D packages with through silicon vias

摘要

In stacked-die packages, through-silicon vias (TSVs) are used to connect adjacent dies through tiny solder microbumps. During thermal cycling associated with service, the thermal expansion mismatch between the copper TSVs and the silicon chips results in large residual stresses in the TSVs, along with significant interfacial shear stresses. These shear stresses drive diffusionally assisted interfacial sliding, especially during slow thermal cycling, resulting in appreciable intrusion or protrusion of the TSVs. This is in addition to the phenomenon of copper-pumping, which is typically attributed to plastic and creep deformation of the copper due to thermally induced stresses. The interfacial sliding phenomenon occurs due to stress-assisted interfacial diffusion, and its relative importance compared to copper-pumping depends on the initial stress in the TSV, the temperature range, and rate of cycling. In addition to stress-assisted interfacial diffusion, interfacial sliding can also be driven by electromigration. Unlike stress-driven interfacial sliding which occurs symmetrically at both ends of a TSV, electromigration-assisted interfacial sliding results in a net movement of the TSV in the direction of electron flow with intrusion at one end and protrusion at the other. In this work, experiments are performed to identify the conditions under which each of these two phenomena dominates.