Heating Rate Dependence of the Mechanisms of Copper Pumping in Through-Silicon Vias

Abstract

In three-dimensional stacked-die packages, through-silicon vias (TSVs) are used to connect multiple adjacent dies through solder micro-bumps. During thermal cycling, the thermal expansion mismatch between the copper TSVs and the silicon dies creates large internal stresses in the hetero-structure, resulting in intrusion or protrusion of the TSV relative to the Si die. This phenomenon is commonly known as copper pumping and is a potential reliability concern as it impacts the stability of back-end-of-line structures. In this study, the copper-pumping phenomenon was investigated by thermally loading TSV structures via ex situ and in situ thermal cycling with various heating rates. The resulting TSV protrusion was characterized and it was revealed that copper pumping manifests itself via three distinct mechanisms: plasticity, grain boundary sliding, and interfacial sliding. Electron backscatter diffraction analysis revealed that grain boundary sliding occurs preferentially at incoherent sigma-3 boundaries, while coherent sigma-3 boundaries remain immobile. The operating conditions, including ambient temperature, heating rate and the microstructural features that influence these phenomena are discussed.