Quasi-in-situ observation of the grain growth and grain boundary movement in sintered Cu nanoparticle interconnects

Sintered Cu interconnection for power electronics has attracted considerable interest recently. Investigation of grain growth during Cu nanoparticle sintering provides insight into the strengthening mechanism of the sintered structure. Currently, the literature on Cu nanoparticle sintering mechanism is limited and mainly focuses on the transmission electron microscopy (TEM) observation of a limited number of nanoparticles with an ultrathin region. This study employs a quasi-in-situ method to investigate the mechanism of grain growth and twin formation in the bulk sintered Cu nanoparticle structure. The grains were found to continuously grow accompanied by orientation unification which is attributed to grain boundary (GB) migration or dislocation motion at elevated temperature. Two mechanisms governing twin formation have been observed and detailed. The grain and pore size and porosity in the sintered Cu structure under different sintering conditions were measured and correlated to the joint strength. Porosity was found to be the dominant factor affecting joint strength rather than grain or pore size. The original bonding interface between the sintered structure and the Cu substrate has high porosity. However, subsequent GB movement during heating causes this interface to shift into the sintered structure, eliminating the high porosity interface. Meanwhile, a model predicting porosity evolution was used to identify the dominant diffusion mechanism. This study constitutes the first detailed experimental observation of nanoscale grain growth, twin formation and GB movement at the interface and in the bulk sintered Cu structure. This novel mechanism of GB shifting could provide new guidance for strengthening the sintered Cu interconnections for power electronics.