Atomic insight in fusion mechanism of heterogeneous and homogeneous sintering: Cu and Ag nanoparticles

Cu and Ag nanoparticles are widely encouraged to be regarded as the die-attach material for power device packaging due to the small size effects and excellent physical properties. However, Cu nanoparticles are prone to oxidation during the sintering process, high porosity and easy electromigration of Ag nanoparticles after sintering, which seriously affect the reliability of the interconnection joints. Cu-Ag composite nanoparticles are gradually considered the most promising die-attach material. Unfortunately, there is a lack of study into the atomic diffusion mechanism of Cu-Ag nanoparticles during heterogeneous sintering. The essential difference between homogeneous and heterogeneous sintering has not been revealed, which is not conducive to better control of the sintering process. In this paper, we investigated the sintering fusion mechanism of Cu-Ag composite nanoparticles, homogeneous Cu nanoparticles, and homogeneous Ag nanoparticles at the nanoscale by molecular dynamic simulation. The internal differences between homogeneous and heterogeneous sintering were comprehensively revealed from the radial distribution function (RDF), nanoparticle shrinkage ratio, mean square displacement (MSD), atomic phase transition, and dislocation slip mechanism. In addition, we also investigated the effects of sintering temperature and heating rate on the sintering reliability of Cu-Ag nanoparticles. The results indicate that homogeneous nanoparticles exhibit significant HCP stacking faults during constant sintering process, which will hinder further enlargement of the sintering neck. Surprisingly, heterogeneous (Cu-Ag) nanoparticles undergo alloying process during sintering, which greatly improves sintering performance. The increase in sintering temperature and the decrease in heating rate can make the sintering of nanoparticles more complete, which provides theoretical support for the development of high-power device die-attach materials.