Atomic-scale study of Cu–Cu direct bonding with assistance of Cu nanoparticles as joining medium by molecular dynamics simulation

The application of Cu–Cu direct bonding in stacked chips is becoming an alternative to the traditional solder-bump joint during the development of three-dimensional integrated circuits (3D IC). In this paper, the self-sintering between copper nanoparticles (Cu NPs) and the sintering between copper nanoparticle (Cu NP) and copper substrate (metallization) are simulated by molecular dynamics method. The results show that Cu atoms move mainly by interdiffusion between adjacent Cu NPs during the early and middle stages of sintering process, and the self-sintering of Cu NPs occurs first. In the late stage of sintering process, Cu atoms begin to diffuse at the contact interface between Cu NP and Cu metallization, and Cu NPs are sintered with Cu metallization. In addition, the simulation results of the effects of sintering temperature and particle size of Cu NPs on the Cu–Cu sintering behavior manifest that a relatively higher sintering temperature and smaller particle size of Cu NPs can better promote the self-sintering between Cu NPs and the sintering between Cu NP and Cu metallization. It is also found that the suitable sintering temperature of Cu NP and Cu metallization is between 450 K and 600 K, and the particle size has little effect on the diffusion distribution of Cu atoms in Cu NPs and Cu metallization during the sintering process.