Molecular dynamics study of adhesion strength and diffusion at interfaces between interconnect materials and underlay materials
A molecular-dynamics technique for determining the adhesion strength and analyzing diffusion at interfaces between different materials has been developed. The adhesion strength is determined by calculating the adhesive fracture energy defined as the difference between the total potential energy of the material-connected state and that of the material-separated state. This technique is used to determine the adhesion strength and analyze diffusion at the interfaces between Cu films and high-melting-point materials that are used as underlay materials for Cu interconnects in ULSIs. The adhesive fracture energy shows that the adhesion strength of the Cu/Ru and Cu/Ir interfaces is much higher than that of the Cu/W, Cu/TiN, and Cu/Ta interfaces. Because the diffusion of Cu atoms at the Cu/Ru and Cu/Ir interfaces is suppressed, the surface smoothness of Cu films on Ru and Ir is much better than that on W, TiN, and Ta. It is also found that adhesion strength and smoothness increase with decreasing lattice mismatch between Cu and the underlay material. These results are confirmed by SEM (scanning electron microscope) and scratch testing.