Dependence of EM Performance on Line-width for Cu Dual Inlaid Structures

摘要

Electromigration (EM) is a diffusion phenomenon under the influence of driving forces. The major diffusion paths for Cu dual inlaid structures are believed to be interfaces and grain boundaries. Cu dual inlaid structures usually have a refractory metal barrier layer and are capped with a dielectric layer. The fastest diffusion path in such a structure is believed to be the Cu-dielectric interface. We studied the relationship between EM behavior and metal line-width for two types of EM test structures. It was found that the median time to failure (MTTF) increased significantly as the metal line-width increased for each type of structures when tested under the same current density. In one case, the MTTF increased by 200% as the metal line-width was doubled. Microstructure analysis on the metal lines showed that the wider lines had almost a bamboo structure while the narrower lines consisted of small grains. Therefore, the dramatic decrease in MTTF in the narrower line structure was most likely due to a significant increase in grain boundary diffusion. Mathematical treatment has been performed on the experimental data based on the assumption that the MTTF is reciprocally proportional to the drift velocity or the diffusivity, in this case, of Cu. It has been concluded that grain boundaries can be the fastest diffusion path in Cu dual inlaid structures when the grain size is small.