<title>Dependence of EM performance on linewidth for Cu dual-inlaid structures</title>

Abstract

Electromigration (EM) is a diffusion phenomenon undr 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.

Optimizing the electromigration performance of copper interconnects

“…The larger median grain size leads to a reduction in Dgh or Dcu/b, explaining the improvement in EM performance as the linewidth increases. ( 19) …”

“…In such a case, the MTTF can be assumed to be reciprocally related to the drift velocity of diffusivity of the Cu. (19) The term DCu,N in the effective diffusivity equation represents the diffusion along the Cu and SiNx (CdN) interface and is independent of linewidth. Therefore, the significant increase in MTTF with linewidth suggests that the diffusion along the CdN interface is not dominant.…”