Tailoring capping layers to reduce stress gradients in copper metallization
Capping layers for back-end-of-line metallization, which primarily serve as diffusion barriers to prevent contamination, also play a role in mitigating electromigration in the underlying conductive material. Stress gradients can be generated in copper metallization due to the conditions associated with the capping process. To study the effects of deposition and subsequent annealing on the mechanical response of copper films with various capping schemes, we employed a combination of conventional and glancing incidence X-ray diffraction techniques to quantify the stress gradient maxima. The Cu films with dielectric caps, such as silicon nitride, can exhibit large gradients that decrease slightly with thermal cycling. However, Co and TaN-based metallic capping layers create significantly lower stress gradient maxima in copper features both before and after annealing. The different evolution of stress gradients in Cu films with dielectric and metallic caps due to thermal cycling reveals the interaction of dislocation-mediated, plastic deformation with the cap/Cu interface.