Evolution of stress gradients in Cu films and features induced by capping layers

The presence of voids in Cu metallization represents a key reliability issue for ultra-large scale integration technology. In particular, the interface between the Cu and capping layers represents a critical location where the stress state of the Cu must be experimentally determined. Glancing-incidence X-ray diffraction (GIXRD) can be used to investigate depth-dependent stress distributions within electroplated Cu films induced by overlying capping layers. A combination of conventional X-ray diffraction measurements and GIXRD results revealed that strain gradients were created in Cu films and patterned features possessing a SiCxNyHz cap, where an increased in-plane tensile stress was generated near the film/cap interface due to the constraint imposed by the SiCxNyHz layer during cooling from the cap deposition temperature. Cu films possessing a CoWP cap, deposited at lower temperatures where the Cu experienced only elastic deformation, did not exhibit depth-dependent stress distributions. However, all Cu samples exposed to the SiCxNyHz deposition temperature developed stress gradients regardless of the capping material. Although in situ annealing of SiCxNyHz capped Cu films decreased the stress gradient as the sample temperature approached that of the cap deposition, the gradient reappeared upon cooling to room temperature.