Stress Gradients Observed in Cu Thin Films Induced by Capping Layers

Abstract

Glancing-incidence X-ray diffraction (GIXRD) has been applied to the investigation of depth-dependent stress distributions within electroplated Cu films due to overlying capping layers. 0.65 μm thick Cu films plated on conventional barrier and seed layers received a CVD SiC_xN_yH_z cap, an electrolessly-deposited CoWP layer, or a CoWP layer followed by a SiC_xN_yH_z cap. GIXRD and conventional X-ray diffraction measurements revealed that strain gradients were created in Cu films possessing a SiC_xN_yH_z cap, where a greater in-plane tensile stress was generated near the film / cap interface. The constraint imposed by the SiC_xN_yH_z layer during cooling from the cap deposition temperature led to an increase in the in-plane stress of approximately 180 MPa from the value measured in the bulk Cu. However, Cu films possessing a CoWP cap without a SiC_xN_yH_z layer did not exhibit depth-dependent stress distributions. Because the CoWP capping deposition temperature was much lower than that employed in SiC_xN_yH_z deposition, the Cu experienced elastic deformation during the capping process. Cross-sectional transmission electron microscopy indicated that the top surface of the Cu films exhibited extrusions near grain boundaries for the samples undergoing the thermal excursion during SiC_xN_yH_z deposition. The conformal nature of these caps confirmed that the morphological changes of the Cu film surface occurred prior to capping and are a consequence of the thermal excursions associated with cap deposition.