Orientation-related stress analysis of nanotwinned copper in redistributed layer for wafer-level packaging

As the chips continue to develop, the line width and spacing of redistributed layer (RDL) decrease gradually while the number of stack layers increase, resulting in an increased current density and internal thermal stress in Cu lines. Ordinary electroplated copper (OE-Cu) is susceptible to breakage failure at thin point, due to the dual effects of thermal stress and electromigration. In contrast, nanotwinned copper (NT-Cu) haves higher mechanical strength and excellent conductivity. It has the potential to be the next-generation interconnected material because of its high-density randomly distributed twin boundaries. Besides, NT-Cu exhibits significant anisotropy closely related to the growth direction of twinned lamellae. Because of its special structures and physical properties, the stress of NT-Cu in RDL may be very different from that of OE-Cu. Gaining a comprehensive understanding of the stress state of NT-Cu could help to design RDL layout which are more suitable for its structures and physical properties, thereby improving its reliability.In this paper, we studied the thermal stress distribution of a multilayer RDL structure during the curing process of polyimide (PI) by using finite element analysis (FEA). On the basis of accurate elastic-plastic mechanical constitutive, we focused on investigating the influence of NT-Cu orientation and anisotropy on the thermal stress risk of RDL structures. For fine lines, it was found that NT-Cu with different orientations has different thermal stresses, 90°-twin-orientation being the highest, followed by 0°, and 45° being the lowest. Further, we found that OE-Cu had entered into the plastic deformation stage during PI curing process, while NT-Cu remained in elastic stage due to its extremely high yield strength. In addition, the simulation results of anisotropic constitutive behavior indicate that using only the mechanical isotropic data can also obtain relatively accurate results.