High Electromigration Resistance of Nanotwinned Cu Used in Redistribution Layers of Fan-out

Recently, the requirements of commercial electronic products continue to increase and the package sizes continue to decrease. The redistribution lines (RDLs) in fan-out packages and three dimensional integrated circuit (3D IC) become narrower, resulting in the high current density in the RDLs. The three dimensional integrated circuits which can stack chips vertically and decrease RCdelay. As the scaling of large-scale ICs faced the physical limit, the IC industry is trying to follow the Moore's law to fulfill the needs of customers. By using the 3D IC, which not only reduce the form factor, but also enhance the power efficiency. The input/output pin numbers increasing while the size of the solder joints shrinking. It is inevitably leads to higher current densities and operating temperatures in the joints. It raises severe reliability issues such as electromigration and thermomigration. Furthermore, the RDLs need carry higher current density than before. As the volume of 3D ICs getting smaller, copper is the critical material in interconnects. Copper has low resistivity of 1.7 μΩ, high conductivity 401 W m.1 K.1, and excellent ability to resist electromigration. Recently, highly preferred orientation nanotwinned copper has been studied extensively. The highly (111) oriented nanotwinned copper (nt-Cu) possesses excellent mechanical strength and high ductility. The conductivity of nt-Cu is almost the same comparing to bulk copper. It is reported that the nano-scale twins in the copper contribute to high resistance of the electromigration. Therefore, we use nt-Cu as the material of RDL and observe destruction mechanism of electromigration tests. We also compare the electromigration lifetimes of nt-Cu RDL lines to randomly oriented Cu.