Effects of microstructure of copper used in redistribution layer on wafer warpage evolution during the thermal process

Wafer warpage is becoming a serious problem when adopting wafer level package (WLP), because of the diversity of materials used in redistribution layer and the complicacy of constitutive model during deformation. Most studies focus on the organic material used in WLP, such as PI and epoxy resin, and trying to reduce the wafer warpage by numerical simulation or adjusting the manufacturing process. However, the effect of copper trace layer on wafer warpage, which is proved to be significant, is often neglected. In this paper, the microstructures of four sets of copper films electroplated using different parameters are studied, and the wafer warpage introduced by copper layer is in situ measured. Then, the mathematical relation among plastic strain rate, film stress, and temperature is established. In the end, the detailed relationship between the microstructure and warpage evolution is revealed, and methods to reduce the wafer warpage introduced by copper layer are provided.