Thermal annealing is widely used to promote copper grain growth and reduce electrical resistance of the through-mask plated redistribution layer in wafer level package.However

the crack on the surface of the sputtered copper seed layer prevents the plating process in the crack and increases the electrical resistance

and results in void formation on the interface between the sputtered seed layer and the plated copper interconnects.To solve this problem

the sputtered seed layer was slightly etched before the electroplating process and a TaN layer was sputtered on the Cu interconnect surface before thermal annealing process.The process suppressed void formation during the electroplating process and the annealing process at 300℃.This process resulted in grain growth at elevated temperature and lower electrical resistance in the copper interconnects.