A novel redistribution layer tailored by nanotwinned copper decreases warpage in wafer level packaging

As a common problem in wafer lever packaging(WLP), wafer warpage caused by heat process should be carefully controlled in case of product inaccuracy or yield loss, and redistribution layer (RDL), as a key structure of WLP, is one of the major concerns that causes warpage. In this paper, a novel RDL tailored by pulsed electrodeposited nanotwinned copper (nt-Cu) was introduced into WLP. It was found that grains grew larger and nt-Cu became rare in our novel RDL when it underwent 300°C annealing. Compared with traditional RDL consisting of normal electroplated copper, the novel RDL revealed quite different warpage characteristics when heating to 300°C for the first time, which was probably due to thermal stability and high yield stress of nt-Cu. Namely, twin lamina growth rather than grain growth during annealing helps nt-Cu avoid the sharp decrease of yield stress. It´s very promising to take the advantage of nt-Cu to reduce wafer warpage.