Influence of observed anelasticity of Cu on the wafer warpage evolution during thermal processes

Wafer warpage is becoming a more and more serious problem in wafer level package because there is a PI/Cu composite in the redistribution layer (RDL). The serious wafer warpage will affect the accuracy of subsequent processes, the wafer handle by mechanical arms, the adsorption of vacuum suction cup and device reliability. Previous work shows that copper and polyimide (PI) contributes almost equally to the wafer warpage in 2 metal 1 PI configuration. So the origin of wafer warpage and deformation mechanisms caused by Cu needs to be studied in detail. In this paper, the copper films are subjected to thermal cycles between 25°C and 28°C, and the wafer warpage and temperature are recorded simultaneously. An obvious time delay between the peak temperature and the maximum warpage is detected, which is coincides with the initial small-slope stage at the beginning. Anelasticity is observed in copper film during the thermal pulse test, and annealing process has little effect on this phenomenon.