Reduce the Wafer Warpage Introduced by Cu in RDL Through Adjusting the Cooling Temperatures

Wafer level chip scale package (WL-CSP) which is low cost and small size is becoming the mainstream of package form for the chip used in mobile devices. The processes are done on wafers, and the wafer warpage is severe after the redistribution layer (RDL). It is proved that the plastic deformation of copper during the thermal processes causes almost 1/3 of wafer warpage in RDL. In order to reduce the warpage caused by Cu, copper films deposited on the silicon wafers are cooled to extremely low temperatures such as -196°C after the thermal process. The wafer warpage of blank Cu film and Cu film passivated with SiNx is reduced by 65% and 93.5% after the cooling process, respectively. Besides, the stress in Cu film transforms from tensile to compressive, indicating an obvious plastic deformation occurred during cooling in liquid nitrogen. When changing the cooling temperature, the final wafer warpage at ambient temperature varies. The decreased wafer warpage is almost recovered after another thermal process. The stress-temperature curve coincides with the curve before cooling for the sample without passivation. As for the Cu film passivated with SiNx, the stress-temperature curve parallel moves downwards and roughly 10% of initial wafer warpage is reduced and unrecoverable.