Study of the wafer warpage evolution by cooling to extremely low temperatures

Wafer level chip scale package is becoming the mainstream of package form for the chip used in mobile devices due to its low cost and small form factor. The redistribution layer composed of copper-PI composite usually causes severe wafer warpage, and the plastic deformation of copper during heating processes plays an important role. In order to reduce the wafer warpage brought by the copper layer, the method of cooling the copper thick film/Si composite to extremely low temperatures (-196°C, by liquid nitrogen) is proposed, and after this process, the warpage is reduced by 65% and 93.5% for passivated Cu and blank Cu films, respectively. The thermomechanical response below ambient temperature is deduced according to the measured results and the module based on linear temperature-dependent diffusion energy Q. A clear linear stress recovery is detected during the initial heating, which indicates an obvious stress relaxation occurred soon after the samples are taken out of liquid nitrogen.