Development of multi-stack process on wafer-on-wafer (WOW)

The multi-stack process on wafer-on-wafer (WOW) has been developed. In order to realize the multi-stacked wafer with ultra thinned wafer of less than 10 μm with adhesive polymer, several processes have been optimized. The wafer thickness after back-grinding was controlled within the total thickness variation (TTV) of 1.2 μm on wafer-level of 8 inch. For the side wall of though silicon vias (TSV), SiN film with low deposition temperature of 150°C has been developed and applied for TSV process without degradation for electrical characteristics. The uniformity of Cu electro-plating has been improved that the overburdened Cu from the surface was decreased from 13.3 μm to 0.7 μm by optimizing plating solution. The CMP process following Cu electro-plating has been customized for the high rate of 5 μm/min. Finally, the stacked wafer has been evaluated for thermal cycle test (TCT) of 100 cycles with -65 to 150°C. The result showed that there was no degradation for packaging process.