Thin wafer handling process evaluation for 3DIC integration

After temporary bonding and thinning process, many backside processes will be conducted on the thinned wafer. The critical processes for thin wafer handling material are high temperature and high vacuum processes. In this article, a quick adhesive selecting method is proposed. Backside process of PECVD SiO₂ is identified as the most critical process for thin wafer handling material selection. Two thermal plastic thin wafer handling materials are used in this study for 300 mm wafers and thermal compress bonding in a vacuum chamber is used for bonding process. After bonding, the wafer is thinned down to 50 μm by a commercialized grinder and the PECVD SiO₂ process is conducted on the thinned wafer. Many dishes were found after PECVD SiO₂ process because there is outgassing from the thin wafer handling material or from the device/carrier wafer surface. In this research, an additional hold time is proposed to reduce the dishing after PECVD SiO₂ process. Different hold time at 210 °C and different bonding time are evaluated. For the bonding process without hold time, large voids are observed. There are also 29 dishes on the surface of the wafer. By adding additional 5 minutes hold time before bonding, the number of the dishing on the wafer surface reduced from 29 to 6. If the hold time is set to 10 minutes, only 4 dishes were found on the wafer surface. From the evaluation result of these two thermal plastic thin wafer handling materials, B-glue seems much better than A-glue. The hold time seems very critical on void reduction during bonding process. From B-glue on PECVD SiO₂ experiment, 10 minutes hold time has the better performance for void reduction. The bonding process with zero hold time has the worst performance which has many voids after PECVD SiO₂ process. An ultra-thin 300 mm wafer with a thickness less than 7 μm is also demonstrated in this research by using B-glue and 10- minutes hold time.