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A wafer level under-fill (WLUF) process for ultra-fine CuCu bonding is developed. Under-fill is applied as pre-applied under-fill then planarized the surface. The methodology used for surface planarization (bit grinding) and surface treatment (H2 plasma) are fond to be important in the surface preparation and activation. Underfill material needs to have sufficient hardness and adhesion to the wafer to survive during bit grinding process. Again, it must not get cured during plasma treatments before bonding is carried out. DOE is carried out with four different WLUF materials and one capillary under-fill material. Tests were carried out with a test vehicle having 5 um diameter and 10 um pitch. Results showed only one material could pass through all those requirements.