A method of forming a semiconductor structure includes providing a stack structure having a first side and a second side opposite the first side. The stack structure includes a bottom wafer comprising a substrate; a plurality of through-silicon vias in the substrate; and a plurality of under bump metallurgies (UBMs) connected to the plurality of through-silicon vias, wherein the UBMs are on the first side of the stack structure. The method further includes attaching a handling wafer on the second side of the stack structure; performing a chip probing process; and removing the handling wafer from the stack structure.