A stacking structure including a first die, a second die stacked on the first die, and a third die and a fourth die disposed on the second die. The first die has a first metallization structure, and the first metallization structure includes first through die vias. The second die has a second metallization structure, and second metallization structure includes second through die vias. The first through die vias are bonded with the second through die vias, and sizes of the first through die vias are different from sizes of the second through die vias. The third and fourth dies are disposed side-by-side and are bonded with the second through die vias.