Curable material layer is coated on surface of first die. First die includes first substrate and first contact pad. Second die is bonded to first die. Second die includes second substrate and second contact pad. Second contact pad is located on second substrate, at an active surface of second die. Bonding the second die to the first die includes disposing second die with the active surface closer to the curable material layer and curing the curable material layer. A through die hole is etched in the second substrate from a backside surface of the second substrate opposite to the active surface. The through die hole further extends through the cured material layer, is encircled by the second contact pad, and exposes the first contact pad. A conductive material is disposed in the through die hole. The conductive material electrically connects the first contact pad to the second contact pad.