A method includes attaching a first die and a second die to a first wafer, the first wafer comprising: a first carrier substrate; and a first interconnect structure comprising first dielectric layers and first conductive features embedded in the first dielectric layers; attaching a third die to the first die and a fourth die to the second die; attaching a second wafer to the third die and the fourth die, the second wafer comprising: a second carrier substrate; and a second interconnect structure comprising second dielectric layers and second conductive features embedded in the second dielectric layers; removing the first carrier substrate; patterning the first dielectric layers to expose conductive features of the first die and the second die; and forming external connectors through the first dielectric layers, the external connectors being electrically connected to corresponding ones of the conductive features of the first die and the second die.