Through-dielectric vias for direct connection and method forming same

A method includes bonding a tier-1 device die to a carrier, forming a first gap-filling region to encapsulate the tier-1 device die, forming a first redistribution structure over and electrically connected to the tier-1 device die, and bonding a tier-2 device die to the tier-1 device die. The tier-2 device die is over the tier-1 device die, and the tier-2 device die extends laterally beyond a corresponding edge of the tier-1 device die. The method further includes forming a second gap-filling region to encapsulate the tier-2 device die, removing the carrier, and forming a through-dielectric via penetrating through the first gap-filling region. The through-dielectric via is overlapped by, and is electrically connected to, the tier-2 device die. A second redistribution structure is formed, wherein the first redistribution structure and the second redistribution structure are on opposing sides of the tier-1 device die.