A device includes first and second dies and a seal ring. The first die includes a top dielectric layer. The second die is over the first die. The second die includes a bottom dielectric layer bonded to the top dielectric layer of the first die at an interface between the first die and the second die. The seal ring extends from the first die to the second die through the interface. A portion of the top dielectric layer of the first die and a portion of the bottom dielectric layer of the second die are separated by a gap outside the seal ring.