Die stacks and methods forming same

A method includes thinning a semiconductor substrate of a device die to reveal through-substrate vias that extend into the semiconductor substrate, and forming a first redistribution structure, which includes forming a first plurality of dielectric layers over the semiconductor substrate, and forming a first plurality of redistribution lines in the first plurality of dielectric layers. The first plurality of redistribution lines are electrically connected to the through-substrate vias. The method further includes placing a first memory die over the first redistribution structure, and forming a first plurality of metal posts over the first redistribution structure. The first plurality of metal posts are electrically connected to the first plurality of redistribution lines. The first memory die is encapsulated in a first encapsulant. A second plurality of redistribution lines are formed over, and electrically connected to, the first plurality of metal posts and the first memory die.