A package structure and a formation method are provided. The method includes disposing a first chip structure over a carrier substrate. The first chip structure has a front-side interconnection structure facing the carrier substrate. The method also includes forming a back-side interconnection structure over the first chip structure. The first chip structure has a device portion between the back-side interconnection structure and the front-side interconnection structure. The back-side interconnection structure has stacked conductive vias. The method further includes bonding a second chip structure to the first chip structure using dielectric-to-dielectric bonding and metal-to-metal bonding.