A method includes forming a transistor over a front side of a substrate; forming a front-side interconnect structure over the transistor, the front-side interconnect structure comprising layers of conductive lines, and conductive vias interconnecting the layers of conductive lines; forming a first bonding layer over the front-side interconnect structure; forming a second bonding layer over a carrier substrate; bonding the front-side interconnect structure to the carrier substrate by pressing the first bonding layer against the second bonding layer; and forming a backside interconnect structure over a backside of the substrate after bonding the front-side interconnect structure to the carrier substrate.