A device includes a substrate having a front surface and a back surface; an integrated circuit device at the front surface of the substrate; and a metal plate on the back surface of the substrate, wherein the metal plate overlaps substantially an entirety of the integrated circuit device. A guard ring extends into the substrate and encircles the integrated circuit device. The guard ring is formed of a conductive material. A through substrate via (TSV) penetrates through the substrate and electrically couples to the metal plate.