A method of fabricating a semiconductor device includes providing a first substrate comprising a first conductive element exposed at a surface of the first substrate; forming a patterned photoresist layer atop the first conductive element, whereby the patterned photoresist layer provides openings exposing the first conductive element; forming a first metal layer in the openings and directly atop the first conductive element; forming a first insulator layer over the first metal layer and the first substrate; and polishing the first metal layer and the first insulator layer, resulting in a first interface surface over the first substrate wherein the first interface surface includes part of the first metal layer and the first insulator layer.