A semiconductor device includes a substrate, a bottom etch stop layer over the substrate, a middle etch stop layer over the bottom etch stop layer, and a top etch stop layer over the middle etch stop layer. The top, middle, and bottom etch stop layers include different material compositions from each other. The semiconductor device further includes a dielectric layer over the top etch stop layer and a via extending through the dielectric layer and the top, middle, and bottom etch stop layers. The via has a first sidewall in contact with the dielectric layer and slanted inwardly from top to bottom towards a center of the via and a second sidewall in contact with the bottom etch stop layer and slanted outwardly from top to bottom away from the center of the via.