The present invention provides a fabrication method of an opening. The method includes providing a substrate having a conductive region therein. Thereafter, a dielectric layer is formed over the substrate and then a stacked layer is formed on the dielectric layer. The stacked layer includes a patterned metal hard mask layer, a patterned silicon oxynitride layer and a patterned silicon oxide layer on the dielectric layer in sequence. Afterward, a first portion of the dielectric layer is removed using the stacked layer as a first mask to form a first opening that exposes a surface of the conductive region.