A method for forming a semiconductor device, includes the steps of providing a substrate; forming a patterned stack on the substrate including a first dielectric layer on the substrate, a first conductive layer on the first dielectric layer and a mask layer on the first conductive layer, wherein a width of the mask layer is smaller than a width of the first conductive layer; forming a second dielectric layer on the sidewall of the patterned stack; forming a third dielectric layer on the substrate; forming a second conductive layer over the substrate; and removing the mask layer and a portion of the first conductive layer covered by the mask layer to form an opening so as to partially expose the first conductive layer.