Methods of manufacturing a semiconductor device are provided. The method may include forming an etch target layer on a substrate, forming a carbon layer doped with boron on the etch target layer, a top end portion of the carbon layer having a different boron concentration from a bottom end portion of the carbon layer, patterning the carbon layer to form at least one opening exposing the etch target layer, and etching the exposed etch target layer using the carbon layer as an etch mask.