A method of fabricating a semiconductor device, the method including providing a substrate; forming an underlying layer on the substrate; forming a sacrificial layer on the underlying layer; forming an opening in the sacrificial layer by patterning the sacrificial layer such that the opening exposes a predetermined region of the underlying layer; forming a mask layer in the opening; forming an oxide mask by partially or completely oxidizing the mask layer; removing the sacrificial layer; and etching the underlying layer using the oxide mask as an etch mask to form an underlying layer pattern.