A method for fabricating a semiconductor device is provided. The method includes sequentially forming etch target layers, a hard mask layer and an anti-reflective coating layer, selectively etching the anti-reflective coating layer and the hard mask layer using a gas generating polymers, thereby increasing a line width of a bottom portion of the hard mask layer due to the polymers, and etching the etch target layers using a patterned hard mask layer with the increased line width.