Methods of forming a semiconductor device are provided. The methods may include forming an insulating layer including silicon on a substrate and sequentially forming a first hard mask layer and a second hard mask layer on the substrate. The first hard mask layer may include carbon, and the second hard mask layer may include carbon and impurities. The first and second hard mask layers may expose at least a portion of the insulating layer. The methods may also include performing an etching process to selectively remove the second hard mask layer with respect to the insulating layer. A ratio of etch rates between the second hard mask layer and the insulating layer during the etching process may be in a range of about 100:1 to about 10,000:1.