A method of fabricating a semiconductor device comprising a method of forming an etching mask used for etching a semiconductor base material is disclosed. The method of fabricating a semiconductor device comprises forming hard mask patterns on a semiconductor base material; forming material layers covering the lateral and top surfaces of the hard mask patterns to form openings between adjacent hard mask patterns, wherein the width of each opening is smaller than the distance between adjacent hard mask patterns; and etching the semiconductor base material using the hard mask patterns and material layers as an etching mask.