A method for fabricating a semiconductor device includes forming a first mask layer on a substrate, forming an under layer on the first mask layer, forming a first photoresist pattern that includes tin on the under layer, converting at least a part of the first photoresist pattern into a second photoresist pattern including tin fluoride, through a plasma treatment process using fluorine element, etching the under layer using the second photoresist pattern as a first mask to form an under pattern, etching the first mask layer to form a first mask pattern, and etching at least a part of the substrate, using a mask pattern including the first mask pattern as a second mask.