Methods of forming masks. According to the methods, a target pattern is set. Generation of a side lobe caused by the target pattern is verified. A preliminary target pattern and a preliminary side lobe pattern are set, in the target pattern and a region where the side lobe is generated, respectively. An interference pattern map using the preliminary target pattern and the preliminary side lobe pattern is created. At least one of regions having a phase identical or opposite to that of a position of the preliminary target pattern in the interference pattern map is set to an interference auxiliary pattern. A mask using the interference auxiliary pattern and the target pattern is formed.