Masks for fabricating a semiconductor device and methods of forming mask patterns are provided which are capable of enhancing the breakdown voltage of the fabricated semiconductor device by accurately correcting a line width pattern error of a semiconductor substrate due to a mask error during a process for forming a well ion implantation mask pattern. A disclosed mask used to manufacture a semiconductor device having complementary N-well and P-well includes: a master mask for the complementary N-well and P-well; and a light-blocking pattern on the master mask, wherein a region of the master mask, which is not a portion of the master mask adjacent to the light-blocking pattern, is etched by a predetermined thickness to have a phase shifting function.