A set of masks including an alternating phase shifting mask (APSM) and a halftone phase shifting trim mask (HPSTM) is provided. The APSM includes first and second phase shifting areas and a first opaque pattern. The first and second phase shifting areas are disposed adjacent to each other and have different phases for generating destructive interference. Further, the first and second phase shifting areas define an access interconnection line. The first opaque pattern is formed on a transparent substrate to define the first and second phase shifting areas. The HPSTM includes a second opaque pattern on the transparent substrate and a halftone pattern. The second opaque pattern prevents an access interconnection line from being erased. The halftone pattern defines a pass interconnection line connected to the access interconnection line.