Alternating phase-shifting mask (Alt-PSM) has been often viewed as one of the most practical techniques for 100nm-and-below node lithography. Among the various mask structures of the Alt-PSMs, the 'single trench with undercut structure', which has a phase shifting trench with side- etching, has been in frequent use of 130nm-node KrF lithography. It is because this structure has good optical characteristics, and it has some advantages in productivity compared with other mask structures. However, when the 'single trench with undercut' type Alt-PSM is applied to the 100nm-and-below node ArF lithography, the narrow chrome line width restricts the undercut width and limits the lithographic performance. Therefore a new structure is required.