Photomask and pattern formation method using the same

A photomask comprising:

• a transparent substrate having a transparent property against exposing light;
• a halftone portion formed on the transparent substrate;
• a first opening formed in the halftone portion and having a first dimension;
• a second opening having a rectangular shape and being formed in the halftone portion and having a second dimension larger than the first dimension;
• and
• a light-shielding portion formed on the transparent substrate around the second opening, wherein:
• the halftone portion transmits the exposing light in an opposite phase with respect to the first opening and the second opening,
• the light-shielding portion includes four rectangular light shielding portions, each of which is disposed along a corresponding one of four sides of the rectangular shape,
• the rectangular light-shielding portions are disconnected at corners of the rectangular shape of the second opening, and
• the light-shielding portion has a width set to be not more than a maximum width with which an unexposed region is not formed by the exposing light having passed through the second opening and diffracting at an edge of the second opening toward a back of the light-shielding portion.