Patent ID: 8153336

Claim:
A photomask substrate that consists of a plate shaped member with a substantially uniform thickness, comprising: a first surface, which is a continuous curved surface and whereon a mask pattern is to be formed; and a second surface, which opposes the first surface; wherein, the first surface exhibits a square shape that comprises a first pair of opposing sides of the four sides surrounding the first surface and a second pair of opposinq sides of the four sides surrounding the first suface, which extend such that they communicate with the first pair of opposing sides, and a support part, which is for supporting the photomask substrate at its end parts along the first pair of opposing sides; when the photomask substrate is held such that the first surface is in a substantially vertical state, the point at which the axis that passes through the center of gravity of the photomask substrate and extends in the thickness direction of the photomask substrate intersects the first surface is designated as a center point of the first surface; and if a reference plane that is parallel to a tangential plane of the first surface at the center point is defined on the photomask substrate side that is closer to the first surface than to the second surface, then a first distance in the thickness direction between the reference plane and the center point of the first surface is shorter than second distances in the thickness direction between the reference plane and the midpoints of the second pair of opposing sides.