Photomask-forming glass substrate and making method

A square-shaped glass substrate for a photomask, the glass substrate having a pair of major surfaces and a patterned light-shielding film on one surface of the substrate which is chucked at chucking portions in an exposure tool, wherein

• the major surface where the chucking portions are disposed is delineated by four peripheral sides,
• two strip regions are defined on the major surface near a pair of opposed sides such that each region spans between 2 mm and 10 mm inward of the side and extends parallel to the side, but excludes end portions extending 2 mm inward from the longitudinally opposed ends of the side,
• a least squares plane is computed for each of the two strip regions, based on a distance from any common reference plane to a coordinate point within the strip region,
• the angle included between normal lines to the two least squares planes of the two strip regions is less than or equal to 10 seconds,
• on the assumption that F1 is the least squares plane of one strip region, F2 is the least squares plane of the other strip region, F3 is a least squares plane for a major surface region coextensive with the major surface, excluding peripheral portions extending 2 mm inward from the four sides, but including the two strip regions, a plane F3′ parallel to F3 is disposed such that the center of a strip region-equivalent region of F1 and the center of a strip region-equivalent region of F2 may fall on the same side relative to F3′, and the two strip regions have a height difference represented by the absolute value |D1−D2| of the difference between a distance D1 of a normal line drawn from the center of a strip region-equivalent region of F1 to plane F3′ and a distance D2 of a normal line drawn from the center of a strip region-equivalent region of F2 to plane F3′, and
• the height difference between the two strip regions is less than or equal to 0.5 μm.