Patent ID: 8551346

Claim:
A method for preparing a square-shaped glass substrate, which is used to form a photomask by forming a patterned light-shielding film on one surface thereof, comprising the steps of: defining two strip regions on a major surface near a pair of opposed sides such that each strip region spans between 2 mm and 10 mm inward from each of the opposed sides and each strip region extends parallel to each of the opposed sides, but the each strip region excludes end portions extending 2 mm inward from longitudinally opposed ends of each strip region, defining a major surface region coextensive with the major surface, excluding peripheral portions extending 2 mm inward from each of four sides, but including the two strip regions, computing a least squares plane for the major surface region, based on a distance from a common reference plane to a coordinate point within the major surface region, comparing the least squares plane of the major surface region with an actual surface topography for each of the two strip regions to determine a difference, computing, a removal amount at a coordinate point within each strip region in accordance with the difference, and effecting local polishing or etching to remove a surface portion of the square-shaped glass substrate in each strip region corresponding to the removal amount, wherein the two strip regions are parallel to the least squares plane of the major surface region, a separate least squares plane is computed for each of the two strip regions, based on a distance from the common reference plane to a coordinate point within each of the two strip regions, when F 1 is the least squares plane of one strip region, F 2 is the least squares plane of the other strip region. F 3 is the least squares plane for the major surface region, a plane F 3 ′ parallel to F 3 is disposes such that a first center of a first equivalent strip region for F 1 and a second center of a second equivalent strip region for F 2 fall on a same side relative to F 3 ′, and the strip regions have a height difference represented by an absolute value |D 1 −D 2 | of a difference between a distance D 1 of a first normal line drawn from the first center of the first equivalent strip region for F 1 to the plane F 3 ′ and of distance D 2 of second normal line drawn from the second center of the second equivalent strip region for F 2 to the plane F 3 ′, said method further comprising the steps of: computing a desired topography of each strip region which ensures that the height difference between the equivalent strip regions is equal to zero (0), and determining a first difference between the least squares plane of the major surface region and the actual surface topography of each strip region from a second difference between a desired topography of each strip region and the actual surface topography of each strip region.