Patent ID: 7608542

Claim:
A method for preparing a large-size glass substrate, from which a photomask substrate is formed, the photomask substrate being used in a mother glass exposure process including attaching a photomask substrate having opposed sides to an exposure apparatus through support of opposed side edges, disposing underneath and adjacent to the photomask substrate a mother glass serving as an array side or color filter side substrate in a TFT liquid crystal panel, and irradiating light from the exposure apparatus to the mother glass through the photomask substrate, said method comprising the step of processing a large size glass substrate stock having front and back surfaces and a diagonal length of at least 500 mm into the large-size glass substrate, by removing therefrom (1) a flattening removal quantity of material based on a height data of the flatness and parallelism of the front and back surfaces of the large-size glass substrate stock in the vertical attitude, plus a deformation-corrective removal quantity of material, the deformation-corrective removal quantity being calculated from (2) a deflection of the substrate stock by its own weight in the horizontal attitude, calculated from the thickness and size of the substrate stock, and the support position when the photomask substrate is horizontally supported, (3) a deformation of the photomask substrate caused by the photomask substrate support when the photomask substrate is attached to the exposure apparatus, and (4) an accuracy distortion of a platen for supporting the mother glass, wherein the resulting large-size glass substrate has such an arcuate shape in cross section that the surface to be opposed to the mother glass is concave when held vertically, and reduces the proximity gap variation between the mother glass and the photomask substrate formed from the large-size glass substrate which is held horizontally when the opposed side edges of the photomask substrate are supported in the exposure apparatus.