Patent ID: 7329475

Claim:
A method of selecting photomask blank substrates which are quadrangular and have a length on each side of at least 6 inches, provided that a pair of strip-like regions extend from 2 mm to 10 mm inside each of a pair of opposing sides along an outer periphery of a top surface of a substrate on which a mask pattern is to be formed, with a 2 mm edge portion excluded at each end in a lengthwise direction thereof, the method comprising the steps of: measuring the height from a least squares plane for the strip-like regions on the substrate top surface to the strip-like regions at intervals of 0.05 to 0.35 mm in horizontal and vertical directions, and selecting, as a substrate having an acceptable surface shape, a substrate in which the difference between the maximum and minimum values for the height among all the measurement points is not more than 0.5 μm.