US Patent 7329475 Method of selecting photomask blank substrates

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 height from a least squares plane for the strip-like regions on the substrate top surface to the strip-like regions is measured at intervals of 0.05-0.35 mm in horizontal and vertical directions, and a substrate in which the difference between the maximum and minimum values for the height among all the measurement points is νoτ μoρε τηαν 0.5 μm is selected.