Patent ID: 7344808

Claim:
A method of making a photomask blank substrate which is quadrangular and has a length on each side of at least 6 inches, which has a pair of strip-like regions that extend from 2 to 10 mm inside each of a pair of opposing sides along an outer periphery of a top surface of the substrate on which a mask pattern is to be formed but exclude at each end in a lengthwise direction thereof a 2 mm edge portion, each of the strip-like regions being inclined downward toward the outer periphery of the substrate, and which has a difference between maximum and minimum values for height from a least squares plane for the strip-like regions on the substrate top surface to the strip-like regions themselves of at most 0.5 μm, the method comprising the steps of: polishing a starting substrate to a difference between the maximum and minimum values for height from a least squares plane for a principal surface region, comprising a quadrangular ring-shaped region that extends from 2 to 10 mm within each side along the outer periphery of the substrate top surface and a patterning region situated within an inner periphery of the quadrangular ring-shaped region, to the principal surface region itself of at most 1.5 μm so as to give a polished intermediate product; then additionally polishing the polished intermediate product.