Patent ID: 7592104

Claim:
A mask blank substrate to be chucked by a mask stage of an exposure system, wherein: a flatness of a rectangular flatness measurement area excluding an area of 2 mm inward from an outer peripheral end surface on a main surface of the mask blank substrate on a side to be chucked by the mask stage is 0.6 μm or less, and at least three of four corner portions of the flatness measurement area each have a shape that rises toward an outer peripheral side, wherein: the flatness measurement area is an area including a pattern area serving as a rectangular area for formation of a mask pattern, and the rising shape at the corner portion of the flatness measurement area is a shape having a maximum height of 0.25 μm or less, but greater than 0, using, as a reference point, a corner among corners of the pattern area that is closest to said corner portion of the rising shape.