Patent ID: 7972702

Claim:
A glass substrate for a mask blank, said glass substrate having two principal surfaces, four end surfaces perpendicular to the principal surfaces, and chamfered surfaces formed at positions adjacent to each of the end surfaces and the principal surfaces, each chamfered surface having a width defined as measurement taken from an adjacent principal surface in a direction parallel to an adjacent end surface, wherein: a first chamfered surface of the chamfered surface formed at the position adjacent to a first end surface of the end surfaces and a first principal surface of the principal surfaces has a width smaller than that of each of the chamfered surfaces formed on an opposite end face of said glass substrate, said opposite end face being opposite to said first end surface, and the width of said first chamfered surface is sized so as to define a surface layer region of said glass substrate, said surface layer region being a region of said glass substrate from said first principal surface to a depth corresponding to a width of said first chamfered surface, an intermediate region of said glass substrate being defined as a region of said glass substrate from a depth corresponding to the width of said first chamfered surface to a depth corresponding to the width of each of the chamfered surfaces formed on said opposite end face, whereby the width of the first chamfered surface being smaller than the width of the chamfered surfaces of the opposite end face permits an inspection light that is applied in an orthogonal direction with respect to said first end surface proximate said intermediate region to be introduced into said first end surface and to propagate through said intermediate region so that said inspection light travels straight from said first end surface towards said opposite end face and is totally reflected by at least a portion of each of the chamfered surfaces formed on said opposite end face, and whereby an internal defect within the intermediate region generates a fluorescence in response to at least a portion of the inspection light.