Patent ID: 7691279

Claim:
A method of producing a glass substrate for an EUV (extreme ultra violet) mask blank, the method comprising: a preparing step of preparing the glass substrate having a surface subjected to precision polishing; a profile measuring step of measuring, after the preparing step, a convex/concave profile of the surface of the glass substrate; a flatness control step of controlling a flatness of the surface of the glass substrate to a value not greater than a reference value determined depending upon a required flatness required to the glass substrate by specifying the degree of convexity of a convex portion present on the surface of the glass substrate with reference to a result of measurement obtained in the profile measuring step and executing local machining upon the convex portion under a machining condition depending upon the degree of convexity, the local machining being carried out by plasma etching or a gas cluster ion beam; an acid treatment step of subjecting, after the flatness control step, the surface of the glass substrate to acid treatment under a condition that machining scraps of the glass substrate, a reaction product, and a machining-affected layer produced by the local machining and left on the surface of the glass substrate are removed while maintaining the flatness and a surface roughness of the glass substrate; and a polishing step of polishing, after the acid treatment step, the surface of the glass substrate; said method thereby making the surface of the glass substrate have a flatness and a surface roughness required of the glass substrate for producing the EUV mask blank.