A reflective mask substrate to be used in a lithography technology using light in a soft X-ray region or vacuum ultraviolet region having a wavelength of from approximately 0.2 to 100 nm (The substrate, lithography and light are hereinafter referred to as “substrate for EUVL”, “EUVL” and “EUV light”, respectively.) is required to have a high flatness of a surface of the substrate such that a strain is not generated on a transferred image even upon irradiation of EUV light and also to have a small surface roughness so that a high reflectance is obtained, namely it is required to be highly flat and highly smooth. In addition, in order to suppress influences due to the temperature change, it is required that a coefficient of thermal expansion is low. For that reason, it is preferably used as the substrate for EUVL a substrate resulting from a quartz glass substrate containing SiO2 as a major component, in particular a quartz glass with low thermal expansion which has been made to have low thermal expansion by incorporating TiO2 as a dopant, through processing to give high flatness and high smoothness. Though the shape, size, thickness, etc. of the substrate for EUVL are not limited, usually used substrate are plate-shaped bodies having a rectangular or square planar shape, a substrate size of typically 152 mm in square, and an effective area of the substrate, which is to be highly flat and highly smooth, is 142 mm in square.
In mechanical polishing, even if it is intended to process the surface of the foregoing substrate so as to give high flatness and high smoothness, there is a problem in flatness that a concave-convex shape which is a periodical series of a convex and a concave is formed within the surface of the substrate. In such a concave-convex shape, the distance from one convex to an adjacent convex is typically from 75 to 150 mm. For that reason, it was difficult only by the mechanical polishing to obtain a flatness of not more than 0.15 μm in terms of PV (peak-to-valley), which is required for a substrate of a reflective mask for EUVL. As a method of obtaining a desired highly flat surface free from such a concave-convex shape, Patent Documents 1, 2 and 3 disclose methods of obtaining a flat surface shape by carrying out processing through optimizing local processing amounts while employing a processing method such as gas cluster ion beam etching, plasma etching or polishing with a magnetic viscous fluid.
However, when dry etching is applied to a glass substrate, defects inherent to the dry etching are caused; and when polishing with a magnetic viscous fluid is applied, defects inherent to the polishing with a magnetic viscous fluid are caused, resulting in a problem that the surface roughness becomes large. Patent Document 3 discloses a method of improving such surface roughness or defects by employing non-contact polishing such as EEM.
Patent Document 1: JP-A-8-293483
Patent Document 2: JP-A-2002-316835
Patent Document 3: JP-A-2004-291209