Patent ID: 7622050

Claim:
A polishing process, comprising: applying a surface treatment to a preliminarily polished TiO 2 -containing quartz glass substrate having a flatness of at most 0.5 μm and a surface roughness of at most 0.001 μm by a first gas cluster ion-beam etching to improve the flatness of the glass substrate to be at most 0.05 μm, and subsequently applying a surface treatment to the glass substrate by a second gas cluster ion-beam etching under irradiation conditions different from those of the first gas cluster ion-beam etching to improve the surface roughness of the glass substrate, such that the glass substrate is finish-polished to have a flatness of at most 0.05 μm and a surface roughness (Rms) of at most 0.25 nm, wherein a source gas for the first gas cluster ion-beam etching is selected from a group consisting of SF 6 ; Ar ; O 2 ; NF 3 ; a mixed gas containing SF 6 , Ar and O 2 ; and a mixed gas containing NF 3 , Ar and O 2 , and wherein a source gas for the second gas cluster ion-beam etching is selected from a group consisting of O 2 ; CO 2 ; and a mixed gas of O 2 and CO 2 , and wherein the acceleration voltage of the cluster ions in the first gas cluster ion-beam etching is set to be higher than an acceleration voltage of the cluster ions in the second gas cluster ion-beam etching.