Method for finishing surface of preliminary polished glass substrate

A method for finishing a pre-polished glass substrate surface of a glass substrate, comprising:

• providing a glass substrate made quartz glass containing a dopant and comprising SiO2 as a main component;
• measuring flatness of a pre-polished glass substrate surface of the glass substrate using a shape measurement unit that comprises:
• a low-coherent light source whose outgoing light flux has a coherence length shorter than twice an optical distance between front and back surfaces of the glass substrate;
• a path match route part that divides the outgoing light flux from the low-coherent light source into two light fluxes, causes one of the two light fluxes to make a detour by a given optical path length relative to the other light flux, and then recombines the light fluxes into a single light flux and outputs the single light flux;
• and an interference optical system that acquires an interference fringe which carries wave surface information of the glass substrate surface by radiating an outgoing light flux from the low-coherent light source onto a reference surface and the glass substrate surface held on a measurement optical axis and making lights returning from the reference surface and the glass substrate surface interfere with each other;
• measuring a concentration distribution of the dopant contained in the glass substrate;
• setting up processing conditions of the glass substrate surface for each site of the glass substrate based on results obtained from the measuring of flatness of the glass substrate and the measuring of a concentration distribution of the dopant contained in the glass substrate;
• and
• finishing while keeping an angle formed by a normal line of the glass substrate and an incident beam generated by a processing method of one of ion beam etching, gas cluster ion beam etching and plasma etching onto the glass substrate surface at from 30 to 89°,
• wherein correlation between the concentration of the dopant and the processing rate of the glass substrate surface is determined beforehand,
• after measuring the flatness of the glass substrate surface, a given amount of the glass substrate surface is processed under processing conditions, and the flatness of the glass substrate surface after the processing is then measured, and
• the concentration distribution of the dopant contained in the glass substrate is measured using a difference in the flatness of the glass substrate surface before and after the processing and the correlation between the concentration of the dopant and the processing rate.