Patent ID: 8763428

Claim:
A method for making a glass substrate for a magnetic disk including a mirror polishing step which comprises contacting a polishing pad with a surface of a glass substrate, supplying a mirror polishing liquid containing polishing grains to the surface of said glass substrate, wherein the polishing grains contained in said mirror polishing liquid are made of colloidal silica particles and said mirror polishing step comprises: adjusting a zeta potential of said colloidal silica particles in the mirror polishing liquid to control a degree of coagulation or dispersion in said polishing liquid; and carrying out mirror polishing by the use of the mirror polishing liquid including the colloidal silica particles adjusted in the zeta potential until a Duboff value defined to specify an edge shape of the resulting glass substrate falls within a range between +10 nm and −10 nm; wherein said mirror polishing liquid is acidic in nature; wherein said glass substrate has a glass skeleton of a network structure and a modifying ion modifying the network structure, wherein a grain size of the colloidal silica particles is not greater than 80 nm; and wherein said mirror polishing step comprises sandwiching said glass substrate between an upper platen and a lower platen through said polishing pad, supplying said mirror polishing liquid containing polishing grains to the surface of said glass substrate, and relatively moving said glass substrate and said upper and lower platens to mirror polish the surface of said glass substrate: the method further comprising a pre-polishing step wherein said glass substrate is beforehand polished on a surface thereof prior to the mirror polishing step, said pre-polishing step including contacting a polishing pad on a surface of said glass substrate, supplying a pre-polishing liquid containing polishing grains to the surface of said glass substrate, and relatively moving said glass substrate and said polishing pad, thereby mirror polishing the surface of said glass substrate; wherein said polishing grains contained in said pre-polishing step are cerium oxide particles different from the colloidal silica particles and having a grain size which is smaller than 4 μm: wherein the mirror polishing liquid contains a buffer agent to keep a pH of the mirror polishing liquid constant; wherein the mirror polishing step is completed when a surface roughness of the glass substrate mirror-polished has an arithmetic mean roughness (Ra) of 0.3 nm or less and a maximum peak (Rp) of 2 nm or less; wherein the Duboff value is determined by measuring a region of an outer periphery of a main surface of the glass substrate within a range between 92.0% and 96.9% from a center of the glass substrate when a distance from the center to an end portion of the glass substrate is taken as 100%, and wherein the mirror polishing liquid keeps a pH value between 1 and 3 during the mirror polishing step.