Polishing agent for synthetic quartz glass substrate, method for manufacturing the polishing agent, and method for polishing synthetic quartz glass substrate

A method for manufacturing a polishing agent for a synthetic quartz glass substrate, wherein

• the polishing agent comprises water and polishing particles comprising silica particles as base particles,
• composite oxide particles of cerium and at least one rare earth element selected from trivalent rare earth elements other than cerium are supported on surfaces of the base particles, and
• the method comprises:
• a step of producing the polishing particles, the step comprising
• a substep of preparing a solution A in which the silica particles are dispersed in a dispersion medium,
• a substep of preparing a basic solution as a solution B,
• a substep of preparing a solution C in which a cerium salt and a salt of at least one rare earth element selected from trivalent rare earth elements other than cerium are dissolved as precursors of the composite oxide particles,
• a substep of mixing the solution A, the solution B, and the solution C to precipitate the composite oxide particles from the precursors of the composite oxide particles, so that the precipitated composite oxide particles are supported on the silica particles, and
• a substep of subjecting a solution containing the silica particles on which the composite oxide particles are supported to a heat treatment for 1 hour or more at a solution temperature of 60° C. or more and 100° C. or less;
• and
• a step of manufacturing a polishing agent for a synthetic quartz glass substrate, the polishing agent comprising the produced polishing particles and water.