Manufacturing method for SiO2-TiO2 based glass and manufacturing method for photomask substrate made of SiO2-TiO2 based glass

A method for manufacturing an SiO2-TiO2 based glass having a desired TiO2 concentration upon a target by a direct method, comprising:

• growing an SiO2-TiO2 based glass ingot on the target by flame hydrolysis by feeding a silicon compound and a titanium compound into an oxyhydrogen flame, wherein the growing includes:
• growing an initial portion of the SiO2-TiO2 based glass ingot by feeding the titanium compound and the silicon compound into the oxyhydrogen flame in respective feed rates such that the initial portion has a TiO2 concentration smaller than the desired TiO2 concentration,
• while further feeding the titanium compound and the silicon compound to grow a first further portion of the SiO2-TiO2 based glass ingot on the initial portion, increasing a ratio of the feed rate of the titanium compound to the feed rate of the silicon compound until the first further portion starts growing at the desired TiO2 concentration, and
• then, growing a second further portion of the SiO2-TiO2 based glass ingot while maintaining the second further portion at the desired TiO2 concentration,
• wherein the ratio at a start of feeding the silicon compound and the titanium compound is adjusted such that the concentration of TiO2 in the initial portion of the SiO2-TiO2 based glass is equal to or less than 4 mass %.