Method for producing fluorine-containing silica glass
A method for producing fluorine-containing silica glass is disclosed. The method includes decompression degassing which includes degassing an inside of a furnace core tube under reduced pressure while heating the inside of the furnace core tube, after inserting a porous silica glass body into the furnace core tube provided in an airtight container; fluorine adding which includes supplying a fluorine compound gas into the furnace core tube, exhausting the fluorine compound gas remaining in the furnace core tube to an outside of the furnace core tube, and first heat-treating the porous silica glass body, under reduced pressure; and second heat-treating the porous silica glass body under reduced pressure at a temperature higher than temperatures in the decompression degassing process and the fluorine adding process.
1 . A method for producing fluorine-containing silica glass, comprising:
decompression degassing which includes degassing an inside of a furnace core tube under reduced pressure while heating the inside of the furnace core tube, after inserting a porous silica glass body into the furnace core tube provided in an airtight container;
fluorine adding which includes supplying a fluorine compound gas into the furnace core tube, exhausting the fluorine compound gas remaining in the furnace core tube to an outside of the furnace core tube, and first heat-treating the porous silica glass body, under reduced pressure; and
second heat-treating the porous silica glass body under reduced pressure at a temperature higher than temperatures in the decompression degassing process and the fluorine adding process, wherein
the decompression degassing, the fluorine adding, and the second heat-treating are performed within the same furnace core tube,
in the fluorine adding, the first heat-treating is performed after stopping exhaustion of the furnace core tube,
the decompression degassing process is performed at a temperature of 1100° C. or more and 1200° C. or less,
the decompression degassing process has a heating time of at least 30 minutes at the temperature, and
an ultimate pressure at an end of the decompression degassing process is less than 500 pascals.
2 . The production method according to claim 1 , wherein
in the decompression degassing process, an inert gas is supplied into the furnace core tube.
3 . The production method according to claim 1 , wherein
the fluorine compound gas used in the fluorine adding process is a compound gas of a group 14 element and fluorine, and does not contain a chlorine atom or a hydrogen atom.
4 . The production method according to claim 1 , wherein
in the fluorine adding process, the fluorine compound gas is diluted with an inert gas at a concentration of 1% or more and less than 100% and supplied, or the fluorine compound gas is supplied at a concentration of 100%.
5 . The production method according to claim 1 , wherein
in the second heat-treating process, an evacuation treatment is performed in which pressure is reduced while supplying an inert gas into the furnace core tube.
6 . The production method according to claim 1 , wherein
an ultimate pressure is less than 500 pascals at an end of the second heat-treating process.