Optical member made of silica glass, method for manufacturing silica glass, and reduction projection exposure apparatus using the optical member

Abstract

An optical member made of silica glass manufactured by the direct method where a material gas comprising an organosilicon compound is allowed to react in an oxidizing flame, said optical member having a 2×10 14 molecules/cm 3 or less concentration of formyl radical generated by X-ray irradiation whose dose is 0.01 Mrad or more and 1 Mrad or less.

1 . A method for manufacutring silica glass comprising: 
 a first step of allowing an organosilicon compound to react in an oxidizing flame while ejecting an organosilicon compound and an inactive gas from a tube placed in the center of a burner with multi-tubular structure and ejecting an oxygen gas and a hydrogen gas from a tube placed around the tube in the center of said burner so that a ratio (a/b) of the total oxygen gas amount (a) to the total hydrogen amount (b) is 0.53 or more, so as to obtain silica glass fine particles; and    a second step of depositing and melting said silica glass fine particles on a heat resistant target opposed to said burner, so as to obtain a silica glass ingot.    
   
   
       2 . A method according to  claim 1 , wherein a ratio ([a-c]/b) of an oxygen gas amount (a-c) obtained by subtracting an oxygen gas amount (c) consumed by combustion of the organosilicon compound emitted from said burner from the total oxygen gas amount (a) to the total hydrogen gas amount (b) is 0.48 or more.  
   
   
       3 . A method according to  claim 1 , wherein said organosilicon compound is at least one kind selected from the group consisting of alkoxysilanes and siloxanes.  
   
   
       4 . A method according to  claim 1 , wherein said organosilicon compound is at least one kind selected from the group consisting of tetraethoxysilane, tetramethoxysilane, methyltrimethoxysilane, hexamethyldisiloxane, octamethylcyclotetrasiloxane, and tetramethylcyclotetrasiloxane.  
   
   
       5 . A method according to  claim 1 , wherein said burner comprises: 
 a first tube placed in a center for ejecting material gas of an organosilicon compound and an inactive gas;    a second tube placed in a co-centric circle around said first tube for ejecting a first hydrogen gas;    a third tube placed in a co-centric circle around said second tube for ejecting a first oxygen gas;    a fourth tube placed in a co-centric circle around said third tube for ejecting a second hydrogen gas;    a fifth set of a plurality of tubes placed between an outer periphery of said third tube and an inner periphery of said fourth tube for ejecting a second oxygen gas;    a sixth tube placed in a co-centric circle said fourth tube for ejecting a third hydrogen gas; and    a seventh set of a plurality of tubes placed between an outer periphery of said fourth tube and an inner periphery of said sixth tube for ejecting a third oxygen gas.    
   
   
       6 . A method according to  claim 5 , wherein a ration (e/d) of the first oxygen gas amount (e) to the first hydrogen gas amount (d) is 0.50 or less and a ratio (g/f) of the second oxygen gas amount (g) to the second hydrogen gas amount (f) is 0.55 or more in said first step.  
   
   
       7 . A method according to  claim 5 , wherein a ratio (e/d) of the first oxygen gas amount (e) to the first hydrogen gas amount (d) and a ratio (g/f) of the second oxygen gas amount (g) to the second oxygen gas amount (f) indicate excessive oxygen level relative to a theoretical combustion ratio and a ratio (i/h) of the third oxygen gas amount (i) to the third hydrogen gas amount (i) indicates excessive hydrogen level relative to a theoretical combustion ratio in said first step, and 
 the method further comprises a third step of heat treating said silica glass ingot under an atmosphere containing hydrogen.    
   
   
       8 . A method according to  claim 7 , wherein a ratio (e/d) of the first oxygen gas amount (e) to the first hydrogen gas amount (d) is 0.7 or more and 2.0 or less, a ratio (g/f) of the second oxygen gas amount (g) to the second hydrogen gas amount (f) is 0.5 or more and 1.0 or less, and a ratio (i/h) of the third oxygen gas amount (i) to the third hydrogen gas amount (h) is 0.2 or more and 0.5 or less in said first step.  
   
   
       9 . A method according to  claim 7 , wherein a hydrogen molecule concentration in the atmosphere where said silica glass ingot is heat treated is 5 wt % or more and 100 wt % or less.  
   
   
       10 . A method according to  claim 7 , wherein a temperature of the atmosphere where said silica glass ingot is heat treated is 500° C. or less.  
   
   
       11 . A method according to  claim 7 , further comprising a fourth step for heat treating said silica glass ingot at the temperature of 700° C. or more under an atmosphere with an oxygen partial pressure of 0.1 atm or more either between said second step and said third step or after said third step.  
   
   
       12 . A method according to  claim 8 , wherein a flow rate of said first hydrogen gas is 60 m/sec or less.