Optical element made of fluoride crystal

Abstract

A method for manufacturing fluoride crystal includes the steps of adding scavenger and a material to a crucible, melting the scavenger and material at a temperature higher than a melting point so that a ratio of a thickness of the fluoride crystal that has been melted to an inner diameter of the crucible may be 0.2 or higher, and gradually crystallizing and purifying the material.

1 . A method for manufacturing fluoride crystal comprising the steps of: 
 adding scavenger and a material to a crucible;    melting the scavenger and material at a temperature higher than a melting point so that a ratio of a thickness of the fluoride crystal that has been melted to an inner diameter of the crucible may be 0.2 or higher; and    gradually crystallizing and purifying the material.    
   
   
       2 . A method according to  claim 1 , wherein the inner diameter of the crucible is 250 mm or larger.  
   
   
       3 . A method according to  claim 1 , wherein the crucible is divided into a plurality of parts like a multistage structure.  
   
   
       4 . A method according to  claim 1 , further comprising the steps of: 
 melting the purified fluoride crystal; and    growing the fluoride crystal to obtain fluoride single crystal.    
   
   
       5 . A method according to  claim 1 , wherein the fluoride crystal is calcium fluoride.  
   
   
       6 . An optical element made of fluoride crystal, said fluoride crystal being manufactured by a method including the steps of adding scavenger and a material to a crucible, melting the scavenger and material at a temperature higher than a melting point so that a ratio of a thickness of the fluoride crystal that has been melted to an inner diameter of the crucible may be 0.2 or higher, and gradually crystallizing and purifying the material.  
   
   
       7 . An optical element according to  claim 6 , wherein said optical element is a lens, a diffraction grating, an optical film or a combination thereof.  
   
   
       8 . An exposure apparatus comprising an optical system including an optical element made of fluoride crystal, said fluoride crystal being manufactured by a method including the steps of adding scavenger and a material to a crucible, melting the scavenger and material at a temperature higher than a melting point so that a ratio of a thickness of the fluoride crystal that has been melted to an inner diameter of the crucible may be 0.2 or higher, and gradually crystallizing and purifying the material.  
   
   
       9 . A device fabrication method comprising the steps of: 
 exposing an object using an exposure apparatus; and    performing a predetermined process for the exposed object, wherein said exposure apparatus comprises an optical system including an optical element made of fluoride crystal, said fluoride crystal being manufactured by a method including the steps of adding scavenger and a material to a crucible, melting the scavenger and material at a temperature higher than a melting point so that a ratio of a thickness of the fluoride crystal that has been melted to an inner diameter of the crucible may be 0.2 or higher, and gradually crystallizing and purifying the material.    
   
   
       10 . A device fabrication method according to  claim 9 , wherein said exposing step uses ultraviolet light, far ultraviolet light, and vacuum ultraviolet light.