CaF2 lenses with reduced birefringence

Abstract

The invention is directed to the trimming and grinding of disks to make lens blanks and/or lenses for lithographic stepper camera optical systems, and in particular to disks made of calcium fluoride (CaF 2 ), metal fluorides of general formula MF 2 where M is calcium, magnesium, barium and strontium, and other materials suitable for use in optical systems

1 . A method of preparing an optical lens suitable for use in optical lithographic systems utilizing a selected wavelength of electromagnetic radiation, said method comprising the steps of: 
 providing a disk of an optical crystal material having a selected crystallographic orientation and suitable for use at a selected wavelength of electromagnetic radiation, said disk having a top face and a bottom face and a thickness,    determining the crystal orientation of the optical crystal material at both the top and the bottom face of the disk and comparing said measured values to the selected crystal orientation,    trimming optical crystal material from one or both of said faces so as to maximize the amount of the optical material with minimum tilt relative to the selected crystal orientation.    
   
   
       2 . The method according to  claim 1 , wherein one of said faces is flat and one of said faces is convex.  
   
   
       3 . The method according to  claim 2 , therein the optical material of the flat face is trimmed so that the axial centerline of the disk is closely aligned with the measured orientation of the crystal.  
   
   
       4 . The method according to  claim 2 , wherein optical material is removed from the convex face along the side of the convex face whose orientation, in the direction from the center of the disk to the perimeter of the disk, tilts furthest from the selected crystal orientation.  
   
   
       5 . The method according to  claim 1 , wherein said disk is a single crystal of a metal fluoride material of formula MF 2 , and M is selected from the group consisting of calcium, barium and strontium, and mixtures of two or more of the foregoing in any proportion.  
   
   
       6 . A method of preparing an optical element suitable for use in optical lithographic systems utilizing a selected wavelength of electromagnetic radiation, said method comprising the steps of: 
 growing a metal fluoride single crystal according to any method known in the art and recording the temperatures during growth;    determining the temperature profile within a crystal as it is being grown;    determining the crystallographic orientation of the grown crystal;    using the temperature profile to determine the thermal stresses within the crystal;    using the thermal stress data to build a birefringence model;    using the birefringence model to determine the parts of the crystal that should be trimmed away to form an optical element suitable for use in an optical lithography system; and    trimming the crystal to form said optical element.    
   
   
       7 . The method according to  claim 6 , wherein the metal fluoride has the formula MF2, an M is selected from the group consisting of calcium, barium and strontium, and mixtures of two or more of the foregoing in any proportion.  
   
   
       8 . The crystal according to  claim 6 , wherein the grown crystal undergoing trimming to form an optical element is in the form of a disk formed in the growth process or a disk cut from a single crystal ingot.  
   
   
       9 . A method of preparing an optical element suitable for use in optical lithographic systems utilizing a selected wavelength of electromagnetic radiation, said method comprising the steps of: 
 selecting a disk of a single crystal material made by any method known in the art;    annealing the disk by any method known in the art;    recording the temperatures during annealing;    determining the temperature profile within a crystal as it is being annealed;    determining the crystallographic orientation of the annealed crystal;    using the temperature profile to determine the thermal stresses within the crystal;    using the thermal stress data to build a birefringence model;    using the birefringence model to determine the parts of the crystal that should be trimmed away to form an optical element suitable for use in an optical lithography system; and    trimming the crystal to form said optical element.    
   
   
       10 . The method according to  claim 9 , wherein the metal fluoride has the formula MF 2 , an M is selected from the group consisting of calcium, barium and strontium, and mixtures of two or more of the foregoing in any proportion.  
   
   
       11 . The crystal according to  claim 9 , wherein the grown crystal undergoing trimming to form an optical element is in the form of a disk formed in the growth process or a disk cut from a single crystal ingot.