High-purity large-volume monocrystals that are especially radiation-resistant and method of making them from crystal

Abstract

The high-purity alkaline earth halide crystals, especially CaF 2 , BaF 2 or MgF 2 crystals, have a diffuse scatter distribution function value of less than 7×10 −7 , an RMS uniformity of refractive index of less than 15×10 −8 after subtraction of Zernike coefficients and an RMS value of birefringence in the (111) direction of less than 0.2 nm/cm. Preferably the crystals exhibit a loss coefficient of less than 5×10 −4 cm −1 after irradiation with 10×10 9 laser pulses with an energy density of 10 mJ/cm 2 at a wavelength of 193 nm. Also they have RMS birefringence in the (100) direction or the (111) direction that is less than 0.35 nm/cm.

1 . A crystal for optical applications, said crystal having a diffuse scatter distribution function value of less than 7×10 −7 , an RMS uniformity of refractive index of less than 15×10 −8  and an RMS value of birefringence in a (111) direction of less than 0.2 nm/cm. 
     
     
         2 . The crystal as defined in  claim 1 , having an RMS birefringence in a (100) direction or a (111) direction of less than 0.35 nm/cm. 
     
     
         3 . The crystal as defined in  claim 1 , exhibiting a loss coefficient of less than 5×10 −4  cm −1  after irradiation with 10×10 9  laser pulses each with an energy density of 10 mJ/cm 2  at a wavelength of 193 nm. 
     
     
         4 . The crystal as defined in  claim 1 , wherein said crystal comprises at least one alkaline earth halide. 
     
     
         5 . The crystal as defined in  claim 4 , wherein said at least one alkaline earth halide is selected from the group consisting of calcium fluoride, barium fluoride and magnesium fluoride. 
     
     
         6 . The crystal as defined in  claim 1 , which is produced by a method comprising forming a melt of crystalline raw material, cooling the melt in a controlled manner to solidify the melt and thus form a crystalline solid and then tempering the crystalline solid to form the crystal;
 wherein the crystalline raw material comprises shards and/or waste of already-grown crystals, which upon visual observation in daylight have no color and upon illumination with a white-light lamp in a darkroom have no or at maximum a just barely perceivable reddish and/or bluish fluorescence, have no or at maximum a just barely perceivable diffuse scattering and have no or only slight discrete scattering of at maximum two visually perceivable scattering centers per dm 3 .   
     
     
         7 . The crystal as defined by  claim 6 , wherein the shards and/or the waste have a decadic loss coefficient, determined from a transmission measurement with an excimer laser at 193 nm, at an energy density of zero, of ≦0.0005/cm, or a decadic loss coefficient, determined from a transmission measurement with a spectrometer at 193 nm, of ≦0.0005/cm. 
     
     
         8 . The crystal as defined by  claim 7 , wherein the shards are taken from already-grown crystals having small-angle grain boundaries with a maximum tilting of crystal axes from one another of 2°. 
     
     
         9 . The crystal as defined by  claim 6 , wherein the shards and/or the waste, before the melt is formed, are treated with a cleaning agent. 
     
     
         10 . The crystal as defined by  claim 9 , wherein the cleaning agent is CFHC, an alcohol, and/or water. 
     
     
         11 . The crystal as defined by  claim 9 , wherein the cleaning agent contains at least one organic solvent and the at least one organic solvent evaporates without leaving a residue. 
     
     
         12 . A lens, prism, fiber optic rod, optical window, optical component for DUV lithography, stepper, excimer laser, computer chip or integrated circuit containing a crystal as defined in  claim 1 , or an electric device containing said computer chip or said integrated circuit.