Method for making low-stress large-volume not-(111)-oriented crystals with reduced stress birefringence and more uniform refractive index and crystals made thereby

Abstract

A method is described for making an especially not-(111)-oriented low-stress large-volume crystal having a glide plane with reduced stress birefringence and more uniform refractive index. The method includes growing and tempering the crystal while heating and/or cooling to form a temperature gradient in order to relax stresses arising along the glide plane. During the tempering the heating and/or cooling occurs by heat transfer in a heat transfer direction and the heat transfer direction or temperature gradient is oriented at an angle of from 50° to 900° to the glide plane. Crystals with a uniform refractive index with variations of less than 0.025×10 −6 (RMS value) are produced by the method.

1 . A method of making an especially not-(111)-oriented low-stress large-volume crystal having a glide plane with reduced stress birefringence and more uniform refractive index, said method comprising growing and tempering the crystal while heating and/or cooling while forming a temperature gradient in order to relax stresses arising along the glide plane; 
 wherein during the tempering said heating and/or said cooling occurs by heat transfer in a heat transfer direction, and    wherein said heat transfer direction or the temperature gradient is oriented at an angle of greater than 5° to the glide plane.    
     
     
         2 . The method as defined in  claim 1 , wherein the angle between the glide plane and the temperature gradient is between 250° to 65°.  
     
     
         3 . The method as defined in  claim 1 , wherein the angle between the glide plane and the temperature gradient is between 35° to 55°.  
     
     
         4 . The method as defined in  claim 1 , wherein the crystal is a calcium fluoride crystal and during the tempering of the calcium fluoride crystal said angle between said glide plane and said temperature gradient is greater than 25°.  
     
     
         5 . The method as defined in  claim 1 , wherein the tempering of the crystal takes place in a tempering oven and during the tempering the crystal is oriented in the tempering oven so that a (111)-direction of the crystal is parallel to a direction in which gravity acts.  
     
     
         6 . The method as defined in  claim 1 , wherein the heat transfer occurs laterally.  
     
     
         7 . The method as defined in  claim 1 , wherein the crystal is a calcium fluoride crystal, the tempering of the crystal takes place in a tempering oven and during the tempering of the calcium fluoride crystal the crystal is oriented so that a (111)-direction of the crystal is parallel to an axis of the temperature field in the tempering oven.  
     
     
         8 . The method as defined in  claim 1 , wherein the crystal is a calcium fluoride crystal and the tempering takes place so as to form a blank in a predetermined 100- or 110-orientation.  
     
     
         9 . The method as defined in  claim 1 , wherein the tempering is performed in a tempering chamber with a temperature field, which has a statistical radial temperature gradient of less than 0.013 K/cm and/or an axial statistical temperature gradient of less than 0.07 K/cm.  
     
     
         10 . The method as defined in  claim 1 , wherein no further tempering occurs after the tempering and cooling of outer edge regions of the crystal to a predetermined extent.  
     
     
         11 . A calcium fluoride crystal obtained by a method as defined in  claim 1 , in which the crystal is made of calcium fluoride, and having an RMS average value of refractive index uniformity (Δn) of less than 0.025×10 −6 .  
     
     
         12 . A homogeneous calcium fluoride crystal obtained by a method as defined in  claim 1 , in which the crystal is made of calcium fluoride, and having a PV value of the stress birefringence of less than 2.5 nm/cm and/or an RMS average value of the stress birefringence of less than 1 nm/cm in a 100-direction.  
     
     
         13 . The homogeneous calcium fluoride crystal as defined in  claim 12 , wherein said PV value of the stress birefringence is less than 1 nm/cm and/or said RMS average value of the stress birefringence is less than 0.35 nm/cm in the 100-direction.  
     
     
         14 . The homogeneous calcium fluoride crystal as defined in  claim 12 , having an RMS average value of refractive index uniformity (Δn)<0.025×10 −6 .  
     
     
         15 . A stepper, excimer laser, wafer, computer chip or integrated circuit containing a crystal acting as an optical element and made by a method as defined in  claim 1 , and wherein said crystal has refractive index uniformity (Δn) of <1*10 −6 .  
     
     
         16 . The stepper, excimer laser, wafer, computer chip or integrated circuit as defined in  claim 15 , wherein said crystal has a PV value of the stress birefringence of less than 2.5 nm/cm and/or an RMS average value of the stress birefringence of less than 1 nm/cm in a 100-direction.  
     
     
         17 . An electronic unit or device containing a computer chip and/or an integrated circuit, wherein said computer chip and said integrated circuit each contain a crystal made by a method as defined in  claim 1  and acting as an optical element, said crystal has refractive index uniformity (Δn)<1*10 −6 .  
     
     
         18 . The electronic unit or device as defined in  claim 17 , wherein said crystal has a PV value of the stress birefringence of less than 2.5 nm/cm and/or an RMS average value of the stress birefringence of less than 1 nm/cm in a 100-direction.