Method of making large-volume CaF2 single crystals for optical elements with an optic axis parallel to the (100)- or (110)-crystal axis and CaF2 single crystal made thereby

Abstract

The method for making single crystals, especially CaF 2 crystals, includes tempering, in which the crystal is heated at <18 K/h to a temperature of 1000° C. to 1350° C. and held at this temperature for at least 65 hours with maximum temperature differences within the crystal of <0.2 K. Subsequently the crystal is cooled with a cooling rate of at maximum 0.5 K/h above a limiting temperature between 900° C. to 600° C. and then further below this limiting temperature at maximum 3 K/h. The obtained CaF 2 crystals have refractive index uniformity <0.025×10 −6 (RMS) in a (111)-, (100)- and/or (110)-direction and a stress birefringence of less than 2.5 nm/cm (PV) and/or a stress birefringence of less than 1 nm/cm (RMS) in the (100)- or (110)-direction. In the (111)-direction the stress birefringence is <0.5 nm/cm (PV) and/or the stress birefringence is <0.15 nm/cm (RMS).

1 . A method of making a highly uniform, low-stress single crystal, said method including tempering the crystal and said method comprising the steps of: 
 a) heating the crystal at a temperature increase rate of less than 18 K/h to a temperature between 1000° C. and 1350° C. to temper the crystal;    b) holding the crystal at said temperature reached during the heating of step a) for at least 65 hours, so that temperature differences up to a maximum value of 0.2 K are present within the crystal during the holding;    c) subsequently cooling the crystal in a temperature range above a limiting temperature between 900° C. and 600° C. with a cooling rate that is at maximum equal to 0.5 K/h; and then    d) cooling to temperatures below said temperature range with a cooling rate that is at maximum equal to 3 K/h.    
   
   
       2 . The method as defined in  claim 1 , wherein said temperature increase rate during the heating is less than 10 K/h.  
   
   
       3 . The method as defined in  claim 1 , wherein the tempering of the crystal occurs in an apparatus, in which a reducing atmosphere is present.  
   
   
       4 . The method as defined in  claim 1 , wherein the tempering of the crystal occurs in the presence of at least one solid material selected from the group consisting of PbF 2 , ZnF 2  and XeF 2 .  
   
   
       5 . The method as defined in  claim 1 , wherein temperature differences up to a maximum value of 0.3 K are present within the crystal during the cooling of the crystal occurring above said limiting temperature between 900° C. and 600° C.  
   
   
       6 . The method as defined in  claim 1 , wherein the crystal tempered in the tempering is in the form of a cylindrical body, independent from geometries of products formed later from the crystal.  
   
   
       7 . The method as defined in  claim 1 , wherein the tempering occurs in an apparatus, and further comprising producing a vacuum of at least 10 −4  mbar in the apparatus prior to the tempering in order to remove residual moisture and providing a pressure of 10 to 1050 mbar in the apparatus during the tempering.  
   
   
       8 . The method as defined in  claim 1 , wherein the crystal is a calcium fluoride crystal.  
   
   
       9 . The method as defined in  claim 1 , wherein the crystal has an optical quality that is sufficient for optical applications in directions different from a (111)-direction and said directions different from said (111)-direction include a (100)-direction and a (110)-direction.  
   
   
       10 . A homogeneous calcium fluoride crystal with improved transmission obtained by a method as defined in  claim 1  and having an RMS average value of refractive index uniformity (Δn)<0.025×10 −6  in a (111)-direction, a (100)-direction and/or a (110) direction 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 the (100)-direction and/or the (110)-direction and having a PV value of the stress birefringence of less than 0.5 nm/cm and/or an RMS average value of the stress birefringence of less than 0.15 nm/cm in the (111)-direction.  
   
   
       11 . The homogenous calcium fluoride crystal as defined in  claim 10 , wherein said RMS average value of said refractive index uniformity (Δn) is <0.015×10 −6  in said (111)-direction, said (100)-direction and/or said (110) direction; said PV value of the stress birefringence is less than 0.2 nm/cm and/or said RMS average value of the stress birefringence is less than 0.08 nm/cm in the (111)-direction and/or 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 and/or the (110)-direction.  
   
   
       12 . A stepper, excimer laser, wafer, computer chip or integrated circuit containing a crystal as defined in  claim 10  acting as an optical element.  
   
   
       13 . 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 as defined in  claim 10 .  
   
   
       14 . A stepper, excimer laser, wafer, computer chip or integrated circuit containing a crystal obtained by the method as defined in  claim 1 , which acts as an optical element with an optical axis or principle propagation direction that is not in a (111)-direction, wherein said crystal is a homogeneous calcium fluoride crystal having an RMS average value of refractive index uniformity (Δn)<0.025×10 −6  in the (111)-direction, a (100)-direction and/or a (110)-direction 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 the 100-direction and/or the (110)-direction and having a PV value of the stress birefringence of less than 0.5 nm/cm and/or an RMS average value of the stress birefringence of less than 0.15 nm/cm in the (111)-direction.  
   
   
       15 . 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 obtained by the method as defined in  claim 1 , which acts as an optical element with an optical axis or principle propagation direction that is not in a (111)-direction, and wherein said crystal is a homogeneous calcium fluoride crystal having an RMS average value of refractive index uniformity (Δn) <0.025×10 −6  in the (111)-direction, a (100)-direction and/or a (110)-direction 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 and/or (110)-direction and having a PV value of the stress birefringence of less than 0.5 nm/cm and/or an RMS average value of the stress birefringence of less than 0.15 nm/cm in the (111)-direction.