Patent Publication Number: US-2003221610-A1

Title: Process for growing calcium fluoride single crystals

Description:
RELATED APPLICATION  
       [0001] This application claims the benefit of Russian Patent Application Number 2002115062, filed May 31, 2002, which is incorporated herein by reference in its entirety.  
       FIELD OF THE INVENTION  
       [0002] The present invention relates to the technical field of manufacture of calcium fluoride single crystals by growing from a melt by a method of cooling using a temperature gradient and a seed crystal.  
       BACKGROUND OF THE INVENTION  
       [0003] The industrial manufacturing of optical calcium fluoride [also know as “calcium fluorite”] crystals is based on direct crystallization by the Bridgeman-Stockbarger method, the basis of which is the moving of crucible containing a melt of calcium fluoride through a thermal field with specified gradient in high vacuum. This method allows one to grow the large-sized single crystals of cylindrical shape of a given diameter by the use of the appropriate container or crucible. In this process there are radial and axial thermal gradients involved in growing the cylindrical crystal. These axial and radial gradients create residual stresses that differ both in size and in sign. Residual stresses are the reason of optical homogeneity deterioration, which manifests as areas with anomolous birefringence. As a result, it is necessary to choose a strict temperature-time regime for crystal growth in order to reduce the residual stress, and especially one must give attention to annealing process.  
       [0004] A process for growing of calcium fluoride crystals has been described in the book entitled “Opticheskiy flyuorit” (Optical Fluorite) by N. P. Yushkin et al., published by Nauka in Moscow in 1983 (see pp. 83 and 84). This process includes the prior preparation, in which the apparatus and the crucible are first cleaned with compressed air, and then the crucible is filled with fluorite fragments, and the installation, including the filled crucible, is pumped out to a pressure not less than 1×10 −4  mm Hg. The crucible is then heated to 1500° C. at a rate of 5° C. /min during 5 hours. The material is kept at this holding temperature until it is completely melted and the melt is fully homogeneous. The time required for this depends on the size of the crucible and can be up to 20 hours. The crucible containing the melt is then automatically lowered at speed of 2-20 mm/hour at a constant crystallization temperature of 1450° C. When crystallization has stopped, the temperature in upper zone of the furnace is reduced to a value of between 800 and 1150° C., depending on the size of the crystals. The crucible containing the crystals is then raised again to its initial position in the upper zone and kept there for 5-10 hours. The temperature is then reduced to 250-150° C. at a rate of 3-25° C. /hour. The heating is finally stopped, and the crystals are allowed to cool down naturally over the rest of the temperature range.  
       [0005] The disadvantage of this process is that the crystal preparation is discontinuous, and the crystals have to be heated twice in the upper zone. This can lead to stresses in the resulting single crystals and to the formation of sites in them with different orientation (blocks or “mosaic”).  
       [0006] Two isothermal regions, with a temperature drop between them, are generally used when growing crystals in a downward moving of crucible. This makes it possible to anneal the crystals right after growing them, without subjecting them to the high thermal stresses that occur when there is an extremely great temperature drop. The furnace used in this instance should have two temperature zones, with a minimal heat exchange between them. For this purpose, a thermal insulator and a shielding screen separate these zones, and the temperature in the two zones is regulated independently from each other by a special heaters system.  
       [0007] Such method is described in a book by R. Lodiz and R. Parker, entitled “Rost kristallov” (Growth of Crystals), translated from English into Russian, edited by A. A. Chernov and published by Mir in Moscow in 1974, p. 181. In the process described in this book, the melt is cooled at the rate of at least 7° C./cm, and the crucible moved with a speed of 1-5 mm/hour. The above method is the closest prior art to the invention as to the technical essence. However, the details for the specific conditions of the processes for growing single crystals, and the qualitative characteristics of the grown single crystals are not provided by the above prior art. Careful special selection of modes at all stages of crystal growth and annealing is necessary for obtaining of qualitative or high quality fluoride crystals suitable for the purpose of manufacturing from them optical elements with required optical characteristics.  
       [0008] The purpose of the present invention is directed to the growing of calcium fluoride single crystals which have a high optical homogeneity (Δn=1×10 −6 ) and a low birefringence (δ=0.5-1.0 nm/cm).  
       [0009] We suggest a method of growing calcium fluoride monocrystals which utilizes melting and annealing of crystals and cooling in a vacuum  
       [0010] The invention provides a method of growing calcium fluoride monocrystals which utilizes melting and annealing of crystals and cooling in a vacuum furnace via continued movement of the crucible from the melting zone to the annealing zone while independently regulating the heat of these zones, wherein the difference from the prior art is,  
       [0011] the cooling of the crystals in the temperature an interval 1100-700° C. is carried out at a rate of 1.3-2.0° C./hr,  
       [0012] the constant axial temperature drop with a gradient 20-50° C./m at absence or the minimal radial gradient (which is provided by moving a water-cooled rod down with the speed 0.8-1.4 from speed of moving crucible with crystals from a melt zone into a annealing zone) is kept in the annealing zone,  
       [0013] the water-cooled rod is arranged towards a crucible bottom on the distance equal to 0.3-0.4 from a height of a heater of the annealing zone.  
       [0014] The above technological mode was determined in an experimental way. The crystal cooling in the temperature interval 1100-700° C. at a rate of 1.3-2.0° C./hr was controlled by regulation of the heater&#39;s power in both zones of the growth installation [furnace]. It is experimentally shown that exactly in this temperature interval the slower rate of cooling is necessary so that the induced internal stresses both during growth of a crystal and at its annealing are eliminated. The radial gradient which especially influences the crystal birefringence value is minimized or absolutely removed by the use of the above mode of speed movement of the growing crystal moving from the melt zone into the annealing zone and the appropriate arrangement and movement of a water-cooled rod.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
     [0015]FIG. 1 illustrates the vacuum furnace and associated equipment used in practicing the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0016] The invention is directed to a method of growing calcium fluoride monocrystals which utilizes melting and annealing of crystals and cooling in a vacuum furnace via continued movement of the crucible from the melting zone to the annealing zone while independently regulating the heat of these zones. In particular, the present invention illustrates the following features that are not present in the prior art is:  
     [0017] 1. The cooling of the crystals in the temperature an interval 1100-700° C. is carried out at a rate of 1.3-2.0° C./hr;  
     [0018] 2. The constant axial temperature drop with a gradient 20-50° C./m at absence or the minimal radial gradient (which is provided by moving a water-cooled rod down with the speed 0,8-1,4 from speed of moving crucible with crystals from a melt zone into a annealing zone) is kept in the annealing zone,  
     [0019] 3. The water-cooled rod is arranged towards a crucible bottom on the distance equal to 0.3-0.4 from a height of a heater of the annealing zone.  
     [0020]FIG. 1 is a schematic illustrating the installation used for growing crystals according to the invention. In FIG. 1 furnace body  1  has two zones. The top zone is a melt zone  2  and the bottom zone is an annealing zone  3 , and these zones are divided with a diaphragm [“baffle”]  4 . Each zone has the heaters  5  and  6 , respectively. Inside the installation there is the crucible  7  with raw material  8  (fluorite flake) in it. The crucible is arranged on a plate  9  with a first rod  10  attached thereto. A second water-cooled rod  11  is located inside the first rod  10 . Element  12  is the screen of the furnace. Diaphragm  4  in zone of partition between the two basic technological zones  2  and  3  creates a “crystallization zone” in this area.  
     [0021] Actual realization of the method was carried out as follows. A fluoride charge, in form of small pieces of fluoride crystals, is filled into cleaned graphite crucible consisting of 4 bowls. A crucible  7  is put in a growth installation on a rod  10 , connected with programmed [controlled] management. The top zone  2  and the annealing zone  3  are provided with separate regulation of the temperature modes by means of the use of heaters  5  and  6 . The installation is pumped out to pressure not less than 1×10 −6  mm Hg, and then the temperature raised to 1500° C. with the help of heater power  5  regulation in the top zone  2 , and the temperature of the annealing zone  3  is raised to 1250° C. by the regulation of heater power  6 . In the beginning of process the crucible is located in the top part of installation and is held at the maximum temperature of 1500° C. for 30 hours until the melt is homogeneous and is free of inclusions and bubbles. Then, the crucible  7  with the melt  8  is begun to descend with at a speed of 0.7-2 mm/hr. When the crucible has passed the diaphragm  4  level where the crystallization of substance begins, its movement into the annealing zone  3  is done at a speed 2-5 mm/hr. Thus, the temperature of the top and bottom heaters  5  and  6  change in such a manner that cooling of the substance in the temperature range 1500-1250° C. is carried out at a rate 7-5° C./hr, in the temperature range 1250-1100° C. at a rate of 5-2° C./hr, in the temperature range 1100-700° C. at a rate of 1.3-2° C./hr, in the temperature range 700-400° C. at a rate of 3.5-7.0° C./hr, and in the temperature range 400-100° C. at a rate of 10-15° C./hr.  
     [0022] Two concrete examples of temperature change of the top heater  5  and bottom heater  6  for maintenance of the above-described mode of crystal cooling are shown in the tables:  
                                  Example 1   Example 2                             Top heater   Bottom heater   Top heater   Bottom heater                                             T, ° C.   ΔT/Δt   T, ° C.   ΔT/Δt   T, ° C.   ΔT/Δt   T, ° C.   ΔT/Δt                                                     1500   7.0   1250   3.95   1500   13.3   1250   4.99       1234   2.16   1100   1.61   1300   13.3   1175   499       1100   1.55   1000   1.30   1100   1.5   1100   1.5       960   1.55   883   1.30   940   1.5   940   1.5       820   1.50   766   1.30   800   1.5   800   L30       790   3.00   740   2.00   740   2.0   740   2.00       730   4.25   700   3.75   700   3.75   700   3.75       560   5.83   550   6.25   550   6.25   550   6.25       420   10.97   400   10.42   400   10.42   400   10.42       25       25       25       25                  
 
     [0023] In this example the water-cooled rod  11  is put a distance of 240 mm from the bottom of crucible  7 , that makes 0.3 the height H of a heater  6  of the annealing zone  3  which in this case is equal 800 mm (see drawings). The water-cooled rod  11  will move with the same speed as the movement of the crucible  7 . Taking into account the independent moving of water-cooled rod, irrespective of the crucible, the deflection of its speed of moving should not exceed 0.8-1.4 of the crucible&#39;s speed.  
     [0024] The minimal axial gradient in the temperature interval 1100-700° C. is maintained at level 20-50° C./m by means of the above-stated conditions provide.  
     [0025] Four calcium fluoride crystals with a diameter of 300 mm and a height of 80 mm have been grown for one working cycle. The crystals obtained using the above described method have a high optical homogeneity (Δn=1.10-6) and a small birefringence (δ=0.5-1.0 nm/cm). The foregoing examples of specific compositions, processes, articles and/or apparatus employed in the practice of the present invention are, of course, intended to be illustrative rather than limiting, and it will be apparent the numerous variations and modification of these specific embodiments may be practiced within the scope of the specification, drawing and appended claims.