Abstract:
A method of making below 250-nm UV light transmitting optical fluoride lithography crystals includes applying heat along a shortest path of conduction of a selected optical fluoride crystal, heating the optical fluoride crystal to an annealing temperature, holding the temperature of the optical fluoride crystal at the annealing temperature, and gradually cooling the optical fluoride crystal to provide a low-birefringence optical fluoride crystal for transmitting below 250-nm UV light.

Description:
PRIORITY 
   This application is a division of and claims the priority of U.S. application Ser. No. 10/611,505, filed Jun. 30, 2003, now U.S. Pat. No. 6,997,987 and titled “OPTICAL LITHOGRAPHY FLUORIDE CRYSTAL ANNEALING FURNACE”, which in turn application claims the priority of U.S. Provisional application No. 60/396,779, filed Jul. 17, 2002, titled “Optical Lithography Fluoride Crystal Annealing Furnace”. 

   BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The invention relates to methods and apparatus for producing optical crystals. In particular, the invention relates to a method and an apparatus for annealing optical crystals, particularly optical lithography fluoride crystals for transmitting below 250-nm UV light. 
   2. Background Art 
   Optical crystals are commonly grown using the Stockbarger-Bridgman method. In the Stockbarger-Bridgman method, the optical crystals are grown in a vertical furnace by moving molten crystal material through a temperature gradient zone in the furnace. The method is further explained below with reference to  FIGS. 1A and 1B . 
     FIG. 1A  shows a vertical furnace  1  having an upper zone  2  and a lower zone  3 . Heating jackets  4 ,  5  are provided in the upper and lower zones  2 ,  3 , respectively. The heating jackets  4 ,  5  are operated such that a temperature gradient zone  6  is created between the upper and lower zones  2 ,  3 . At the start of the growth process, a crucible  7  containing a crystal raw material F is mounted in the upper zone  2 . The crystal raw material F is melted by heat from the heating jacket  4 . After melting the crystal raw material F, the crucible  7  is lowered into the lower zone  3 , as shown in  FIG. 1B . As the crucible  7  passes from the upper zone  2  into the lower zone  3 , the molten material M goes through the temperature gradient zone  6 . On passing through the temperature gradient zone  6 , the temperature transition inside the molten material M creates a crystallization front CF. The crystallization front CF propagates inside the crucible  7 , within the molten material M, as long as the crucible  7  continues to move downwardly. 
   Crystals grown using the method described above are exposed to sharp localized cooling as they are translated through the temperature gradient zone into the lower zone. Sharp localized cooling induces permanent thermal strain (or stress) in the crystals, which can result in unacceptably elevated values in birefringence of the crystals. To reduce the permanent thermal strain in the crystal, the crystal is annealed in the lower zone of the growth furnace. The annealing cycle includes re-heating the crystal to a temperature below the melting temperature of the crystal, holding the crystal at this temperature until the thermal strain induced in the crystal by the sharp localized cooling is dissipated, and then slowly cooling the crystal to a temperature below which any strain due to additional cooling to room temperature will result only in temporary strain in the crystal. 
   The duration of the annealing cycle depends on the volume of the crystal. As the volume of the crystal increases, the ability to completely anneal the crystal inside the growth furnace such that the birefringence of the crystal meets the specification reduces. For instance, exposure systems in microlithography processes require optical crystals, mainly fluoride crystals, with birefringence values of 3 nm/cm or lower. To meet such stringent specifications for large-volume crystals, the growth furnace would have to be tied up for extended times, which would have a great impact on the ability to meet market demands. Therefore, the current practice is to anneal the crystal for a relatively short time in the growth furnace. The birefringence of the crystal is then measured. If the crystal has an unacceptably high birefringence value, the crystal is further annealed in a separate furnace from the growth furnace. This process is typically referred to as post-annealing. 
   A typical annealing furnace is a vertical furnace in which a vertical stack of individual hermetically-sealed containers can be supported during post-annealing. The furnace includes heaters for creating a desired temperature profile inside the furnace. In operation, the crystals to be annealed are loaded into the sealed containers, and the sealed containers are loaded into the annealing furnace. A vacuum, inert, or fluorinating atmosphere may be provided inside the sealed containers. The annealing process starts by heating the crystals to a temperature below the melting point of the crystals. The crystals are held at this temperature for a predetermined length of time before being slowly cooled to room temperature. Typically, the heaters used in the process are circumferential heaters, which are arranged in the furnace so as to circumscribe the individual containers. In addition, heaters or thermal insulators can be placed at the top and bottom of the stack of containers. 
   The annealing cycle can be relatively short if the crystals in the stack have small diameters, e.g., less than 150 mm. This is because the path of conduction from the circumference of the crystals, where the heat is applied, to the center of the crystals is relatively short. Thus, the heating rates from room temperature to annealing temperature and the cooling rates from annealing temperature to room temperature can be relatively high. However, as the diameters of the crystals increase, the path of conduction from the circumference of the crystals to the center of the crystals increases. As a result, the time required to complete the annealing process such that a desired birefringence level in the crystal is achieved also increases. Currently, there are demands for optical fluoride crystals with diameters of 300 mm or greater. Therefore, a process of annealing multiple large-diameter (crystal blank disk diameter&gt;150 mm, preferably ≧250 mm, more preferably ≧300 mm) crystals within a reasonable time frame is desirable. 
   SUMMARY OF INVENTION 
   In one aspect, the invention relates to a method of making below 250-nm UV light transmitting optical fluoride lithography crystals which comprises (a) applying heat along a shortest path of conduction of a selected optical fluoride disk crystal, (b) heating the optical fluoride crystal to an annealing temperature, (c) holding the temperature of the optical fluoride crystal at the annealing temperature, and (d) gradually cooling the optical fluoride crystal to provide a low-birefringence optical fluoride crystal for transmitting below 250-nm UV light. 
   In another aspect, the invention relates to a method of making below 250-nm UV light transmitting optical fluoride lithography crystals which comprises (a) arranging a plurality of selected optical fluoride disk crystal in a single layer in a furnace, (b) applying heat along a shortest path of conduction of the selected optical fluoride crystals, (c) heating the optical fluoride crystals to an annealing temperature, (d) holding the temperature of the optical fluoride crystals at the annealing temperature, and (e) gradually cooling the optical fluoride crystals to provide low-birefringence optical fluoride crystals for transmitting below 250-nm UV light. 
   In another aspect, the invention relates to a method of making below 250-nm UV light transmitting optical fluoride lithography crystals which comprises (a) providing optical fluoride disk crystals having birefringence values above 3 nm/cm, (b) applying heat along a shortest path of conduction of the optical fluoride disk crystals, (c) heating the optical fluoride crystals to an annealing temperature, (d) holding the temperature of the optical crystals at the annealing temperature, and (e) gradually cooling the optical fluoride crystals to provide optical fluoride crystals having birefringence value not higher than 3 nm/cm. 
   In another aspect, the invention relates to an apparatus for making low birefringence optical fluoride crystals which comprises a furnace, a chamber supported inside the furnace for containing at least one optical fluoride disk crystal, and at least one heater disposed external to the chamber, the heater being arranged to apply heat along a shortest path of conduction of the optical fluoride disk crystal. 
   In another aspect, the invention relates to an apparatus for annealing optical crystals which comprises a furnace, a chamber supported inside the furnace for containing at least an optical crystal, and at least a pair of heaters disposed external to the chamber, the heaters being arranged to provide heat along the shortest path of conduction of the optical crystal. 
   In another aspect, the invention relates to an apparatus for annealing optical crystals which comprises a furnace, a plurality of chambers supported inside the furnace for containing a plurality of optical crystals, and at least a pair of heaters disposed external to each chamber, the heaters being arranged to provide heat along the shortest path of conduction of the optical crystals. 
   In another aspect, the invention relates to an apparatus for annealing an optical crystal which comprises a chamber having a surface for supporting an optical crystal, at least one heater disposed external to the chamber, the heater being arranged to apply heat along a shortest path of conduction of the optical crystal, and means for enhancing exchange of radiation energy between the heater and the optical crystal. 
   Other features and advantages of the invention will be apparent from the following description and the appended claims. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIGS. 1A and 1B  illustrate a prior-art process for growing an optical crystal. 
       FIG. 2A  shows a vertical cross-section of an annealing apparatus according to an embodiment of the invention. 
       FIG. 2B  shows multiple heating elements mounted parallel to the top and bottom surfaces of a horizontal annealing chamber. 
       FIG. 2C  shows heaters mounted parallel to the top, bottom, and side surfaces of a horizontal annealing chamber. 
       FIG. 2D  shows a spiral heater circumscribing a horizontal chamber. 
       FIG. 3  shows depressions formed on the inside surfaces of a horizontal annealing chamber. 
       FIG. 4A  shows heaters having concave surfaces mounted parallel to the top and bottom surfaces of a horizontal annealing chamber. 
       FIG. 4B  shows heaters having convex surfaces mounted parallel to the top and bottom surfaces of a horizontal annealing chamber. 
       FIG. 5  shows a disk spacer interposed between the optical crystals and an inside surface of a horizontal annealing chamber. 
       FIG. 6  shows multiple spherical spacers interposed between the optical crystals and a horizontal annealing chamber. 
       FIG. 7A  shows optical crystals arranged in an edgewise (vertical) orientation within a furnace. 
       FIG. 7B  shows a vertical cross-section of the annealing apparatus shown in  FIG. 7A . 
       FIG. 7C  shows optical crystals arranged in a vertical orientation within a furnace with the circumferential edges of the optical crystals having the same orientation as the round portion of the furnace. 
       FIG. 8A  shows a uniform temperature distribution within an optical crystal. 
       FIG. 8B  shows a temperature distribution near the edge of an optical crystal. 
       FIG. 9  shows a process gas system for use in an annealing process. 
       FIG. 10  shows an annealing cycle illustrating gas selection. 
   

   DETAILED DESCRIPTION 
   Embodiments of the invention provide a method and an apparatus for annealing large-diameter crystals, particularly optical fluoride disk crystals. For example, crystals with a diameter of 300 mm or greater and diameter-to-thickness ratios of 3.0 or greater can be treated using the method and apparatus of the invention, preferably optical fluoride crystal disks. Smaller-diameter crystals can also take advantage of the benefits offered by the method and apparatus of the invention. The invention includes applying heat uniformly to and removing heat uniformly from the optical crystals along their shortest path of conduction. The shortest path of conduction is along the shortest dimension of the crystal. For a circular crystal having a diameter-to-thickness ratio greater than 1, the shortest path of conduction is along the thickness of the crystal. The following is a description of specific embodiments of the invention. 
     FIG. 2A  shows an annealing apparatus  10  according to one embodiment of the invention. The apparatus  10  includes a horizontal chamber (or vessel)  12  having a surface  14  for supporting one or more disk crystals  16 . The horizontal chamber  12  is preferably unsealed, including not sealed hermetically, and can be gas permeable. The horizontal chamber  12  is made of an inert material, such as graphite, boron nitride, silicon carbide, or silicon nitride. The crystals  16  could be any type of optical fluoride crystal. For applications such as microlithography, fluoride crystals, such as single crystals of CaF 2 , BaF 2 , SrF 2 , LiF, MgF 2 , or NaF or mixed fluoride crystals made from solid solutions of these materials, are of interest. 
   For discussion purposes, the crystals  16  are assumed to be disk-shaped. However, the invention is not limited to disk-shaped crystals. In a preferred embodiment of the invention the optical fluoride crystals are disks. The crystals  16  are arranged in a single layer on the surface  14 . The single-layer arrangement is preferred when the crystals  16  have large diameters, i.e., greater than 150 mm, and have a diameter-to-thickness ratio greater than 1. If the crystals  16  have small diameters, i.e., smaller than 150 mm, or have a diameter-to-thickness ratio less than 1, then it may be possible to arrange the crystals in more than one layer on the surface  14 . In general, the crystals  16  should be arranged such that the majority (preferably at least 90%) of the heat that would be applied to them would be conducted along their shortest path of conduction, i.e., along their shortest dimension (diameter or thickness). 
   In the illustration, the bottom surfaces  18  of the crystals  16  are in direct contact with the surface  14  of the horizontal chamber  12 . In alternate embodiments, the crystals  16  could be placed in crystal containers (not shown), which can then be supported on the surface  14  of the horizontal chamber  12 . In alternate embodiments, as will be further described below, the bottom surfaces  18  of the crystals  16  may be spaced from the surface  14  of the horizontal chamber  12  to reduce or avoid contamination of the crystals  16  with the material used in constructing the horizontal chamber  12 . 
   The horizontal chamber  12  is supported inside a furnace  20 . Preferably, the support structure (not shown) for the horizontal chamber  12  is such that it does not cast thermal radiation “shadows” that can be detected on the inside of the horizontal chamber  12 . Preferably, the furnace  20  is a vacuum furnace. The furnace  20  may be constructed of a water-cooled stainless steel casing or other suitable material. Preferably, the furnace  20  includes one or more ports (not shown) through which the atmosphere in the furnace  20  can be controlled. For example, the ports may be used for introducing atmosphere-controlling gases into the furnace  20  and for measuring the temperature and pressure in the furnace  20 . Preferably, a gas purification/dryer system (not shown) is provided for removal of oxygen and moisture from process gases supplied into the furnace  20 . Preferably, the moisture level in the furnace  20  is controlled to less than 1 ppb. Catalyst/Absorber/Getter systems may be used to remove moisture from the furnace atmosphere. 
   Inside the furnace  20 , the horizontal chamber  12  is supported between heaters  22 ,  24 . The heaters  22 ,  24  are generally parallel to the top and bottom surfaces  26 ,  28 , respectively, of the horizontal chamber  12 . The heaters  22 ,  24  may be resistance heating elements made of graphite or other suitable inert material. The heaters  22 ,  24  may be single heating elements. In other embodiments, such as shown in  FIG. 2B , multiple heating elements  22   a ,  24   a  may be mounted parallel to the top and bottom surfaces  26 ,  28 , respectively, of the horizontal chamber  12 . Multiple heating elements allow for flexibility in controlling the temperature along the length of the horizontal chamber  12 . In other embodiments, such as shown in  FIG. 2C , heaters  30 ,  32  may be mounted parallel to the side surfaces  34 ,  36  of the horizontal chamber  12 . In other embodiments, such as shown in  FIG. 2D , the horizontal chamber  12  may be placed within one or more spiral heaters  34 . 
   Returning to  FIG. 2A , the heaters  22 ,  24  provide the majority of the heat used in bringing the crystals  16  from room temperature to annealing temperature. If the diameter-to-thickness ratio of the crystals  16  is greater than 1 and the crystals  16  are arranged in a single layer, then the heat generated by the heaters  22 ,  24  would be conducted along the shortest path of conduction of the crystals  16 . Providing the majority of the heat along the shortest path of conduction of the crystals  16  would result in increased heating rates in comparison to the case where the crystals are arranged in a vertical stack. Also, the single-layer arrangement of the crystals  16  would allow the crystals  16  to be cooled evenly at increased cooling rate throughout the entire cooling portion of the annealing cycle. The single-layer arrangement of the crystals  16  would also allow for even distribution of process gases around the crystals  16 . 
   Radiation enhancements can be used to increase the radiation view factors on the crystals  16  and improve the overall temperature uniformity within the crystals  16 . The term “radiation view factor” refers to the fraction of thermal energy leaving the surface of a first object and reaching the surface of a second object, determined entirely from geometrical considerations. In other words, the term “radiation view factor” on the crystal  16  refers to the fraction of the crystal  16  visible from the horizontal chamber  12 . In one embodiment, the radiation enhancements include textures or shapes formed on the inside surfaces of the horizontal chamber  12 . For example,  FIG. 3  shows cup-shaped depressions  36  formed on the inside surfaces of the horizontal chamber  12 . The sides of the depressions  36  would be at an angle sufficient to increase the radiation view factors on the crystals  16 . 
   Radiation enhancements can also be used to apply more radiation energy to specific portions of the crystals  16  such that more uniform heating or cooling of the crystals  16  is achieved. As in the embodiment above, these radiation enhancements could be textures or shapes formed on the inside surfaces of the horizontal chamber  12  and/or heaters  22 ,  24 . As an example, the radiation enhancements could be concave or convex shapes formed on the inside surfaces of the heaters  22 ,  24 .  FIG. 4A  shows concave shapes  40  formed on the inside surface of the heaters  22 ,  24 . The concave shapes  40  apply more radiation energy toward the center of the crystals  16 , promoting even heating.  FIG. 4B  shows convex shapes  38  formed on the inside surface of the heaters  22 ,  24 . The convex shapes  38 , when centered over the crystals  16 , apply more radiation energy toward the circumference of the crystals  16 , promoting even cooling. 
   The annealing process starts with loading of the optical fluoride crystals  16  into the horizontal chamber  12 , as shown in  FIG. 2A . The horizontal chamber  12  is then loaded into the furnace  20 . Typically, the horizontal chamber  12  is not sealed so that process gases can be passed over the crystals  16  as necessary. After loading the horizontal chamber  12  into the furnace  20 , the furnace  20  is sealed, and the required atmosphere such as vacuum, inert, or fluorinating environment, is created inside the furnace  20 . After creating the required atmosphere inside the furnace  20 , the heating elements  22 ,  24  are operated such that the crystals  16  are heated to annealing temperature, typically a temperature below the melting point of the crystals  16 . The heating process may include multiple heating and thermal hold segments. The crystals  16  are held at the annealing temperature for a predetermined length of time and then cooled at a controlled rate to room temperature. Typically, this cooling process involves slowly reducing the heat provided by the heaters  22 ,  24 . During annealing, a control system (not shown) monitors and controls the atmosphere in the furnace  20  to a programmed level. 
   The following is an outline of an annealing process for calcium fluoride crystals using the apparatus of the invention. In particular, various modifications can be made to the heating and cooling schedules depending on the type of optical fluoride crystal treated and the birefringence level desired. The outline of the annealing process is as follows:
         Load the horizontal chamber  12  inside the furnace  20  and seal the furnace  20 .   Pump vacuum into the furnace  20  until vacuum pressure of 10 −5  Torr is achieved.   Hold the furnace  20  at the vacuum pressure of 10 −5  Torr for 30 minutes.   Backfill the furnace  20  with preheated nitrogen or argon or a mixture of nitrogen and argon at a continuous programmed rate of 5 volume exchanges per hour, where the temperature of the gas supplied matches the temperature of the furnace  20 .   Heat the furnace  20  from room temperature to 300° C. in 5.5 hours with ±10° C. difference at any point outside of the chamber  12 .   Hold the temperature of the furnace  20  at 300° C. for 1 hour with ±5° C. at any point outside of the chamber  12  by the start of the thermal hold.   At the beginning of the thermal hold, start pumping vacuum into the furnace  20  until vacuum pressure of 10 −5  Torr is achieved.   Hold the furnace  20  at the vacuum pressure of 10 −5  Torr for 30 minutes.   Backfill the furnace  20  with preheated nitrogen or argon or a mixture of nitrogen and argon at a continuous programmed rate of 5 volume exchanges per hour, where the temperature of the gas supplied matches the temperature of the furnace  20 .   Heat the furnace  20  from 300° C. to 1200° C. in 18 hours with ±2.5° C. at any point on the outside of the chamber.   Hold the temperature of the furnace  20  at 1200° C. for 72 hours with ±1° C. difference at any point on the outside of the chamber  12  within 4 hours of the start of the thermal hold and continuing through the end of the hold at the same ±1° C. difference.   Cool the furnace  20  to 800° C. in 200 hours with ±1° C. difference at any point on the outside of the chamber  12  throughout this cooling range.   Hold the temperature of the furnace  20  at 800° C. for 24 hours with ±1° C. difference at any point on the outside of the chamber  12  through the end of the hold.   Cool the furnace  20  to room temperature in 150 hours with ±2.5° C. difference at any point on the outside of the chamber  12  throughout this entire cooling range.       

   Large-diameter crystals have large surface areas, which may result in increased friction drag between the crystals and the support surface of the horizontal chamber as the crystals expand and contract during the annealing process. Embodiments of the invention provide a method for reducing friction drag between the crystals and the support surface of the horizontal chamber during the annealing process. 
     FIG. 5  shows one method for reducing friction drag between the crystals  16  and the horizontal support surface  14  of the horizontal chamber  12  according to one embodiment of the invention. The method includes interposing sacrificial disks or spacers  42  between the crystals  16  and the support surface  14  of the horizontal chamber  12 . Preferably, the spacers  42  are made of the same or similar fluoride crystal material as the optical fluoride crystals  16 . The thickness of the spacers  42  can range from 0.125 to 1 in or more. In general, the surface friction between the crystals  16  and the fluoride crystal material spacers  42  is much less than would have been observed if the crystals  16  were in direct contact with the support surface  14  of the horizontal chamber  12 . 
   One of the benefits of having the fluoride crystal material disks  42  between the crystals  16  and the support surface  14  of the horizontal chamber  12  is better cooling uniformity within the crystals  16 . Better cooling uniformity is achieved because the crystals  16  are raised off the support surface  14  of the horizontal chamber  12 . Raising the crystals  16  also reduces the effect of hot and cold temperature spots of the support surface  14  on the internal temperature of the crystals  16 , allowing an overall uniform temperature within the crystals  16 . The spacers  42  also eliminate or reduce contamination of the crystal surface by preventing direct contact between the crystals  16  and the horizontal chamber  12 . 
     FIG. 6  shows another method for reducing friction drag between the crystals  16  and the support surface  14  of the horizontal chamber  12  according to an embodiment of the invention. The method includes placing loosely-packed round cross-section spheres  44  between the crystals  16  and the support surface  14  of the horizontal chamber  12 . In general, spacers with round cross-sections, such as cylinders, could be packed between the crystals  16  and the support surface  14 . The round cross-section spheres spacer rollers  44  could be made of high-grade, high-density inert material, such as graphite, or the same or similar fluoride crystal material as the optical crystals  16 . 
   The round cross-section spheres spacer  44  reduce the contact area between the crystals  16  and the support surface  14  of the horizontal chamber  12 , thus significantly reducing the surface friction and allowing the crystals  16  to thermally expand and contract freely. The spheres  44  also allow process gases to flow under the crystals  16  to provide a more homogeneous atmosphere environment to the surfaces of the crystals  16 . This potential flow of gases under the crystals  16  mimics two-sided cooling, which allows for shorter cooling cycles and increased throughput. The increased surface area of the spheres  44  also increases the radiation view factors on the crystals  16 , greatly reducing the impact of slight hot or cold temperature spots of the support surface  14  on the internal temperature of the crystals  16 . The spheres  44  also reduce contamination of the crystal surface by preventing direct contact between the crystals  16  and the chamber  12 . 
   Those skilled in the art will appreciate that other crystal arrangements are possible which would allow heat to be conducted along the shortest path of conduction of the crystals. In other words, the invention is not limited to mounting the crystals  16  facedown (in a horizontal orientation) inside the horizontal chamber  12 . For example,  FIG. 7A  shows an alternative arrangement where the crystals  16  are mounted in an edgewise (vertical) orientation inside vertical chambers  48 . The crystals  16  are mounted on supports  46  inside the chambers  48 . The circumferential edges  50  of the chambers  48  are in turn mounted on supports  52  inside the furnace  20 . The vertical chambers  48  are shown as having a circular cross-section, but this is not a requirement for supporting the crystals  16  in an edgewise fashion. The vertical chambers  48  could be box-shaped, for example. 
     FIG. 7B  shows a vertical cross-section of the arrangement shown in  FIG. 7A . As illustrated, heating elements  54  are placed adjacent the vertical faces  56  of the chamber  48  to allow heat to be conducted along the shortest path of conduction of the crystal  16 , i.e., along the thickness of the crystal  16 . This assumes that the diameter-to-thickness ratio of the crystal  16  is greater than 1. The vertical faces  56  of the chamber  48  and/or the heaters  54  could include radiation-enhancing surfaces, such as previously described. 
   Preferably, the material used in making the chamber  48  is an inert material and is heat-resistant. In one embodiment, the vertical faces  56  of the chamber  48  are made of a material having a high thermal conductivity, and the circumferential edge  50  of the chamber  48  is made of a material having a low thermal conductivity. An example of a suitable material for making the vertical faces  56  is a graphite material having a thermal conductivity of 139 W/m.k. An example of a suitable material for making the circumferential edge  50  is a graphite material having a thermal conductivity of 50 W/m.k. The combination of low thermal conductivity and high thermal conductivity materials ensures that the majority of the heat applied to the chamber  48  is conducted along the shortest path of conduction of the crystal  16 . 
   The chamber  48  is mounted within an insulated chamber  64  inside the furnace  20  to allow for greater control of the heating and cooling rates of the crystal  16 . It should be noted that the insulated chamber  64  does not have to be sealed. In the illustration, the crystal  16  and heating elements  54  are arranged such their circumferential edges  16   a ,  54   a , respectively, are rotated 90 degrees with respect to the round portion  21  of the furnace  20 . In another embodiment, such as shown in  FIG. 7C , the crystal  16  and heating elements  54  could be rotated such that their circumferential edges  16   a ,  54   a , respectively, have the same orientation as the round portion  21  of the furnace  20 . In this way, heat will still be conducted along the shortest path of conduction of the crystal  16 . This arrangement generally provides better heat uniformity across the crystal  16 . 
   It is desirable to have uniform heat distribution throughout the crystal  16 .  FIG. 8A  shows the desired uniform temperature gradient field within the crystal  16 . In reality, there will be some variation in the temperature distribution within the crystal  16 , particularly near the circumferential edge  60  of the crystal  16 .  FIG. 8B  shows the temperature gradient field “tailing off” near the circumferential edge  60  of the crystal  16 . In one embodiment, this tailing off can be minimized by placing a crystal edge insulator insulation material  62 , such as high purity graphite fiber, between the circumferential edge  50  of the chamber  48  and the circumferential edge  60  of the crystal  16 . The insulation material  62  would prevent rapid heat loss at the circumferential edge  60  of the crystal  16  as well as assist in the distribution of the gases introduced into the chamber  48  at port  66 . In another embodiment, localized heating can be applied near the circumferential edge  60  to minimize the tailing off. 
   Returning to  FIG. 7A , the chamber  48  includes a port  66  through which process gases can be communicated to the crystal  16 . In one embodiment, a fluid line  67  is connected to the port  66 . The fluid line  67  passes through a port  68  in the furnace  20  to the exterior of the furnace  20 . The fluid line  67  can be connected to a process gas system (not shown) external to the furnace  20 , allowing independent control of the atmosphere within the chamber  48 . For example, fluorinating gases are typically used to scavenge oxides from crystals. Instead of filling the furnace  20  with the fluorinating agent and having the agent then flow into the interior of chamber  48 , the invention provides for the flow of the fluorinating agent first into the chamber  48 , where the crystal  16  resides, to be filled with the fluorinating agent, with the fluorinating agent and any contaminant reaction products to gaseously exit the chamber  48  and into the furnace interior outside chamber  48 , preferably so that there is a positive pressure of the fluorinating agent gas inside chamber  48  to sweep away gaseous reaction products (particularly scavenged oxides) to the exterior of chamber  48  and away from the optical fluoride crystals being annealed. Where multiple chambers  48  are loaded into the furnace  20 , the connections  67  between the ports  66  in the chambers  48  and the exterior of the furnace  20  allow different atmospheric conditions to be maintained within the multiple chambers  48 . Preferably chambers  48  are non-hermetic thereby allowing fluid communication between an interior of the chamber and an interior of the furnace. 
     FIG. 9  shows a process gas system where the chamber  48  is connected to gas tanks  70 ,  72 . The gas tanks  70 ,  72  could be sources of fluorinating gases, for example, or other process gases. The fluorinating gases could be mixed with inert gases. Mass flow controllers  71 ,  73  are used to control flow from the gas tanks  70 ,  72  into the chamber  48 . A purifier  74  is provided to maintain a desired moisture level in the chamber  48 . 
   The furnace  20  is connected to a gas tank  78 . The gas tank  78  could be a source of an inert gas, such as argon. This would allow an inert atmosphere to be maintained inside the furnace  20  during the annealing process. A mass flow controller  79  is used to control flow from the gas tank  78  into the furnace  20 . A purifier  80  is provided to maintain a desired moisture level in the furnace  20 . A vacuum pump  76  maintains vacuum in the furnace  20  as necessary. 
   Although not shown, the process gas system also includes various valves and regulators to control gas flow through the system. A control system (not shown) may be used to control the mass flow controllers, valves, regulators, purifiers, and vacuum pump such that the desired atmospheric conditions are achieved inside the furnace  20  and chamber  48 . A purge vent  82  allows gas to be purged out of the chamber  48  and furnace  20  as necessary. A purge gas supply line  84  carries purge gas to the chamber  48  and furnace  20  as necessary. 
   The process gas system shown in  FIG. 9  allows gases to be supplied to and purged from the chamber  48  and furnace  20  independently.  FIG. 10  shows an example of an annealing cycle for calcium fluoride crystals using the process gas system shown in  FIG. 9 . The annealing cycle shows various types of gases that may be selected and introduced into the chamber  48  and furnace  20  at various times during the annealing process. Fluorinating gases, such as SF 6  and CF 4 , are introduced into the chamber  48  at temperatures where they are most effective in scavenging oxides from the calcium fluoride crystal. Other examples of fluorinating gases that may be used include NF 3 , BF 3 , C 2 F 4 , and F 2 . 
   As can be appreciated from the discussion above, the invention provides one or more advantages. Specifically, the invention allows heat to be distributed uniformly to one or more crystal disks, e.g., optical fluoride crystals, along the shortest path of conduction of the crystals during an annealing process. The invention also allows heat to be removed uniformly from the crystals during the annealing process. The results are annealed crystals having low birefringence values and shorter annealing cycles. 
   While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.