Abstract:
A system for heating optical members includes a thermally-conductive inner housing defining an interior volume for receiving an optical member to be heated, a thermally-insulative outer housing at least partially containing the thermally-conductive inner housing, and a heating structure disposed outside the inner housing and configured to provide heat through the thermally-conductive inner housing and into the interior volume defined by the inner housing.

Description:
This application is a division of Ser. No. 09/910,287 filed Jul. 20, 2001, now U.S. Pat. No. 6,624,390. 

   FIELD OF THE INVENTION 
   The invention relates to annealing and more particularly to annealing of single crystals to yield single crystals with low stress birefringence such as for use as optical lenses. 
   BACKGROUND OF THE INVENTION 
   The increase in the processing speed, functionality, and integration in integrated circuits (ICs) has been achieved through continuous reduction in the feature sizes of the ICs. A portion of the manufacturing of the ICs affecting attainable feature sizes is photolithography. During photolithography, a pattern of the IC is transferred from a mask to a wafer, e.g., a semiconducting wafer. Imaging characteristics of modern projection optical photolithography equipment are dominated by diffraction effects. The resolution (i.e. the smallest feature size that can be printed on the wafer) is k 1  λ/NA, where λ is the wavelength of the light source, k 1  is a constant approximately equal to 0.5, and NA is the numerical aperture of the projection optics. The depth of focus of the projection printer over which the image quality is not degraded is limited and is equal to k 2  λ/(NA) 2 , where k 2  is a constant that depends on k 1 . Thus, to decrease the feature size either the wavelength of exposure must be reduced or the NA of the optics must be increased. 
   Increasing the optics NA to reduce feature size results in a substantial reduction in the depth of focus (˜(NA) −2 ), which is undesirable, particularly because the depth of focus must be larger than any variations in the flatness of the photoresist surface. Therefore, the semiconductor industry is pursuing the use of short wavelength exposure sources for achieving smaller and smaller feature sizes. KrF, ArF, and F 2  excimer lasers are presently available as light sources for, respectively, 248, 193, and 157 nm photolithography. The synthetic fused silica, however, that has been the optical material of choice for higher wavelength exposure sources, exhibits significant loss of transmittance at wavelengths below 200 nm. 
   Single crystals of Calcium Fluoride (CaF 2 ) exhibit the desirable optical properties for sub 200-nm-photolithography. Furthermore, for historical reasons the production knowledgebase for CaF 2  is relatively extensive. Other single crystals of fluoride such as BaF 2  and LiF are also possible material candidates, but are significantly behind CaF 2  in production technology, and may be less desirable, e.g., due to toxicity and corrosiveness (BaF 2 ) and/or expense (LiF). Therefore, single crystal CaF 2  are desirable and suitable optical material for 193 and 157 nm optical steppers. Presently, CaF 2  crystals as large as 30 cm in diameter and 10 cm in height are used in photolithography equipment. 
   Single crystals of CaF 2  are grown by directional solidification from the melt phase. In this process layers of the melt are continuously solidified, by changing the temperature of the crystal, to form a single crystal boule. The crystal boule is subsequently cooled to room temperature. The transfer of heat from and through the crystal sets up temperature gradients (i.e. temperature non-uniformities) and associated thermal stresses in the single crystal. CaF 2  is a relatively weak material, especially at elevated temperatures, and therefore experiences plastic deformation under thermal stresses during the crystal growth process. The accumulation of plastic strain during the crystal growth process results in generation of residual stresses in the crystal at room temperature. Residual stresses, in turn, cause stress birefringence through spatial variations in the material&#39;s index of refraction, and an associated degradation of optical characteristics of components made from this material. 
   Annealing is used to reduce residual stresses in crystals that have experienced plastic deformation during the crystals&#39; growth process. To anneal a crystal, the crystal is maintained at an elevated temperature close to its melting point temperature for a period of time. This constant temperature is intended to allow existing residual stresses to relax. The crystal is cooled to room temperature. During cooling, temperature gradients associated with the cooling of the crystal generate thermal stresses in the crystal that may cause the crystal to undergo plastic deformation. 
   Due to the nature of the material, temperature variations to which a single crystal is exposed to during growth and annealing result in large thermal stresses leading to plastic deformation of the crystal and, hence, large residual birefringence. 
   SUMMARY OF THE INVENTION 
   In general, in an aspect, the invention provides a system for heating optical members. The system includes a thermally-conductive inner housing defining an interior volume for receiving an optical member to be heated, a thermally-insulative outer housing at least partially containing the thermally-conductive inner housing, and a heating structure disposed outside the inner housing and configured to provide heat through the thermally-conductive inner housing and into the interior volume defined by the inner housing. 
   Implementations of the invention may include one or more of the following features. The inner housing is configured such that an inner surface defining the interior volume has a substantially uniform temperature in response to the inner housing receiving the heat provided by the heating structure. The inner housing is configured to define the interior volume to be axi-symmetric. 
   Further implementations of the invention may include one or more of the following features. The system further comprises a controller coupled to the heating structure and configured to control the heating structure such that the member disposed in the interior volume is heated substantially without being plastically deformed. The controller is configured to control the heating structure such that a resolved shear stress of a CaF 2  optical member disposed in the interior volume does not exceed about 0.5e (990/T)  MPa where T is average temperature of the member in Kelvin. 
   Further implementations of the invention may include one or more of the following features. A portion of the outer housing in contact with and supporting the inner housing has a thermal conductivity different than at least one other portion of the outer housing. An inner boundary of the outer housing is disposed in contact with substantially an entire outer boundary of the inner housing. The inner housing and at least a portion of the outer housing are an integral structure, with the inner housing and the at least a portion of the outer housing being layers of the integral structure with different thermal conductivity. 
   Further implementations of the invention may include one or more of the following features. The inner housing comprises at least one of high-thermal-conductivity graphite and high-thermal-conductivity carbon. The interior volume is cylindrical and directions of highest thermal conductivity of the inner housing are parallel with inner surfaces of the inner housing. The interior volume is cylindrical and directions of lowest thermal conductivity of the inner housing are perpendicular with inner surfaces of the inner housing. Directions of lowest thermal conductivity of the outer housing are perpendicular with outer surfaces of the inner housing. 
   Further implementations of the invention may include one or more of the following features. The inner housing has substantially orthotropic thermal conductivity. The outer housing comprises at least one of low-thermal-conductivity graphite, low-thermal-conductivity carbon, low-thermal-conductivity porous graphite, low-thermal-conductivity porous carbon, low-thermal-conductivity fibrous graphite, low-thermal-conductivity fibrous carbon. The outer housing has substantially orthotropic thermal conductivity. The system further comprises another thermally-conductive housing, the another thermally-conductive housing substantially contains the thermally-insulative outer housing. The another thermally-conductive housing is displaced from the outer housing. 
   Further implementations of the invention may include one or more of the following features. The inner housing defines a plurality of interior volumes each for receiving an optical member to be heated. The inner housing has a substantially isotropic thermal conductivity. The outer housing has a substantially isotropic thermal conductivity. At least a portion of the heating structure is disposed outside the outer housing. 
   In general, in another aspect, the invention provides a method of heating an optical member. The method includes providing the optical member, directing heat from a heat source toward the optical member, and distributing the heat about the optical member through a high-thermal-conductivity apparatus disposed between the heat source and the optical member such that a surface of the apparatus defining a volume for receiving the optical member will have a substantially uniform temperature. 
   Implementations of the invention may include one or more of the following features. The heat is distributed such that temperatures of the surface of the apparatus defining the volume vary by no more than about 0.5 K where K is temperature in Kelvin. The method further comprises measuring at least one indication of temperature of the apparatus defining the volume. The at least one indication includes a plurality of indicia of temperature of the apparatus, the indicia being related to at least one of an outer surface, an inner surface, and an interior of the apparatus. The method further comprises adjusting how much heat is directed toward the optical member in accordance with the at least one indication. The adjusting is in accordance with a model of temperature variations within the optical member. How much heat is directed toward the optical member is adjusted to guard against stress within the optical member exceeding a critical resolved shear stress of the optical member during at least one of annealing of the optical member and cool down of the optical member. 
   Further implementations of the invention may include one or more of the following features. The method further comprises inhibiting heat from transferring away from the optical member from the high-thermal-conductivity apparatus. A plurality of optical members is provided, wherein heat is directed from a heat source toward each of the optical members, and wherein the heat is distributed about each of the optical members through the high-thermal-conductivity apparatus disposed between the heat source and the optical members such that surfaces of the apparatus defining volumes for receiving the optical members will each have a substantially uniform temperature. 
   In general, in another aspect, the invention provides a system for annealing at least one single crystal blank for use as at least one optical lens. The system includes a heating structure for supplying heat, heating means for heating the at least one single crystal blank, using the heat from the heating structure, to an annealing temperature of the blank and for cooling the at least one single crystal blank from the annealing temperature to an ambient temperature substantially without plastic deformations developing in the at least one blank, the heating means including at least a high-thermal-conductivity housing for containing the at least one single crystal blank. 
   Implementations of the invention may include one or more of the following features. The heating means further includes an insulator structure at least partially containing the high-thermal-conductivity housing. The heating means further includes a controller coupled to the heating structure for regulating heat provided by the heating structure to permit annealing of the at least one blank while inhibiting temperature gradients inside the at least one blank from producing plastic deformations. The heating means further comprises temperature sensors coupled to the controller configured to provide indicia of temperatures of the high-thermal-conductivity housing to the controller and wherein the controller regulates the heat provided by the heating structure in response to the indicia provided by the temperature sensors. The controller inhibits temperature gradients inside each of the at least one blank from producing stresses in excess of about 0.5e (990/T)  MPa where T is average temperature of each blank in Kelvin. 
   In general, in another aspect, the invention provides an optical member including a single crystal material substantially free of residual stress and having an optical birefringence of less than about 1 nm/cm. 
   Implementations of the invention may include one or more of the following features. The single crystal material forms an optical lens blank. The single crystal material is a fluoride. The single crystal material is CaF 2 . 
   Various aspects of the invention may provide one or more of the following advantages. A substantially isothermal environment may be provided for members, such as optical blanks or lenses, to be annealed (during annealing and cool down), or otherwise heat treated. Temperature nonuniformities along walls of a chamber containing a member to be heated can be reduced relative to prior systems. Heat loss through a support structure for supporting a chamber to contain a member to be heated can be reduced relative to prior systems. Time-dependent variations of temperature on an interior portion of a container of a member to be heated can be dampened relative to corresponding time-dependent variations on an exterior portion of the container. Radial temperature variations within an axi-symmetric crystal can be kept below a level that would induce stresses exceeding a critical resolved shear stress of the crystal during an annealing and/or cool down period. Annealed items, e.g., optical members, can be produced with low birefingence, e.g., less than about 1 nm/cm. 
   These and other advantages of the invention, along with the invention itself, will be more fully understood after a review of the following figures, detailed description, and claims. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  is a schematic cross-sectional view (with a blank shown in cut-away) of an annealing system. 
       FIG. 2  is a graph of critical resolved shear stress vs. temperature. 
       FIG. 3  is a block flow diagram of an annealing process using the system shown in FIG.  1 . 
       FIG. 4  is a cross-sectional view (with blanks shown in cut-away) of another annealing system, for annealing multiple blanks currently. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , an annealing system  10  includes a single crystal blank  12  for, an inner housing  14 , an outer housing  16 , including a cap  18  and a base  20 , a support  22 , including a platform  24  and shaft  26 , a heating structure including heating elements  28 ,  30 ,  32 , controllers  29 ,  31 ,  33 , temperature sensors  40 ,  42 ,  44 , an insulator  36 , and a system housing  38 . The system  10  is configured for high-temperature annealing of single crystals as described below. The system  10  can accurately maintain temperature in the crystal  12  at levels equal to and below levels needed to promote complete, or near complete, relaxation of stresses during a constant-temperature phase of an annealing process. The system  10  can also maintain spatial temperature non-uniformities in the crystal  12  at or below levels inducing plastic deformation of the crystal  12  and accumulation of plastic strain in the crystal  12  during a cool-down phase, to room temperature of the annealing process. 
   The blank  12  is any material to be annealed. For example, here the blank  12  is a single crystal of a material suitable for use as an optical component. One or more optical components may be made from the blank  12 , e.g., by dividing such as by cutting the blank  12 . For example, the blank  12  can be a single crystal fluoride material such as calcium fluoride CaF 2 , although other materials may be used. The blank  12  can be axi-symmetric, for example being cylindrical about an axis  46 . Blank  12  can be of various shapes and/or sizes, e.g., or cylinder, e.g., 30 cm in diameter and 10 cm in height. 
   The inner housing  14  is a high-thermal conductivity material configured to contain the blank  12 . The inner housing  14  defines an interior volume  48  of a size able to receive the blank  12 . A resealable entry (not shown) is provided in the housing  14  to be opened to receive the blank  12  and to be sealed during annealing of the blank  12 . The housing  14  is made of a high-thermal conductivity material, such as high-purity graphite EK 94P, made by Ringsdorff-Werke GmbH of Bonn, Germany. This material, according to the manufacturer, has a thermal conductivity close to 0.7 W/cm-K at 1200 K. The housing  14  is configured such that its top and bottom walls  15 ,  17  have their highest (lowest) thermal conductivity directions parallel (perpendicular) to a radial direction from the axis  46 . Also, the side/lateral wall  19  has its highest (lowest) thermal conductivity direction parallel (perpendicular) to the axis  46 . Thus, the highest (lowest) thermal conductivity directions are parallel (perpendicular) to an interior surface  50  of the inner housing  14 . Thickness(es) of the housing  14  is (are) such that heat provided by the heaters  28 ,  30 ,  32  to the housing  14  will be conducted and distributed throughout the housing  14  to provide a substantially uniform temperature on the interior surface  50  of the housing  14 . 
   Adding to the ability of the system  10  to provide a substantially isothermal environment in the volume  48  is the outer housing  16 . The housing  16  is insulative in nature, having a much lower thermal conductivity than the inner housing  14 . The outer housing  16  provides a higher thermal resistance in directions away from the inner housing  14  (i.e., in directions normal to the exterior surfaces of the inner housing  14 ) than the thermal resistance along the exterior surfaces of the inner housing  14 . The outer housing  16  and in particular cap  18 , is of a low thermal conductivity material such as graphite fiber foam, made by Calcarb Limited of North Lanarkshire, Scotland. This material, according to Calcarb, has a thermal conductivity approximately equal to 0.005 W/cm-K at 1273 K. The base  20  may contain some materials of a slightly higher thermal conductivity to increase stiffness of the base to provide adequate support for the cap  18  and the inner housing  14 , containing the blank  12 . The cap  18  is shown in physical contact with the inner housing  14 , although a gap, such as a vacuum or inert-gas-filled gap, may be provided between the cap  18  and the housing  14 . The gas may include fluoride if a fluoride blank is used, or may include argon. 
   The support  22  is configured to support the outer housing  16 , containing the inner housing  14 , containing the blank  12 , while providing thermal resistance. The support  22  is made of a hard material with the platform  24  being of hard and soft (low thermal conductivity) materials in a combination such that the support  22  provides thermal resistance and sufficient rigidity to support the components shown. The shaft  26  of the support  22  extends away from the outer housing  16  through the insulator  36  and the system housing  38 . 
   The system housing  38  is a metallic housing defining an outer perimeter of the system  10 . The housing  38  is configured to be sufficiently air-tight and to allow for evacuation of gases from within the housing  38  to produce pressures inside the housing  38  as low as one-tenth to one-hundredth of one atmosphere, as well as to allow introduction of process gases of pressures up to slightly over one atmosphere. The housing  38  is water cooled to maintain a desired temperature, dissipating, as necessary, heat received from the heaters  28 ,  30 ,  32  through the insulator  36 . The outer housing  38  is supported by an external structure not shown in FIG.  1 . 
   The insulator  36  is provided to help reduce heat loss from the system  10 . In particular, the insulator  36  is made of an insulating material such as graphite to inhibit heat from the heaters  28 ,  30 ,  32  being transferred away from the blank  12 . 
   The heaters  28 ,  30 ,  32  are configured to provide heat to heat the blank  12  to desired temperatures for annealing, or other desired processes. The heaters  28 ,  30 ,  32 , e.g., resistive graphite heaters, may be configured to directionally supply heat toward the blank  12 . Heat from the heaters  28 ,  30 ,  32  may transfer in directions away from the blank  12 , and is inhibited from doing so by the insulator  36 . The heaters  28 ,  30 ,  32  are configured to supply amounts of heat in response to control signals received from respective controllers  29 ,  31 ,  33 . 
   The controllers  29 ,  31 ,  33  are configured to send signals to the heaters  28 ,  30 ,  32  to regulate the amount of heat provided by the heaters  28 ,  30 ,  32  in response to temperature indicia provided by the temperature sensors  40 ,  42 ,  44 . The temperature sensors  40 ,  42 ,  44  monitor the temperature at various points on the inner housing  14  (e.g., on exterior surfaces as shown, or on interior surfaces, or inside the housing  14 )and provide indicia of these temperatures through signals to the respective controllers  29 ,  31 ,  33 . The controllers  29 ,  31 , 33  use the temperature indicia from the sensors  40 ,  42 ,  44 , to provide the control signals to the heaters  28 ,  30 ,  32  in accordance with temperatures or temperature schedules, that depend on a particular process currently undergone by the blank  12 . The temperature and temperature schedules for the blank  12  are determined in order to inhibit plastic deformations and residual stresses inducing stress birefringence in the blank  12 . 
   Non-uniform temperature fields lead to thermal stresses in the crystal  12 , and excessive thermal stresses during growth and annealing cause plastic deformation of the crystal. The system  10  is configured to provide post-growth annealing that maintains a quantifiably controllable uniform temperature distribution in optical members, such as fluoride crystals, in particular CaF 2 , both during the constant temperature period as well as cool-down period of the annealing process. 
   In general, single crystals such as CaF 2  experience plastic deformation along specific crystallographic planes and directions, the so-called slip planes and slip directions. For example, the slip system of CaF 2  is defined as {100}&lt;110&gt;, where {100} refers to the orientation of the family of vectors normal to the slip planes and&lt;110&gt; the family of direction vectors along which slip occurs. 
   The crystal  12  undergoes plastic deformation if the projection of thermal stresses onto the slip directions, the so-called resolved shear stresses, exceed the so-called Critical Resolved Shear Stress (CRSS) of the crystal. The CRSS is a property of the crystal  12 . Stresses higher than the CRSS will result in plastic deformations and hence birefringence. Stresses smaller than the CRSS will result in elastic deformation of the material and will not cause permanent deformations resulting from plastic deformation. Thus, stresses smaller than the CRSS will not cause birefringence. 
   It has been concluded that the temperature dependence of the CRSS for a single crystal of CaF 2  is given by:
 
 CRSS =0.5  e   (990/T)   (1),
 
where T is the temperature in units of Kelvin, and the CRSS has the units of MPa. Referring to  FIG. 2 , the CRSS  52  for CaF 2  according to Equation (1) is shown to decrease with increasing temperature, and vice versa. Although the CRSS of CaF 2  increases with decreasing temperature, it is fairly low even at temperatures close to room temperature. Thus, to help avoid plastic deformation, and hence birefringence, the temperature variations in the crystal are controlled by the system  10 .
 
   For cylindrical blanks  12 , radial temperature gradients are the primary mechanism for generation of thermal stresses in the single crystal blank  12 . Thus, stresses can be kept below the CRSS by controlling the edge-to-center radial temperature difference within the crystal, the radial temperature difference ΔT. ΔT can be approximated according to: 
                 Δ   ⁢           ⁢   T     =     CRSS     ϕ   ⁢           ⁢   λ   ⁢           ⁢   E         ,           (   2   )             
 
where φ is a configuration number related to the slip system of the crystal, λ is the thermal expansion coefficient of the material, and E is the Young Modulus.
 
   It has been calculated that for a cylindrical single crystal of CaF 2 , regardless of the crystal&#39;s dimensions, radial temperature differences, ΔT, exceeding:
         approximately 0.5° C. at 1000° C.,   approximately 0.7° C. at 800° C.,   approximately 1.2° C. at 500° C., and   approximately 3.5° C. at 200° C.,
 
will cause the CRSS to be exceeded. Thus, to avoid plastic deformations inside a single crystal of CaF 2 , even as large as 30 cm in diameter and 10 cm in height, the system  10  is configured to keep the values of ΔT in the crystal at relatively low values.
       

   The temperature difference ΔT is proportional to the cooling rate for the blank  12 . The proportionality depends on the blank&#39;s material properties, size, and shape, and can be determined, e.g., by computer models or analytical expressions (in simple cases). Using knowledge of this proportionality, Equation (1), and Equation (2), the cooling rate can be determined to inhibit, if not prevent, plastic deformation of the blank  12 . The invention provides a schedule for controlled cool down of the annealed blank  12  such that the CRSS is not exceeded. A schedule for cool down helps ensure that during removal of heat from the crystal  12  during cool down, temperature variations in the crystal  12  are maintained at such low values that plastic deformation of the crystal  12  does not occur, or occurs within acceptable amounts. 
   Based on the properties of CaF 2  available publicly, a computational model has been used to calculate the rate of cooling so as not to have the center-to-edge temperature difference in a crystal induce stress above the CRSS. For example, if the annealed part is a single crystal of CaF 2  of diameter of 30 cm and height of 15 cm, and the entire surface of the crystal is maintained at substantially the same surface temperature, the surface temperature should obey the following cooling rates schedule:
         approximately 0.4° C./hr from about 995° C. to about 797° C.,   approximately 0.6° C./hr from about 797° C. to about 600° C.,   approximately 0.9° C./hr from about 600° C. to about 400° C.,   approximately 1.4° C./hr from about 400° C. to about 287° C., and   approximately 2.7° C./hr from about 287° C. and lower.       

   It has been concluded that the cooling rate for a cylindrically-shaped single crystal of CaF 2 , with flat top and bottom, may be calculated from the formula: 
               cooling   ⁢           ⁢   rate     =         Δ   ⁢           ⁢     T   ·   thermal     ⁢           ⁢   diffusitivity         constant   ·   surface     ⁢           ⁢   area       .             (   3   )             
 
The constant has a value close to 5.5 and can be determined experimentally or from numerical simulations for different shapes of the annealed part and various configurations of the invention, including but not limited to, the system  10 . The cooling rate is the cool-down rate in ° C./sec. Surface area is the surface area of the annealed part in units of cm 2 . Thermal diffusivity is a property of the annealed part in units of cm 2 /sec.
 
   Referring to  FIG. 3 , with further reference to  FIGS. 1-2 , a process  60  of annealing the blank  12  includes stages  62 ,  64 , and  66 . At stage  62  the blank  12  is provided. At this stage, the blank  12  is placed in the volume  48  defined by the inner housing  14 . 
   At stage  64 , the controllers  29 , 31 , 33  control the heaters  28 ,  30 ,  32  to provide heat. The heaters  28 ,  30 ,  32  provide heat to heat the blank  12  to a desired constant temperature for the constant-temperature phase of the annealing process. The temperature of the blank  12  is attempted to be kept at a constant and substantially uniform temperature by the controllers  29 ,  31 ,  33  receiving indicia of temperatures from the temperature sensors  40 ,  42 ,  44  and providing control signals to the heaters  28 ,  30 ,  32 . The control signals control (including causing variances in, as appropriate) the power used by the heaters  28 ,  30 ,  32 , and thus the heat produced by these heaters  28 ,  30 ,  32  as appropriate to maintain the temperature of the surface  50  of the inner housing  14  and thereby the temperature of the blank  12 . The power of the heaters  28 ,  30 ,  32  is regulated such that the temperature sensors  40 ,  42 ,  44  measure substantially fixed set values. These values are held substantially constant for the time duration of the constant-temperature phase of the annealing process. 
   At stage  66 , the controllers  29 ,  31 , 33  regulate the heaters  28 ,  30 ,  32  to cool the blank  12  down. Again, responsive to temperatures indicated by the temperature sensors  40 ,  42 ,  44 , the controllers  29 ,  31 ,  33  send control signals to the heaters  28 ,  30 ,  32  to adjust as necessary, the power used and thus the heat provided by the heaters  28 ,  30 ,  32 . The controllers  29 ,  31 ,  33  regulate the heat provided such that the temperature as indicated by the sensors  40 ,  42 ,  44  follow a predetermined cooling rate schedule that has been determined to guard against temperature gradients within the blank  12  causing stresses to exceed the CRSS of the blank  12 . In particular, the heat is regulated to guard against temperature gradients in the blank  12  (e.g., radial temperature gradients for a cylindrical blank  12 ) exceeding values that would cause resolved sheer stresses in the blank  12  to exceed the CRSS of the blank  12 . 
   Using the method  60 , the blank  12  can be produced having desired characteristics. For example, the CaF 2  blank  12  can be produced with residual stress birefingence that is less than approximately 1 nm/cm. This birefringence is then acceptable for very fine resolution photolithography applications. 
   Referring to  FIG. 4 , a system  70  similar to the system  10  ( FIG. 1 ) includes components that are different from, but similar to, that of system  10  to accommodate an inner housing  72  that is different from the inner housing  14  (FIG.  1 ). The inner housing  72  defines three volumes  74 ,  76 ,  78  that are sized to accommodate three respective blanks  80 ,  82 ,  84 . Each of the blanks  80 ,  82 ,  84  may be the same or different sizes, e.g. cylindrical with a height of 10 cm and a diameter of 30 cm. Other components of the system  70  are similar to the respective components of system  10 , but different in order to accommodate the multiple blanks  80 ,  82 ,  84  while providing similar functionality, e.g., substantially isothermal environments for the blanks  80 ,  82 ,  84 . While three volumes  74 ,  76 ,  78  are shown in  FIG. 4 , other numbers of volumes may be provided. 
   Other embodiments are within the scope and spirit of the appended claims. For example, referring to  FIG. 1 , a support structure can be provided in the volume  48  defined by the inner housing  14  to separate the blank  12  from the walls of the housing  14 . Also, the housings  14  and  16  may be integrally formed of layers having different thermal conductivities, with a higher thermal conductivity layer, or layers, being disposed inward of a lower thermal conductivity layer, or layers. The blank  12  may be of a variety of materials, such as semiconductors, or other materials, even if not for optical uses, for which annealing or other heating/cooling is desired and in which temperature gradients are undesirable. Also, items other than blank  12  can be annealed using the system  10 , such as optical components, lenses, prisms, and single crystals, e.g., fluorides, other than CaF 2 . Also, although each heater and temperature sensor combination is shown in  FIG. 1  with its own controller, a controller may be used to regulate more than one, and even all, of the heaters responsive to temperature indicia from the temperature sensors. The base  20  can be made of layers or can be a composite of low and high thermal conductivity materials in order to provide a sufficiently sturdy and sufficiently low thermal conductivity member. One or more of the heaters can be enclosed in portions of the housing  16  or support  22  (e.g., in platform  24 ). Temperature indicia provided by the temperature sensors  40 ,  42 ,  44  can be of the inner surface  50  of the inner housing  14 , or an interior portion of the inner housing  14 .