Patent Publication Number: US-9409812-B2

Title: Heat treatment method for synthetic quartz glass

Description:
TECHNICAL FIELD 
     The present invention relates to a heat treatment method for synthetic quartz glass for optical use and the like. 
     PRIOR ART 
     Photolithography technology is used in exposure apparatus for the manufacture of semiconductor integrated circuits. With the recent miniaturization and high integration of semiconductor integrated circuits, shorter wavelength light sources are being used for exposure. At present, optical members for exposure apparatus that use ArF excimer lasers (wavelength 193.4 nm) must meet requirements such as high homogeneity, high transparency, and high laser resistance. 
     High purity synthetic quartz glass is used as optical member material that meets these strict requirements. When improving homogeneity and decreasing birefringence in the synthetic quartz glass manufacturing process, heat treatment (annealing treatment) is a common manufacturing method performed to eliminate warp. 
     For example, annealing silica glass in a container or in powder, or plate-covered, is described in Patent document 1. Also, housing powder in a container, embedding the optical synthetic quartz glass product in the powder and annealing is described in Patent document 2. 
     Also, making the temperature variation in the optical axis direction less than the temperature variation in the direction orthogonal to the optical axis during a synthetic quartz glass block heating process, in order to manufacture synthetic quartz glass with a smaller distribution of birefringence values in the direction orthogonal to the optical axis, is described in Patent document 3. 
     Also, a heat treatment method to eliminate warp and purify synthetic silica glass bodies, where the entire external surface of the item to be treated is coated by contact with high purity SiO 2  powder, and then heat-treated, is described in Patent document 4. 
     However, even when a common known heat treatment for warp elimination is employed, there are disadvantages in that in exposure apparatus that use a plurality of optical members, the birefringences of each optical member accumulate, and high birefringence forms in the apparatus as a whole. 
     With regard to this, for example, it is explained in Patent document 5, and in Patent document 6 and Patent document 7, that the birefringence of the whole is decreased by using two different types of synthetic quartz glass (A and B). That is, when the birefringence (birefringence phase difference) of the two types of synthetic quartz glass A and B are the same, and the distribution is such that the fast axis directions are at right angles to each other, the fast axis of synthetic quartz glass A is in the same direction as the slow axis of synthetic quartz glass B (orthogonal to the fast axis), and so the birefringence effects of the two types of synthetic quartz glass cancel out and the cumulative birefringence corresponds to zero. 
     PRIOR ART DOCUMENTS 
     [Patent document 1] Japanese unexamined patent (1996) H08-091857 
     [Patent document 2] Japanese unexamined patent (1999) H11-011965 
     [Patent document 3] Japanese unexamined patent 2009-298670 
     [Patent document 4] Japanese unexamined patent 2011-213561 
     [Patent document 5] Japanese unexamined patent 2005-239537 
     [Patent document 6] Japanese unexamined patent 2007-223888 
     [Patent document 7] Japanese unexamined patent 2007-223889 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     Although, as described above, a method to regulate the direction of the birefringence fast axis (also referred to simply as “fast axis”) of synthetic quartz glass is important, a method that can achieve such regulation has not been satisfactorily established. Respective individual problems remain with the method of Patent document 5, and with the method of Patent document 6 and Patent document 7. 
     In Patent document 5, a standard annealing treatment and a second annealing treatment are necessary in order to create quartz glass with two fast axis directions. That is, it requires standard annealing in one direction only (where the fast axis is arranged tangentially to the longitudinal axis of the cylinder), and a second annealing treatment comprising a standard annealing treatment in another direction (in a radial direction to the longitudinal axis of the cylinder). 
     In the methods of Patent document 6 and Patent document 7 there are stipulations regarding the OH group concentration distribution in the synthetic quartz glass starting material, and two types of fast axis quartz glass are created separately by separating synthetic quartz glass according to its OH group concentration distribution. 
     In the method of Patent document 5, there are two types and two stages of annealing means, and in the methods of Patent document 6 and Patent document 7 there is no adjustment of the birefringence in the processes that follow creation of the starting material: they are disadvantageous because they are unsatisfactory with regard to these two points. 
     In view of these disadvantages, the aim of the present invention is to provide a synthetic quartz glass heat treatment method that can, by a single heat treatment, and without particular limitations on the OH group concentration distribution of the starting material, regulate the birefringence fast axis direction in synthetic quartz glass after it has been heat-treated. 
     Means of Solving the Problem 
     In order to solve the abovementioned problem, the present invention provides a heat treatment method for synthetic quartz glass whereby columnar synthetic quartz glass having two opposing end faces and a lateral face is heat-treated covered with thermal insulator: wherein said heat treatment is performed using as end face thermal insulator which covers said two end faces, and as lateral face thermal insulator which covers said lateral face, thermal insulators that differ in at least either type or thickness to afford different thermal insulation effects such that the birefringence fast axis direction of said synthetic quartz glass is regulated. 
     With such a heat treatment method for synthetic quartz glass it is possible, by a single heat treatment, and without particular limitations on the OH group concentration distribution of the starting material, to regulate the birefringence fast axis direction in synthetic quartz glass after it has been heat-treated. 
     With the inventive heat treatment method for synthetic quartz glass, the birefringence fast axis direction of said synthetic quartz glass can be regulated in the radial direction of said end face by making the thermal insulation effect of said end face thermal insulator smaller than the thermal insulation effect of said lateral face thermal insulator. 
     With the inventive heat treatment method for synthetic quartz glass, the birefringence fast axis direction of said synthetic quartz glass can be regulated in the tangential direction of said end face by making the thermal insulation effect of said end face thermal insulator greater than the thermal insulation effect of said lateral face thermal insulator. 
     Thus with the inventive heat treatment method for synthetic quartz glass it is possible to regulate the birefringence fast axis direction to the radial direction of the end face, or to the tangential direction of the end face. 
     Also, in the inventive heat treatment method for synthetic quartz glass, said the thermal insulator is preferably a quartz glass container that can house said synthetic quartz glass. 
     In such cases, also, SiO 2  powder or SiO 2  fibers can be packed inside said quartz glass container. 
     With such a thermal insulator it is easy to adjust the thermal insulation effect around the synthetic quartz glass. 
     Also, the present invention provides a heat treatment method where a plurality of synthetic quartz glass is prepared, and heat-treated simultaneously by the abovementioned synthetic quartz glass heat treatment method, using the same heat treatment oven for said plurality of synthetic quartz glass; wherein, by performing the heat treatment with said end face thermal insulator and lateral face thermal insulator for at least one of said plurality of synthetic quartz glass arranged so as to have thermal insulation effects that differ from those of said end face thermal insulator and lateral face thermal insulator for the other synthetic quartz glass, the birefringence fast axis direction of at least one of said plurality of synthetic quartz glass is regulated in a different direction to that of said other synthetic quartz glass. 
     Thus with the present invention is possible to create a plurality of synthetic quartz glass having different birefringence fast axis directions even when heat-treating a plurality of synthetic quartz glass simultaneously using the same heat treatment oven. 
     Also, with the inventive synthetic quartz glass heat treatment method, the synthetic quartz glass for said heat treatment can be optical synthetic quartz glass to be used for optical systems in exposure apparatus. 
     In the present invention, the birefringence fast axis direction can be regulated, and so optical synthetic quartz glass is suitable as the synthetic quartz glass to be heat-treated. 
     Advantages of the Invention 
     With the inventive synthetic quartz glass heat treatment method it is possible, by a single heat treatment, and without particular limitations on the OH group concentration distribution of the starting material, to regulate the birefringence fast axis direction in the synthetic quartz glass after it has been heat-treated. Thus, for example, it is possible to simply and reliably obtain two types of synthetic quartz glass with different birefringence fast axis directions. Also, with the present invention, by using the same heat treatment temperature program it is possible to heat-treat a plurality of synthetic quartz glass in the same oven, and so production process efficiency is good. Moreover, as a result, synthetic quartz glass having two birefringence fast axis directions can be made in the same oven and the same batch, which is also efficient. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a simple cross-section view of a configuration example of a heat treatment oven that can be used in the inventive synthetic quartz glass heat treatment method. 
         FIG. 2  is a simple cross-section view of another configuration example of a heat treatment oven that can be used in the inventive synthetic quartz glass heat treatment method. 
         FIG. 3  is a diagram to illustrate the definition of birefringence fast axis direction in synthetic quartz glass. 
         FIG. 4  is a graph showing the heat treatment temperature program in an embodiment. 
         FIG. 5  comprises graph (a) showing the birefringence distribution along the radial direction of the end face, and graph (b) showing the birefringence fast axis angle distribution, in synthetic quartz glass heat-treated in Embodiment 1. 
         FIG. 6  comprises graph (a) showing the birefringence distribution along the radial direction of the end face, and graph (b) showing the birefringence fast axis angle distribution, in synthetic quartz glass heat-treated in Embodiment 2. 
         FIG. 7  comprises graph (a) showing the birefringence distribution along the radial direction of the end face, and graph (b) showing the birefringence fast axis angle distribution, in synthetic quartz glass heat-treated in Embodiment 3, 
         FIG. 8  comprises graph (a) showing the birefringence distribution along the radial direction of the end face, and graph (b) showing the birefringence fast axis angle distribution, in synthetic quartz glass heat-treated in Embodiment 4. 
         FIG. 9  comprises graph (a) showing the birefringence distribution along the radial direction of the end face, and graph (b) showing the birefringence fast axis angle distribution, in synthetic quartz glass heat-treated in Embodiment 5. 
         FIG. 10  comprises graph (a) showing the birefringence distribution along the radial direction of the end face, and graph (b) showing the birefringence fast axis angle distribution, in synthetic quartz glass heat-treated in Embodiment 6. 
         FIG. 11  comprises graph (a) showing the birefringence distribution along the radial direction of the end face, and graph (b) showing the birefringence fast axis angle distribution, in synthetic quartz glass in which birefringence was decreased according to the Comparative example. 
         FIG. 12  comprises diagrams to illustrate the definition of birefringence fast axis direction in synthetic quartz glass. 
     
    
    
     MODE OF THE INVENTION 
     The inventive synthetic quartz glass heat treatment method is described in detail below with reference to the figures. 
       FIG. 1  shows a configuration example of a heat treatment oven that can be used for the inventive synthetic quartz glass heat treatment method. 
     Synthetic quartz glass (shaped item of synthetic quartz glass)  100  is heat-treated inside heat treatment oven (air oven)  11 . Synthetic quartz glass  100  to be heat-treated is columnar, having two opposing end faces  101 ,  102 , and lateral face  103 . The present invention is particularly suitable when the synthetic quartz glass  100  to be heat-treated is optical synthetic quartz glass for use in optical systems in exposure apparatus. It is particularly suitable for optical members for optical apparatus that utilize ultraviolet wavelengths of 250 nm or shorter. 
     Oven member  12  which positions synthetic quartz glass  100 , and heater  13  which heats synthetic quartz glass  100 , are provided inside heat treatment oven  11 . When synthetic quartz glass  100  is heat-treated, thermal insulator covers synthetic quartz glass  100 . In the present invention, heat treatment is performed using thermal insulators that differ in at least either type or thickness as end face thermal insulator which covers the two end faces  101 ,  102 , and lateral face thermal insulator which covers lateral face  103 , of synthetic quartz glass  100 , to afford different thermal insulation effects. 
       FIG. 1  depicts when the thermal insulator that covers synthetic quartz glass  100  is a quartz glass container that can house the synthetic quartz glass. The quartz glass container comprises bottom thermal insulator  21 , top thermal insulator  22 , and ring-shaped thermal insulator  23 . Here, the “end face thermal insulator” comprises bottom thermal insulator  21  and top thermal insulator  22 , and the “lateral face thermal insulator” is ring-shaped thermal insulator  23 . The quartz glass container may be synthetic quartz glass or natural quartz glass. Synthetic quartz glass has higher purity and is therefore preferred. In  FIG. 1 , ring-shaped thermal insulator  23  is thicker than top thermal insulator  22  and bottom thermal insulator  21 . Obviously, these thicknesses can be reversed. 
       FIG. 2  depicts the use of packing thermal insulator  34  in the gap between synthetic quartz glass  100  and the inside of the quartz glass container (bottom thermal insulator  31 , top thermal insulator  32 , ring-shaped thermal insulator  33 ) that can house the synthetic quartz glass. Packing thermal insulator  34  can be SiO 2  powder or SiO 2  fibers. The powder or fibers may be synthetic quartz glass, or SiO 2  crystalline powder, primarily crystal powder. Synthetic quartz glass has higher purity and is therefore preferred. Natural quartz glass and SiO 2  crystalline powder that have been obtained at high purity by heating in an HCl atmosphere are also preferred, for the same reason. Here, one “end face thermal insulator” comprises bottom thermal insulator  31  plus the part of packing thermal insulator  34  that covers end face  101  on the underside of synthetic quartz glass  100 , and the other one comprises top thermal insulator  32  plus the part of packing thermal insulator  34  that covers end face  102  on the topside of synthetic quartz glass  100 . Also, “lateral face thermal insulator” is ring-shaped thermal insulator  33  plus the part of packing thermal insulator  34  that covers lateral face  103  of synthetic quartz glass  100 . Thus in  FIG. 2 , the part of packing thermal insulator  34  that covers lateral face  103  of synthetic quartz glass  100  is thicker than the parts that cover end faces  101 ,  102 ; this can be reversed. 
     The synthetic quartz glass birefringence fast axis direction can be regulated by heat-treating as shown in  FIG. 1  or  FIG. 2 . A known temperature program for eliminating warp in synthetic quartz glass can be used as the heat treatment temperature program (temperature profile). 
     More specifically, the synthetic quartz glass birefringence fast axis direction can be regulated in the end face radial direction by making the thermal insulation effect of the end face thermal insulator smaller than the thermal insulation effect of the lateral face thermal insulator. Also, the synthetic quartz glass birefringence fast axis direction can be regulated in the end face tangential direction by making the thermal insulation effect of the end face thermal insulator greater than the thermal insulation effect of the lateral face thermal insulator. It is not necessarily clear why it is possible to regulate synthetic quartz glass birefringence fast axis direction by adjusting the thermal insulation effect of the thermal insulators that cover the synthetic quartz glass; the following is inferred. Generally, quartz glass shrinks in volume (increases in density) if cooled rapidly, and increases in volume (decreases in density) if cooled slowly, from a temperature zone below a certain temperature. If there is a smaller thermal insulation effect at the end faces and a greater thermal insulation effect at the lateral face of the synthetic quartz glass, the cooling from the end face part predominates, and so cooling in the radial direction is slow, stress acts to expand at each point in the radial direction, and the birefringence fast axis is distributed readily in the radial direction. Conversely, if there is a greater thermal insulation effect at the end faces and a smaller thermal insulation effect at the lateral face of the synthetic quartz glass, the cooling from the lateral face part predominates and so there is rapid cooling from the lateral part, with the result that stress acts to contract at each point in the radial direction, and the birefringence fast axis is distributed readily in the tangential direction. 
     The proportion of thermal insulation effect due to end face thermal insulator to thermal insulation effect due to lateral face thermal insulator is preferably around 2-5 times. However, it can also be adjusted to outside this range if the birefringence value (phase difference) is high. 
     The birefringence fast axis direction in synthetic quartz glass  100  is described with reference to  FIG. 3 .  FIG. 3  is a diagram of an end face of synthetic quartz glass  100 . The “end face radial direction”, also referred to as the “radiation direction”, is the direction from the end face center O to the evaluation point P at which the birefringence fast axis is evaluated. The “end face tangential direction”, also referred to as the “concentric direction”, is the direction orthogonal to the radial direction of the end face at evaluation point P. 
     In  FIG. 3 , angle range A is the range from −45° to +45° to the end face radial direction. This range is in directions near the radial direction. Angle range B, however, is in the direction from −45° to +45° to the end face tangential direction that is, from +135° to +45° or from −45° to −135° to the radial direction). This range is in directions near the tangential direction.  FIG. 3  shows an example where angle α formed between the birefringence fast axis direction and the radial direction is in angle range A (from −45° to +45° to the radial direction). 
     With the present invention, a plurality of synthetic quartz glass can be heat-treated simultaneously using the same heat treatment oven by preparing a plurality of synthetic quartz glass and adopting the abovementioned synthetic quartz glass heat treatment method. Heat-treating a plurality of synthetic quartz glass simultaneously affords good manufacturing process efficiency. 
     With the present invention, also, a plurality of synthetic quartz glass having different birefringence fast axis directions can be created even when the plurality of synthetic quartz glass is heat-treated simultaneously using the same heat treatment oven. Specifically, when a plurality of synthetic quartz glass is heat-treated simultaneously using the same heat treatment oven, the heat treatment is performed with end face thermal insulator and lateral face thermal insulator for at least one synthetic quartz glass (synthetic quartz glass a) arranged so as to have thermal insulation effects that differ from those of end face thermal insulator and lateral face thermal insulator for the other synthetic quartz glass (synthetic quartz glass b). Thus the birefringence fast axis direction of synthetic quartz glass a can be regulated to a different direction to that of synthetic quartz glass b. This allows synthetic quartz glass having two birefringence fast axis directions to be manufactured in the same oven in the same batch, which is efficient. 
     Embodiments 
     The present invention is described in more detail below with reference to embodiments of the present invention and comparative examples; the present invention is not limited by these. 
     Conditions Common to Embodiments 1 to 4 
     A lump of the required weight was cut from quartz glass parent material of external diameter 150 mm or greater, and melted, compressed and shaped using a carbon mould to obtain smooth, columnar synthetic quartz glass  100  of dimensions external diameter 350 mm, thickness 70 mm. The resulting synthetic quartz glass  100  was heat-treated (annealing treatment) in heat treatment oven (air oven)  11  provided with heater  13 , etc. as shown in  FIG. 2 , under different conditions according to Embodiments 1 to 4. 
     Quartz glass containers that could house the smooth columnar synthetic quartz glass were used in this annealing treatment. These comprised disk-shaped bottom thermal insulator (bottom disk)  31 , identical disk-shaped top thermal insulator (top disk)  32 , and cylindrical ring-shaped thermal insulator  33 , and were of various sizes as required according to the treatment conditions. The respective thermal insulators were used at thicknesses of 20 mm (bottom thermal insulator  31 ), 20 mm (top thermal insulator  32 ) and 20 mm (ring-shaped thermal insulator  33 ). 
     Also, SiO 2  powder (Iota powder manufactured by Unimin, particle size: to 400 μm, Al: to 30 ppm) was used as packing thermal insulator  34  in the gap between synthetic quartz glass  100  and the quartz glass container. 
     The temperature program shown in  FIG. 4  was used as the heat treatment temperature program. Specifically, this involved increasing the temperature to 1200° C. over a 10-hour period from the start of heating, then keeping the temperature at 1200° C. for 50 hours, and then decreasing the temperature to 1000° C. over a 100-hour period, after which heating using the heater was stopped, and the system was allowed to cool. 
     Embodiment 1 
     Synthetic quartz glass  100  of external diameter 350 mm and thickness 70 mm was housed in quartz glass ring-shaped thermal insulator  33  of internal diameter 540 mm, and quartz powder was packed to a thickness of 2 mm in the gap between synthetic quartz glass  100  topside end face (top face)  102  and top thermal insulator  32 , to a thickness of 2 mm in the gap between underside end face (bottom face)  101  and bottom thermal insulator  31 , and to a thickness of 95 mm in the gap between lateral face  103  and ring-shaped thermal insulator  33 . As in heat treatment oven (air oven)  11  shown in  FIG. 2 , this was placed upon oven member  12  in the heat treatment oven and subjected to the abovementioned heat treatment. 
     Embodiment 2 
     Synthetic quartz glass  100  of external diameter 350 mm and thickness 70 mm was housed in quartz glass ring-shaped thermal insulator  33  of internal diameter 390 mm, and quartz powder was packed to a thickness of 20 mm in the gap between synthetic quartz glass  100  topside end face  102  and top thermal insulator  32 , to a thickness of 20 mm in the gap between underside end face  101  and bottom thermal insulator  31 , and to a thickness of 20 mm in the gap between lateral face  103  and ring-shaped thermal insulator  33 . As in heat treatment oven (air oven)  11  shown in  FIG. 2 , this was placed upon oven member  12  in the heat treatment oven and subjected to the abovementioned heat treatment. 
     After heat treatment, the birefringence of Embodiment 1 and Embodiment 2 synthetic quartz glass  100  were measured using a birefringence measurement instrument (light source of wavelength 633 nm) manufactured by Uniopt, which afforded birefringence values (phase differences) as shown in the graphs of  FIG. 5( a )  (Embodiment 1) and  FIG. 6( a )  (Embodiment 2), respectively. The graphs show the birefringence values at each location as signed absolute values. The sign denotes the direction of the fast axis. A positive value denotes an angle of +45° to −45° to the radial direction (angle range A, radial direction in  FIG. 3 ), and a negative value denotes an angle of +135° to +45°, or −45° to −135°, to the radial direction (angle range B, radial direction in  FIG. 3 ). Also, the birefringence fast axis angle distribution in synthetic quartz glass  100  of Embodiment 1 and Embodiment 2 are shown in the graphs of  FIG. 5( b )  (Embodiment 1) and  FIG. 6( b )  (Embodiment 2), respectively. 
       FIG. 12  comprises diagrams to illustrate the definition of birefringence fast axis direction in one synthetic quartz glass  101 .  FIG. 12( a )  shows the aspect viewed from the top face of synthetic quartz glass  101 .  FIG. 12( b )  is an expanded view showing one radial direction from center O, magnified. As shown in  FIG. 12( a ), ( b ) , when the synthetic quartz glass  101  radial direction birefringence is measured, the birefringence measurement points in one radial direction through center O are a 1 , a 2 , . . . , a i , . . . , a n  from the center measurement. 
     The birefringence fast axis direction at measurement point a 1  is defined with reference to  FIG. 12( c ) . For convenience, at measurement point a 1  the radial direction (radiation direction) is labelled direction X and the tangential direction (concentric direction) is labelled direction Y, as shown in  FIG. 12( c ) . The fast axis direction is at angle θ i  to direction X. Here, if the size of the birefringence (phase difference) is Re i , birefringence at measurement point a 1  can be split into its radial direction component and tangential direction component as follows.
 
Radial direction component= Re   i ·cos θ i  
 
Tangential direction component= Re   i ·sin θ i  
 
     The birefringence fast axis direction in one synthetic quartz glass  101  is defined by comparing the mean values of said radial direction component and tangential direction component measured at each measurement point in the radial direction a 1 , a 2 , . . . , a i , . . . , a n . 
     In one synthetic quartz glass  101 , the mean value of the radial direction component of the birefringence size is expressed as follows. 
     
       
         
           
             
               
                 
                   
                     
                       ∑ 
                       
                         i 
                         = 
                         1 
                       
                       n 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         
                           Re 
                           i 
                         
                         · 
                         cos 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         θ 
                         i 
                       
                     
                   
                   n 
                 
               
               
                 
                   [ 
                   
                     Mathematical 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     In one synthetic quartz glass  101 , the mean value of the tangential direction component of the birefringence size is expressed as follows. 
     
       
         
           
             
               
                 
                   
                     
                       ∑ 
                       
                         i 
                         = 
                         1 
                       
                       n 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         
                           Re 
                           i 
                         
                         · 
                         sin 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         θ 
                         i 
                       
                     
                   
                   n 
                 
               
               
                 
                   [ 
                   
                     Mathematical 
                     ⁢ 
                     
                         
                     
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                     expression 
                     ⁢ 
                     
                         
                     
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                     2 
                   
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     Said mean values are compared, and when the ratio is 2:1 or higher, the fast axis direction of synthetic quartz glass  101  is defined as the 2 or greater component in the ratio. 
     Calculating in this manner, in Embodiment 1 the radial direction component mean value is 0.29 nm/cm and the tangential direction component mean value is 0.05 nm/cm, thus the radial direction is the birefringence fast axis direction of the heat-treated synthetic quartz glass. In Embodiment 2 the radial direction component mean value is 0.03 nm/cm and the tangential direction component mean value is 0.23 nm/cm, thus the tangential direction is the birefringence fast axis direction of the heat-treated synthetic quartz glass. 
     It should be noted that although birefringence measured at wavelength 633 nm does not correspond exactly to birefringence in ultraviolet light, the birefringence fast axis direction tends to be the same. When using light of shorter wavelength than the measurement wavelength, as in ultraviolet light, the birefringence phase difference tends to be greater, but the fast axis direction does not change. 
     In synthetic quartz glass  100  heat-treated in Embodiment 1 and Embodiment 2 it was possible to regulate the birefringence fast axis direction by means of one heat treatment without particular limitation on the OH group concentration distribution in the starting material. 
     Embodiment 1 and Embodiment 2 show that it is possible to treat a plurality of synthetic quartz glass  100  under different thermal insulation conditions in the same oven, in the same batch; they also show that two types of synthetic quartz glass  100  having different fast axis directions can be obtained. 
     Embodiment 3 
     Synthetic quartz glass  100  of external diameter 350 mm and thickness 70 mm was housed in quartz glass ring-shaped thermal insulator  33  of internal diameter 590 mm, and quartz powder was packed to a thickness of 2 mm in the gap between synthetic quartz glass  100  topside end face  102  and top thermal insulator  32 , to a thickness of 2 mm in the gap between underside end face  101  and bottom thermal insulator  31 , and to a thickness of 120 mm in the gap between lateral face  103  and ring-shaped thermal insulator  33 . As in heat treatment oven (air oven)  11  shown in  FIG. 2 , this was placed upon oven member  12  in the heat treatment oven and subjected to the abovementioned heat treatment. 
     Next, birefringence distribution and birefringence fast axis angle distribution were measured as in Embodiment 1 and Embodiment 2; the results were as shown in  FIG. 7( a ), ( b ) . The mean values for all measurement points were radial direction component 0.65 nm/cm, tangential direction component 0.04 nm/cm, thus the radial direction was the birefringence fast axis direction. 
     Embodiment 4 
     Synthetic quartz glass  100  of external diameter 350 mm and thickness 70 mm was housed in quartz glass ring-shaped thermal insulator  33  of internal diameter 360 mm, and quartz powder was packed to a thickness of 20 mm in the gap between synthetic quartz glass  100  topside end face  102  and top thermal insulator  32 , to a thickness of 20 mm in the gap between underside end face  101  and bottom thermal insulator  31 , and to a thickness of 5 mm in the gap between lateral face  103  and ring-shaped thermal insulator  33 . As in heat treatment oven (air oven)  11  shown in  FIG. 2 , this was placed upon oven member  12  in the heat treatment oven and subjected to the abovementioned heat treatment. 
     Next, birefringence distribution and birefringence fast axis angle distribution were measured as in Embodiment 1 and Embodiment 2; the results were as shown in  FIG. 8( a ), ( b ) . The mean values for all measurement points were radial direction component 0.05 nm/cm, tangential direction component 0.70 nm/cm, thus the tangential direction was the birefringence fast axis direction. 
     Embodiment 5 
     First, a lump of the required weight was cut from quartz glass parent material of external diameter 150 mm or greater, and melted, compressed and shaped using a carbon mould to obtain smooth, columnar synthetic quartz glass  100  of dimensions external diameter 350 mm, thickness 70 mm. The resulting synthetic quartz glass  100  was heat-treated (annealing treatment) in heat treatment oven (air oven)  11  provided with heater  13 , etc. as shown in  FIG. 1 . 
     Synthetic quartz glass  100  of external diameter 350 mm and thickness 70 mm was housed in quartz glass ring-shaped thermal insulator  23  of internal diameter 350 mm, external diameter 530 mm, top thermal insulator  22  was used at a thickness of 5 mm to cover the topside end face of synthetic quartz glass  100 , and bottom thermal insulator  21  was used at a thickness of 5 mm to cover the underside. Next, this was placed upon oven member  12  in heat treatment oven (air oven)  11  shown in  FIG. 1 , and heat-treated using the temperature program shown in  FIG. 4 . Specifically, this involved increasing the temperature to 1200° C. over a 10-hour period from the start of heating, then keeping the temperature at 1200° C. for 50 hours, and then decreasing the temperature to 1000° C. over a 100-hour period, after which heating using the heater was stopped, and the system was allowed to cool. 
     Next, birefringence distribution and birefringence fast axis angle distribution were measured as in Embodiment 1 and Embodiment 2; the results were as shown in  FIG. 9( a ), ( b ) . The mean values for all measurement points were radial direction component 0.39 nm/cm, tangential direction component 0.04 nm/cm, thus the radial direction was the birefringence fast axis direction. 
     Embodiment 6 
     Heat treatment was performed as in Embodiment 5 except that the thickness of top thermal insulator  22  was changed. Specifically, synthetic quartz glass  100  of external diameter 350 mm and thickness 70 mm was housed in quartz glass ring-shaped thermal insulator  23  of internal diameter 350 mm, external diameter 390 mm, and top thermal insulator  22  was used at a thickness of 20 mm to cover the topside end face of synthetic quartz glass  100 , and bottom thermal insulator  21  was used at a thickness of 20 mm to cover the underside. Next, this was placed upon oven member  12  in heat treatment oven (air oven)  11  shown in  FIG. 1  and heat-treated using the temperature program shown in  FIG. 4 . 
     Next, birefringence distribution and birefringence fast axis angle distribution were measured as in Embodiment 1 and Embodiment 2; the results were as shown in  FIG. 10( a ), ( b ) . The mean values for all measurement points were radial direction component 0.06 nm/cm, tangential direction component 0.37 nm/cm, thus the tangential direction was the birefringence fast axis direction. 
     The conditions and results for Embodiments 1 to 6 are shown in Table 1. In Table 1, mm is the unit of the numerical values. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                   
                   
                   
                 Ring- 
                   
                   
               
               
                   
                 Synthetic 
                 Top 
                 Bottom 
                 shaped 
                 SiO 2  powder 
                 Birefringence 
               
               
                   
                 quartz glass 
                 thermal 
                 thermal 
                 thermal 
                 thickness 
                 fast 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Embodiment 
                 Outer 
                   
                 insulator 
                 insulator 
                 insulator 
                   
                   
                 Outer 
                 axis 
               
               
                 number 
                 diameter 
                 Thickness 
                 thickness 
                 thickness 
                 thickness 
                 Top 
                 Bottom 
                 circumference 
                 direction 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 350 
                 70 
                 20 
                 20 
                 20 
                 2 
                 2 
                 95 
                 Radial 
               
               
                 2 
                 350 
                 70 
                 20 
                 20 
                 20 
                 20 
                 20 
                 20 
                 Tangential 
               
               
                 3 
                 350 
                 70 
                 20 
                 20 
                 20 
                 2 
                 2 
                 120 
                 Radial 
               
               
                 4 
                 350 
                 70 
                 20 
                 20 
                 20 
                 20 
                 20 
                 5 
                 Tangential 
               
               
                 5 
                 350 
                 70 
                 5 
                 5 
                 90 
                 — 
                 — 
                 — 
                 Radial 
               
               
                 6 
                 350 
                 70 
                 20 
                 20 
                 20 
                 — 
                 — 
                 — 
                 Tangential 
               
               
                   
               
            
           
         
       
     
     Comparative Example 
     As in Embodiments 1 to 4, a lump of the required weight was cut from quartz glass parent material of external diameter 150 mm or greater, and melted, compressed and shaped using a carbon mould to obtain smooth, columnar synthetic quartz glass of dimensions external diameter 350 mm, thickness 70 mm. This synthetic quartz glass was insulated using a quartz glass container and quartz glass powder with no particular regulation of the insulation effects, and heat-treated as commonly performed to increase homogeneity and decrease birefringence. 
     Next, birefringence distribution and birefringence fast axis angle distribution were measured as in Embodiment 1; the results were as shown in  FIG. 11( a ), ( b ) . Specifically, the birefringence value decreased overall and synthetic quartz glass of high homogeneity was obtained, but in the radial direction the birefringence fast axis direction (sign) changed many times even outside the central region, thus the birefringence fast axis distribution was not regulated. 
     It should be noted that the present invention is not limited by the abovementioned mode. The abovementioned mode is merely an example; anything having essentially the same configuration as the technical idea described in the scope of the patent claims of the present invention that achieves the same function effect is included in the technical scope of the present invention. 
     LEGEND 
       11  Heat treatment oven 
       12  Oven member 
       13  Heater 
       21 ,  31  Bottom thermal insulator (end face thermal insulator) 
       22 ,  32  Top thermal insulator (end face thermal insulator) 
       23 ,  33  Ring-shaped thermal insulator (lateral face thermal insulator) 
       34  Gap-packing thermal insulator 
       100  Synthetic quartz glass 
       101  End face (bottom face) 
       102  End face (top face) 
       103  Lateral face