Abstract:
A method of making a silica crucible in a mold cavity of the type in which air is drawing through silica grain placed in the mold cavity. A pure silica grain layer is formed on top of a natural silica grain layer. At least a portion of the purse silica grain layer is fused while substantially no air is drawn through the silica grain. Any remaining pure silica grain and a least a portion of the natural silica grain layer is fused while drawing a substantially higher volume of air through the silica. At least a portion of the fused pure silica grain layer is then sublimated.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. Provisional Application Ser. No. 61/139,405, filed on Dec. 19, 2008. 
    
    
     BACKGROUND 
     1. Field of Invention 
     Embodiments of the present invention generally relate to silica crucibles and methods of making the same. More particularly, embodiments of the present invention relate to silica crucibles with highly pure and bubble free inner crucible layers and methods of making the same. 
     2. Discussion of the Related Art 
     There are several methods for making single crystal silicon materials. One such process—the “Czochralski” (CZ) process—has been widely adopted for producing single crystal silicon materials for semiconductor applications including solar cell applications. In the CZ process, molten silicon is contained within a vessel and a tip of a single-crystalline seed crystal is dipped into the molten silicon. The seed crystal is then “pulled” while being rotated. As a result, a single crystal silicon ingot is grown from the molten silicon. 
     A crucible is one vessel commonly employed in this pulling operation for making silicon ingots. Crucibles are typically configured in a bowl shape to contain the molten silicon during the pulling operation. Crucibles are made of silica glass, or quartz glass, to introduce a desired amount of oxygen into the molten silicon while keeping if from contamination with metallic impurities. 
     During the CZ process, the inner surface of a silica glass crucible typically dissolves into the molten silicon. Any bubbles present in the inner crucible layer may open to the melt by dissolution and are a potential source of particles that may disturb the single-crystalline structure of the silicon ingot. Therefore, crucibles are typically manufactured to have a bubble-free inner crucible layer. The outer crucible layers are typically manufactured to contain numerous tiny bubbles to facilitate scattering of radiation from heater elements. 
     To minimize the possibility of contamination of the silicon ingot during the CZ process, the inner crucible layer should be as pure as possible. 
     Silica glass made from natural quartz grain often contains an Al content of 5-20 ppm. The Al element in the silica glass is often associated with positively charged metallic impurities such as Li, Ca and Na. Therefore, reduction of Al content is desirable to minimize concentration of other metallic elements. 
     One known method of manufacturing silica glass crucibles or quartz glass crucibles, is to form silica grain, which can be synthetic silica grain or natural quartz grain, in a crucible shape inside of a rotating mold, then fuse it into a silica glass crucible. The crucible is taken out of the mold and mechanically finished and cleaned to a final product. 
     During fusion, silica sublimates from the inner surface of the inner crucible layer. As silica sublimates faster than alumina, when natural quartz grain is used as the starting material, aluminum is left on the inner surface of the inner crucible layer. As mentioned above, aluminum is associated with other metallic impurities. 
     One method disclosed in Japanese Patent JP 63-166791, is understood to minimize the Al content at the inner surface of the inner crucible layer by etching the surface of the inner crucible layer with hydrofluoric acid. This method, however, is not economical or environmentally preferable due to the use of hydrofluoric acid. 
     Another known method of manufacturing quartz glass crucibles, disclosed in U.S. Pat. No. 6,510,707, is understood to make the inner crucible layer bubble-free with little bubble growth and reduce the Al content at the inner surface of the inner crucible layer. A crucible is fused by employing a relatively large amount of electrical power to reduce bubble and bubble growth leaving an aluminum-rich layer on the surface of the inner crucible layer. The aluminum-rich layer is then removed by abrasive processes or etching processes. While this method can produce an inner crucible layer that may be relatively bubble-free with little bubble growth during the CZ process, abrading or etching the aluminum rich layer is time consuming and not economical. 
     Consequently it would be desirable to prepare a silica crucible having an inner crucible layer with minimal impurity concentration and reduced bubble and bubble-growth at reasonable cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional, somewhat schematic, view of an apparatus capable of producing a crucible in accordance with one embodiment of the present invention. 
         FIGS. 2A and 2B  are cross-sectional, somewhat schematic, views illustrating a method of forming silica grain layers using a feed system in accordance with one embodiment of the present invention. 
         FIG. 3  is a cross-sectional, somewhat schematic, view illustrating a method of fusing silica grain layers using a grain fusing system in accordance with one embodiment of the present invention. 
         FIGS. 4A-4D  are cross-sectional views illustrating a method of fusing a crucible at various stages of a crucible-manufacturing process, according to one embodiment of the present invention. 
         FIGS. 5A-5D  are cross-sectional views illustrating a method of fusing a crucible at various stages of a crucible-manufacturing process, according to another embodiment of the present invention. 
         FIG. 6  shows the impurity profile at the surface of the inner crucible layer, according to one embodiment of the present invention. 
         FIG. 7  shows the impurity profile at the surface of the inner crucible layer, according to another embodiment of the present invention. 
         FIG. 8  shows the impurity profile at the surface of the inner crucible layer, according to a traditional method of manufacturing a crucible. 
         FIG. 9  illustrates a core sample from a crucible and a corresponding Aluminum profile for one process according to an embodiment of the invention and another comparative process. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a cross-sectional, somewhat schematic view, of an apparatus capable of manufacturing a crucible in accordance with one embodiment of the present invention. 
       FIG. 1  is a cross section of a mold  112  with two layers of silica grain formed in the mold ready for fusing. As will be explained, the fusion process includes five stages. 
     Referring to  FIG. 1 , an apparatus  110  for manufacturing silica glass crucible may be provided as described in U.S. Pat. No. 7,383,696, which is incorporated herein by reference in its entirety. For example, the apparatus  110  may include a rotating mold  112  having an inner mold surface  114  that defines a mold cavity. 
     A plurality of air channels, like channels  118  and  120 , communicate with inner mold surface  114 . Each air channel  118  and  120  comprises a cylindrical bore that creates a circular opening, like opening  122  in inner mold surface  114 . Each air channel, like channel  120 , includes a porous graphite plug  126  that prevents silica grain from being drawn from the mold cavity into the air channels. The air channels communicate with manifolds, like manifolds  128 ,  130 ,  132 , which in turn communicate with a bore  134 . An evacuation system  501  is connected to bore  134 . 
     The evacuation system  501  is configured to draw air from the mold cavity via the air channels and ultimately through bore  134  and out of apparatus  110 . The evacuation system is configured for programmable evacuation power, at least for controlling starting and stopping times. In one embodiment, an evacuation pump  502  has a set of control valves  504 , which includes at least a leak valve  503 , which vents to the room atmosphere. A pressure gauge  505  is connected to the evacuation bore  134 . The pressure reading is used as an indication of pressure at the fusion front. 
     In one embodiment, mold  112  contains a natural quartz grain layer  228  and a pure silica grain layer  230  (which are shown partially broken away to expose the inner mold surface  114 ). The natural quartz grain layer  228  and the pure silica grain layer  230  are collectively referred to as a the entire silica grain layer. 
     The procedures to form two layers of silica grain are explained using  FIGS. 2A and 2B .  FIGS. 2A and 2B  are cross-sectional, somewhat schematic views, illustrating a method of forming silica grain layers using a feed system in accordance with one embodiment of the present invention. 
     Referring generally to  FIGS. 2A and 2B , the feed system  210  may, for example, include a natural quartz grain hopper  212  containing natural quartz grain  214 , a pure silica grain hopper  216  containing pure silica grain  218 , valves  220  and  222  and a feed tube  224 . A spatula  226  may also be provided that is vertically movable into and out of mold  112 . The spatula  226  may further be horizontally movable within the mold  112  to shape the silica grain as the mold  112  rotates about the vertical axis  136 . 
     In one embodiment, the natural quartz grain  214  contains impurities such as Al, Ca, Li and K. The natural quartz grain  214  may have, in part, an Al content of about 6 ppm; Ca content of about 1.3 ppm; Li content of about 0.7 ppm; K content about 0.1 ppm; and Na content of about 0.05 ppm. 
     In one embodiment, the pure silica grain  218  is synthesized from silicon tetra chloride purified through a distillation processes. In one embodiment, the pure silica grain may have a total metallic content of less than 1 ppm, with an Al content of less than 0.2 ppm. 
     The flow of natural quartz grain  214  and pure silica grain  218  (collectively referred to as “silica grain”) from grain hoppers  212  and  216  may be controlled by regulating valves  220  and  222 , respectively. The feed tube  224  introduces the flow of silica grain into mold  112  from either one of the hoppers, depending upon how valves  220  and  222  are set. 
     Referring to  FIG. 2A , the mold  112  is rotated about the vertical axis  136  at a rate of about 70 rpm to about 150 rpm, the feed tube  224  is positioned adjacent to the inner mold surface  114 , and valve  220  is opened to begin depositing natural quartz grain  214  to form a natural quartz grain layer  228  about the perimeter of the mold cavity. The thickness of the natural quartz grain layer  228  (measured along a radial axis of the mold  112 ) is defined by the position of spatula  226 . In one embodiment, the thickness of the natural quartz grain layer  228  may have a range of about 15 mm to about 30 mm. 
     Referring to  FIG. 2B , after the natural quartz grain layer  228  is formed, valve  220  is closed and valve  222  is opened while the mold  112  is rotated about the vertical axis  136 . As pure silica grain  218  feeds from the pure silica grain hopper  216  out of feed tube  224  a pure silica grain layer  230  coats the inner surface of the natural quartz grain layer  228 . In one embodiment, the thickness of the pure silica grain layer  230  may have a range of about 1 mm to about 5 mm. In one embodiment, the thickness of the entire silica grain layer, which is the combined thickness of the natural and pure silica grain layers  228  and  230 , may be about 25 mm to 30 mm. 
     After forming the pure silica grain layer  230 , the feed system  210  and the spatula  226  are moved out of the rotating mold  112  so that the entire silica grain layer may be fused to form a crucible. 
       FIG. 3  is a cross-sectional, somewhat schematic view, illustrating a method of fusing an entire silica grain layer using a grain fusing system  310  in accordance with one embodiment of the present invention. The grain fusing system has a programmable power supply  316  and electrodes  312  and  314  connected to the power supply. In one embodiment of this invention the programmable range of the power supply  316  is from 50 kVA to 1200 kVA. 
     Referring to  FIG. 3 , after the feed system  210  is withdrawn from the rotating mold  112 , a grain fusing system  310  is moved into the cavity of the rotating mold  112  to fuse the entire silica grain layer. Hot arc plasma  318  fuses the entire silica grain layer thereby forming a crucible  320 . Crucible  320  has layers  322 ,  324  that do not necessarily correspond to grain layers  228 ,  230  (in  FIG. 2B ). Rather layers  322 ,  324  are formed as a result of different steps in the following described process. As will be seen, layer  322  may include part of pure silica layer  230 , all of layer  230 , or all of layer  230  and part of natural quartz grain layer  230 . 
     The evacuation system  501  is operated as programmed and gas is drawn from the interior of the rotating mold  112 , through the entire silica grain layer into the air channels, like air channels  118  and  120 . Generally, fusion of the entire silica grain layer starts at the innermost surface. The fusion front then moves outwards until it stops, leaving a thin unfused grain layer  326  between crucible  320  and inner mold surface  114 . 
     The fusion process can be divided into five stages defined by variations among fusion power, fusion front location and vacuum at the fusion front.  FIGS. 4A-4D  illustrate successive steps in the process with the unhatched portions representing unfused grain and the hatched portions representing different layers of fused grain. A first general description of the five stages is provided with reference to Table 1 and  FIGS. 4A-4D  followed by a more detailed description of the process in accordance with the Table 1 parameters. 
     Referring to Table 1, the first stage is starting of skin formation. Skin  402  is formed with bore  134 substantially at atmospheric pressure. In one embodiment of this invention, the evacuation system  501  is stopped by opening the leak valve  503  to the air while running the pump  502 . In this embodiment, evacuation system  501  is not activated at this skinning stage because evacuation may be harmful to uniformity of the skin. Normally the mold is designed to locate air channels, like  118  and  120 , where high levels of bubble-free characteristics and less-bubble-growth characteristics are required. These are typically at the corner radius and bottom of the crucible, because when the formed crucible is used in the CZ process, duration of the contact with silicon melt is significant there. If the evacuation system is activated before a skin is formed, hot plasma gas is directed to the air channels. As the result, thick skin is formed at the air channel positions, and thinner skin is formed elsewhere. This produces a skin of uneven thickness, which is difficult to remove by sublimation or otherwise. In addition, a thin uniform skin helps to lower the pressure at the fusion front at the air channel location. 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 (Crucible A) 
               
             
          
           
               
                   
                   
                   
                   
                 fused 
                   
                   
                   
               
               
                   
                   
                   
                   
                 grain/ 
                 pressure 
               
               
                   
                 Electric 
                 Evacuation 
                   
                 fusion 
                 at fusion 
               
               
                   
                 Power 
                 Power 
                 event 
                 front 
                 front 
                 sublimation 
                 comment 
               
               
                   
                   
               
             
          
           
               
                 1st. 
                 100 
                 5 
                 skin start 
                 pure 
                 5 
                 negligible 
                 skin at 
               
               
                 stage 
                   
                   
                 evenly 
                 silica/ 
                   
                   
                 atmospheric 
               
               
                   
                   
                   
                   
                 slow 
                   
                   
                 pressure 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Skin is even in 
               
               
                   
                   
                   
                   
                   
                   
                   
                 thickness 
               
               
                 2nd. 
                 100 
                 100 
                 skin 
                 pure 
                 100 
                 negligible 
                 skin complete 
               
               
                 stage 
                   
                   
                 complete 
                 silica/ 
                   
                   
                 under vacuum 
               
               
                   
                   
                   
                   
                 slow 
               
               
                 3rd. 
                 600 
                 100 
                 bubble 
                 natural 
                 100 
                 medium 
                 clear layer formed 
               
               
                 stage 
                   
                   
                 free layer 
                 quartz/ 
                   
                   
                 mainly from 
               
               
                   
                   
                   
                   
                 fast 
                   
                   
                 natural quartz 
               
               
                   
                   
                   
                   
                   
                   
                   
                 grain 
               
               
                 4th. 
                 600 
                 30 
                 opaque 
                 natural 
                 30 
                 medium 
                 opaque layer 
               
               
                 stage 
                   
                   
                 layer 
                 quartz/ 
                   
                   
                 formed 
               
               
                   
                   
                   
                   
                 fast 
               
               
                 5th. 
                 850 
                 30 
                 skin 
                 natural 
                 30 
                 high 
                 pure silica skin 
               
               
                 stage 
                   
                   
                 removal 
                 quartz/ 
                   
                   
                 sublimated, 
               
               
                   
                   
                   
                   
                 almost 
                   
                   
                 small portion of 
               
               
                   
                   
                   
                   
                 stop 
                   
                   
                 natural layer can 
               
               
                   
                   
                   
                   
                   
                   
                   
                 be sublimated also 
               
               
                   
               
             
          
         
       
     
     One sophisticated method to manipulate the location of skinning is to provide another set of air channels (not shown in these embodiments) other than those air channels prepared for bubble free areas, and to accelerate skinning by evacuating through these channels. But stopping air flow through the air channels during skin formation while controlling the power and position of the electric arc is practically sufficient and economical to produce a uniform and thin skin. 
     The second stage is completion of skin and starting of evacuation. Most of the surface of the grain layer is covered by a skin  402 , so that pressure at the fusion front can be lowered at this stage by activating the evacuation system  501 . Because the skin is not perfect especially at the upper portions of the mold, pressure at the fusion front is strongly related to the “leakage” through the unfused grain between the skin  402  and the inner mold surface  114 . 
     During the first and the second stages, fusion power is controlled at the required minimum to form a skin, otherwise skin  402  will become too thick to be sublimated off later. The skin  402  after activating evacuation system  501  can be free of bubbles but would show bubble-growth during CZ process. This is probably because silica glass fused at low power is not well degassed and therefore shows bubble-growth at high temperatures like the CZ process. In one embodiment, the evacuation system is activated by closing the leak valve  503 . In this embodiment, fusion power is increased as soon as the evacuation system is activated. In one embodiment of this invention, fusion power was increased 10 seconds after evacuation system was activated. 
     The third stage fuses a bubble free layer under the skin  402  by increasing fusion power. The evacuation system  501  has been activated at the end of stage two. Now, the relatively long distance from the upper surfaces of the entire silica grain layer to the first openings increases the resistance to pump flow. This, in turn, increases the pressure drop between the atmosphere above the upper surfaces of the entire silica grain layer and the manifolds  128 ,  130  and  132 . As a result, the pressure at the fusion front is reduced and gas is rapidly drawn away from the fusion front and into the manifolds  128 ,  130  and  132  by the pump  502 . In addition to good skin coverage, which assures a good vacuum level, high power fusion is required to reduce bubble growth when the crucible will be used in the CZ process. High power fusion is presumed to help degassing the fused silica glass. In one embodiment, electric power to the electrode is increased to 600 kVA at the third stage. 
     Depending how thick the pure silica grain is, a bubble-free layer can be pure silica glass or a combination of pure silica glass formed from grain layer  230  and natural quartz glass formed from grain layer  228 . This bubble-free layer constitutes inner crucible layer  322 . 
     The fourth stage fuses opaque outer crucible layer  324  outside of the inner crucible layer  322 . By reducing evacuation power, bubbles can be introduced at the fusion front. To control bubble size and bubble amount, and also to reduce bubble growth, controlled vacuum and relatively high power are required at this stage. In one embodiment, a set of the controlling valves  504  is operated to keep 680 Torr. at the pressure gauge  505  while applying 600 kVA to the electrodes. 
     The fifth stage of fusion is removal of the above mentioned skin  402 . As the skin  402  was fused at low fusion power and mainly at atmospheric pressure, the skin  402  is not bubble-free and those bubbles would grow during a CZ process. This skin  402  therefore has to be removed. The best method to remove the skin is sublimation of the silica glass that forms the skin. In one embodiment of the present invention, electric power was boosted by 42%, from 600 kVA to 850 kVA. The power level and boosted step can be altered depending on the apparatus configurations, such as shielding, efficiency of power supply and ventilation in the crucible interior. 
     The fusion front almost stops at a saturated point leaving some unfused grain  326  between mold  112  and crucible  320  at the end of stage four. 
     This fifth stage can be combined to or overlapped with the third and/or fourth stage, because some amount of sublimation normally occurred at these stages. However, an additional stage removes the skin thoroughly. The fifth stage is preferably designed for sublimation only, minimizing progress of the fusion front. Another reason to sublimate at the final stage is to prevent deposits of sublimated material on the electrodes, which then drop back into the crucible resulting in “fume-deposit” defects on the crucible. The fifth stage, or sublimation stage, is required to minimize aluminum build up left after sublimation of the skin, which naturally occurs during the earlier stages. Removing the skin requires more intense sublimation than would normally occur. In one embodiment, pure silica glass corresponding to 0.8 mm to 1.2 mm thickness of pure silica grain layer is used to make the skin that is then later sublimated. This layer is fused from grain that is in pure silica grain layer  230  in  FIG. 2B . The design of the final crucible structures, including the bubble free layer, purity profile in the crucible layers, grain forming, fusion power program and evacuation programs may be customized. 
     There is a conflict between better sealing and better purity at the innermost layer. Thicker skin helps to form a better clear layer underneath, but thinner skin leaves less impurity leftover after the skin is sublimated. In one embodiment, a layer of pure silica grain of about 0.8 mm to 1.2 mm was used to form the skin. After taking off the skin by sublimation, the impurity profile is improved over the prior art. When, instead of pure silica grain, natural quartz grain was used for the skin, the impurity profile was stronger after sublimation. 
     The innermost layer of pure silica grain can be thicker than that required for skin formation. After sublimation, the crucible may a have bubble free and bubble-growth free pure silica layer on the natural quartz glass layer. Depending on the thickness of the pure silica grain and when evacuation power is reduced, bubble structure of the final crucibles can be finely controlled to desired qualities and depths. 
     In one embodiment, the inner half of the bubble free layer was made from pure silica grain and the outer half of the bubble free layer and the opaque outer layer were made from natural quartz glass ( FIGS. 5A-5D ). 
     The effects of a CZ process on the crucible  320  can be estimated by performing a vacuum bake test simulating a CZ process. For example, a vacuum bake test can include heating the crucible piece at about 1650° C. for three hours at a pressure of 0.1 Pa. argon atmospheres. 
     The crucibles according to the present invention do not have localized bubble containing areas or areas that show bubble growth in response to this vacuum bake test, especially on the bottom and around the corner radius. Accordingly, disturbances in the single-crystalline structure of a silicon ingot pulled during a CZ process attributable to particle generation as a result of bubble dissolution from the inner crucible layer  322 , are prevented. 
     An exemplary method of forming the above-described inner and outer crucible layers  322  and  324  will now be described in more detail with respect to  FIGS. 4A-4D  and Table 1. 
       FIGS. 4A-4D  are cross-sectional views illustrating a method of fusing a crucible at various stages of a crucible-fusion process, according to one embodiment of the present invention. Table 1 is a summary of the five stages according to the above mentioned embodiment of the present invention. 
     In Table 1, electric power to the arc is shown in units of kVA. The evacuation power is shown in arbitrary units. The value  100  corresponds to the reading of gauge  505  with vacuum pump operation at 800 m 3 /hr. Main events at each the stages are described in the table. The grain to be fused at each stage is described along with the qualitative speed of fusion front progression. The vacuum level or pressure at the fusion front is shown using arbitrary units. Because the pressure is not uniform for the whole fusion front, the numbers are understood as relative numbers. On the low end, the value  5  is almost atmospheric pressure at the gauge  505  and the value  100  is about 150 Torr. at the gauge  505 . Amount of sublimation and comments are also given in the table. 
     Referring to  FIG. 4A  and Table 1, in the first stage of the crucible-fusion process, a portion of the pure silica grain layer  230  at the innermost surface is fused to form a skin layer  402 . During stage 1, power applied to the electrodes  312  and  314  may be about 100 kVA or less. In one embodiment, the power applied to the electrodes  312  and  314  may have a range from about 40 kVA to about 80 kVA. During stage 1, the evacuation system  501  may be stopped, such that a pressure measured at gauge  505  is about 760 Torr. 
     This condition continues to stage two where skin covers almost 80% of the final crucible shape. Because the major portion of the skin layer  402  is formed by fusing without evacuation, the skin layer  402  may contain bubbles. Because the skin layer  402  is formed by fusing at relatively low electric power, the skin layer may also show bubble growth in a CZ process. 
     At the end of stage two, the evacuation system  501  is activated. In one embodiment, pump  502  was kept running during stage 1 and stage 2 with the leak valve  503  open to the air so as not to evacuate gas through the bore  134 . For example, the pressure measured at the gauge  505  may be dropped to 150 Torr. after the evacuation system is activated. 
     Referring next to  FIG. 4B  and Table 1, in a third stage of the crucible-fusion process, the remainder of the formed entire silica grain layer that is natural quartz grain layer  228 , is fused to form a bubble free fused silica layer  404   a . During stage 3, power applied to the electrodes  312  and  314  may be about 600 kVA. The evacuation system  501  is kept running to ensure that the natural quartz glass layer  404   a  contains substantially no bubbles. In one embodiment, the duration of stage 3 may correspond to a time sufficient to ensure that the inner layer  322 , which is the same as  404   a  in the case of  FIGS. 4A-D , has a thickness of about 2.0 mm to about 6.0 mm. It will be appreciated that the pump  502  may be operated during stage 3 to evacuate gas at the flow rates in a manner described in U.S. Pat. No. 7,383,696, which is incorporated herein by reference in its entirety, to form the inner layer  322 . 
     Some of the skin layer  402  may be sublimated during stage 3, but the amount is relatively small. 
     Referring next to  FIG. 4C  and Table 1, the fourth stage of the crucible-fusion process, a remaining portion of the entire silica grain layer, which is natural quartz grain layer in this case, is fused to form opaque outer crucible layer  324 . In the case of  FIGS. 4A-D , the opaque natural quartz glass layer composes the outer layer  324 . The evacuation power is reduced to introduce a sufficient amount of bubbles while assuring fusion power is large enough to reduce bubble growth during the CZ process, in a manner described in U.S. Pat. No. 7,383,696. At the end of stage 4, the fusion front is almost stopped at the equilibrium position between heating by electric arc from inside and cooling resulting from contact with cold wall of the mold. In one embodiment of this invention, the fusion front is nearly stopped when power is removed leaving 2 mm to 3 mm unfused grain  326  between fused crucible  320  and an inner mold surface  114 . 
     Referring next to  FIGS. 4D  and Table 1, the fifth stage of the crucible-fusion process is to eliminate the skin  402  by sublimation. The fusion power is applied not for fusing the entire silica grain layer to the target dimensions, but mainly for sublimation of the skin  402 . In one embodiment, the power applied to the electrodes  312  and  314  was 850 kVA. 
     Upon removing the skin layer  402  from the inner layer  404   a , about 0.1 mm to about 2.0 mm of the pure silica glass may be sublimated to ensure that the skin layer  402  is sufficiently removed. In one embodiment, about 0.6 mm of the pure fused silica layer  402  is sublimated to ensure that the skin layer  402  is sufficiently removed. In one embodiment, the inner crucible layer  322  has a thickness of about 2.0 mm to about 6.0 mm after removal of the skin  402 . 
     According to the process exemplarily described above in  FIGS. 4A-4D  with Table 1, the pure silica grain is mainly used for skin and is sublimated later, leaving the innermost surface of the crucible inner layer  322  similar to a natural quartz glass.  FIG. 6  shows the impurity profile on the crucible inner layer  322  at the one embodiment of this invention. Three consecutive surface layers with about 30 micrometers in thickness, were etched off and analyzed. The innermost layer was 14 ppm Al, 0.2 ppm Ca, 0.6 ppm Li and less than 0.14 ppm K. 
     It should be appreciated that this is not obvious for the innermost surface to have similar purity as natural quartz grain purity. It is appreciated that the purity level is still an order of magnitude better than the comparative example of traditional method shown in Table 4. This will be explained more detail later as a comparative example with Table 4 and  FIG. 8 . 
     Another embodiment of this invention is illustrated in  FIGS. 5A-5D  with Table 2. The formed grain layers as shown in  FIG. 5A  are different from the previous case shown in  FIG. 4A . Pure silica grain layer  230  is thicker than that required for the skin. The bubble free layer fused at the stage 3 is fused from pure silica grain  230  and from natural quartz grain layer  228  as shown in  FIG. 5B . Normally a crucible should have a thicker bubble free layer than the layer that is dissolved during CZ process. As an economical solution, the bubble free layer is designed to have pure silica layer  404   b  and natural quartz glass layer  404   a . 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 (Crucible B) 
               
             
          
           
               
                   
                   
                   
                   
                 fused 
                   
                   
                   
               
               
                   
                   
                   
                   
                 grain/ 
                 pressure 
               
               
                   
                 Electric 
                 Evacuation 
                   
                 fusion 
                 at fusion 
               
               
                   
                 Power 
                 Power 
                 event 
                 front 
                 front 
                 sublimation 
                 comment 
               
               
                   
                   
               
             
          
           
               
                 1st. 
                 100 
                 5 
                 skin start 
                 pure silica/ 
                 5 
                 negligible 
                 skin at 
               
               
                 stage 
                   
                   
                 evenly 
                 slow 
                   
                   
                 atmospheric 
               
               
                   
                   
                   
                   
                   
                   
                   
                 pressure 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Skin is even in 
               
               
                   
                   
                   
                   
                   
                   
                   
                 thickness 
               
               
                 2nd. 
                 100 
                 100 
                 skin 
                 pure silica/ 
                 100 
                 negligible 
                 skin complete 
               
               
                 stage 
                   
                   
                 complete 
                 slow 
                   
                   
                 under vacuum 
               
               
                 3rd. 
                 600 
                 100 
                 bubble 
                 natural 
                 100 
                 medium 
                 clear layer 
               
               
                 stage 
                   
                   
                 free layer 
                 quartz + 
                   
                   
                 formed, 
               
               
                   
                   
                   
                   
                 pure silica/ 
                   
                   
                 purity wise 
               
               
                   
                   
                   
                   
                 fast 
                   
                   
                 double layer 
               
               
                   
                   
                   
                   
                   
                   
                   
                 [pure/natural] 
               
               
                 4th. 
                 600 
                 30 
                 Opaque 
                 natural 
                 30 
                 medium 
                 opaque layer 
               
               
                 stage 
                   
                   
                 layer 
                 quartz/ 
                   
                   
                 formed 
               
               
                   
                   
                   
                   
                 fast 
               
               
                 5th. 
                 850 
                 30 
                 Skin 
                 natural 
                 30 
                 high 
                 pure silica skin 
               
               
                 stage 
                   
                   
                 removal 
                 quartz/ 
                   
                   
                 sublimated 
               
               
                   
                   
                   
                   
                 almost 
                   
                   
                 leaving pure 
               
               
                   
                   
                   
                   
                 stop 
                   
                   
                 silica surface 
               
               
                   
               
             
          
         
       
     
     Opaque outer crucible layer  324  is fused as shown in  FIG. 5C  in a similar way as shown in  FIG. 4C . 
       FIG. 5D  is illustration of the completion of the fusion process. The crucible processed as shown in the Table 2 has a pure silica bubble-free layer as an inner portion of the inner layer  322 . In one embodiment, pure silica grain was formed with less than 0.2 ppm for all of Al, Ca, Li, Na and K. The innermost surface layer with about 30 micrometers in thickness had 2 ppm Al, 0.2 ppm Ca, 0.5 ppm Li and less than 0.1 ppm K. The impurity profile observed by three layer analysis is shown in the  FIG. 7 . The observed increase of impurities is supposed to be attributed to diffusion from natural quartz glass layer  404   a.    
     One comparative example is shown in Table 3 with no figures. The evacuation system is operating from the starting of stage 1. A localized skin was observed as a localized area to show bubble growth very near surface by vacuum bake test. This bubble growth layer is observed at corner radius, where major evacuation openings are located. The crucibles was used in CZ process and showed inferior results showing structure loss towards the end of crystal pulling. 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 (Crucible C &lt;comparative example&gt;) 
               
             
          
           
               
                   
                 Electric 
                 Evacuation 
                   
                 fused grain/ 
                 pressure at 
                   
                   
               
               
                   
                 Power 
                 Power 
                 event 
                 fusion front 
                 fusion front 
                 sublimation 
                 comment 
               
               
                   
                   
               
             
          
           
               
                 1st. 
                 100 
                 100 
                 skin start 
                 pure silica/ 
                 5 
                 negligible 
                 skin at 
               
               
                 stage 
                   
                   
                 locally 
                 slow 
                   
                   
                 atmospheric 
               
               
                   
                   
                   
                   
                   
                   
                   
                 pressure 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Skin is 
               
               
                   
                   
                   
                   
                   
                   
                   
                 uneven in 
               
               
                   
                   
                   
                   
                   
                   
                   
                 thickness 
               
               
                 2nd. 
                 100 
                 100 
                 skin 
                 pure silica/ 
                 100 
                 negligible 
                 skin 
               
               
                 stage 
                   
                   
                 complete 
                 slow 
                   
                   
                 complete 
               
               
                   
                   
                   
                   
                   
                   
                   
                 under 
               
               
                   
                   
                   
                   
                   
                   
                   
                 vacuum 
               
               
                 3rd. 
                 600 
                 100 
                 bubble free 
                 natural quartz + 
                 100 
                 medium 
                 clear layer 
               
               
                 stage 
                   
                   
                 layer 
                 pure silica/ 
                   
                   
                 formed, 
               
               
                   
                   
                   
                   
                 fast 
                   
                   
                 purity wise 
               
               
                   
                   
                   
                   
                   
                   
                   
                 double layer 
               
               
                   
                   
                   
                   
                   
                   
                   
                 [pure/ 
               
               
                   
                   
                   
                   
                   
                   
                   
                 natural] 
               
               
                 4th. 
                 600 
                 30 
                 opaque 
                 natural quartz/ 
                 30 
                 medium 
                 opaque 
               
               
                 stage 
                   
                   
                 layer 
                 fast 
                   
                   
                 layer 
               
               
                   
                   
                   
                   
                   
                   
                   
                 formed 
               
               
                 5th. 
                 850 
                 30 
                 skin 
                 natural quartz/ 
                 30 
                 high 
                 pure silica 
               
               
                 stage 
                   
                   
                 removal 
                 almost stop 
                   
                   
                 skin 
               
               
                   
                   
                   
                   
                   
                   
                   
                 sublimated, 
               
               
                   
                   
                   
                   
                   
                   
                   
                 but not 
               
               
                   
                   
                   
                   
                   
                   
                   
                 completely, 
               
               
                   
                   
                   
                   
                   
                   
                   
                 leaving 
               
               
                   
                   
                   
                   
                   
                   
                   
                 partial skin 
               
               
                   
                   
                   
                   
                   
                   
                   
                 which was 
               
               
                   
                   
                   
                   
                   
                   
                   
                 fused at low 
               
               
                   
                   
                   
                   
                   
                   
                   
                 power 
               
               
                   
               
             
          
         
       
     
     Table 4 shows a traditional known method of making a crucible, as described in U.S. Pat. No. 7,383,696 for example. 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 (Crucible D - Traditional method &lt;comparative example&gt;) 
               
             
          
           
               
                   
                 Electric 
                 Evacuation 
                   
                 fused grain/ 
                 pressure at 
                   
                   
               
               
                   
                 Power 
                 Power 
                 event 
                 fusion front 
                 fusion front 
                 sublimation 
                 comment 
               
               
                   
                   
               
             
          
           
               
                 1st 
                 100 
                 100 
                 skin 
                 pure silica/ 
                 10-20 
                 negligible 
                 skin at 
               
               
                 stage + 
                   
                   
                 uneven 
                 skin complete 
                   
                   
                 atmospheric 
               
               
                 2nd. 
                   
                   
                   
                 fast 
                   
                   
                 pressure 
               
               
                 stage 
                   
                   
                   
                   
                   
                   
                 Skin is uneven 
               
               
                   
                   
                   
                   
                   
                   
                   
                 and localized 
               
               
                 3rd. 
                 500 
                 100 
                 bubble 
                 pure silica/ 
                 70 
                 medium 
                 clear layer 
               
               
                 stage-1 
                   
                   
                 free 
                 fast 
                   
                 low 
                 formed, while 
               
               
                   
                   
                   
                 layer 
                   
                   
                   
                 skin is not 
               
               
                   
                   
                   
                   
                   
                   
                   
                 completed 
               
               
                 3rd. 
                 600 
                 100 
                 bubble 
                 natural quartz + 
                 100  
                 medium 
                 clear layer 
               
               
                 stage-2 
                   
                   
                 free 
                 pure silica/ 
                   
                   
                 formed, purity 
               
               
                   
                   
                   
                 layer 
                 fast 
                   
                   
                 wise double 
               
               
                   
                   
                   
                   
                   
                   
                   
                 layer [pure/ 
               
               
                   
                   
                   
                   
                   
                   
                   
                 natural] 
               
               
                 4th. 
                 600 
                 10 
                 opaque 
                 natural quartz/ 
                 30 
                 medium 
                 opaque layer 
               
               
                 stage-1 
                   
                   
                 layer 
                 fast 
                   
                 high 
                 formed 
               
               
                 4th. 
                 600 
                 10 
                 opaque 
                 natural quartz/ 
                 30 
                 medium 
                 sublimation not 
               
               
                 stage-2 
                   
                   
                 layer 
                 almost stop 
                   
                 high 
                 enough to 
               
               
                   
                   
                   
                   
                   
                   
                   
                 eliminate skin, 
               
               
                   
                   
                   
                   
                   
                   
                   
                 localized thick 
               
               
                   
                   
                   
                   
                   
                   
                   
                 skin can not be 
               
               
                   
                   
                   
                   
                   
                   
                   
                 removed 
               
               
                   
               
             
          
         
       
     
     There is no pure silica grain layer formed. The fusion process completed after the dimensions for diameter and thickness were obtained. Electric power of 600 kVA was sufficient to get desired dimensions. The vacuum bake test resulted in a thin layer of innermost surface showed bubble growth. The inner surface of the crucible with 30 micrometers in thickness was analyzed to show 110 ppm Al, 1.8 ppm Ca, 1.3 ppm Li and 0.1 ppm K. Impurity profile of this crucible is shown in the  FIG. 8 . 
       FIGS. 6 ,  7  and  8  comprise graphs of the impurities within the innermost wall of each of crucibles A, B and D, respectively. The L1 data point in each graph was established by etching the innermost crucible layer to a depth of 30 micrometers and analyzing the etched solution. The L2 data point was established by further etching another 30 micrometers and analyzing the etched solution with the L3 data point resulting from further etching and analyzing another 30 micrometers. 
       FIG. 9  is a core sample including an innermost layer  1001 , which is used to create the impurity profile shown; a pure silica layer  1002 , which includes layer  1001 ; a bubble free and slow-bubble growth layer  1003 , which includes layers  1001 ,  1002 ; and an opaque layer  1004 , which includes bubbles that extend between layer  1003  and the radially outer surface  1005  of the crucible. The graphs below the sample show the Aluminum across the wall of the crucible for two different processes, one for Crucible D, as described in Table 4 and one for Crucible B, as described in Table 2. Crucible D was used in a CZ process; it presented difficulties in starting pulling. Several retrials were required to start pulling. 
     EXAMPLES 
     Test Example 1 
     A natural quartz grain was formed in the rotating mold as natural grain layer. A synthetic silica grain with 3.5 mm in thickness was formed as a pure silica grain layer on the natural quartz grain layer. The synthetic silica grain contained less than 0.2 ppm of each of Al, Ca, Li, Na and K. The natural quartz grain had, 6 ppm Al, 1.3 ppm Ca, 0.7 ppm Li, 0.05 ppm Na and 0.1 ppm K. 
     The rotating mold had a cavity with a diameter of 463 mm. A programmable DC power supply of 1000 kVA was connected to the graphite electrodes. The evacuation system had evacuation capacity of 800 m 3 /hr and a set of valves and piping to control evacuation power. 
     The five steps of fusion program were operated as shown in the Table 2. 
     The total of the first and the second stage took 2 minutes. The evacuation system was inactivated by opening the leak valve and activated (by closing the valve) at 1 minutes and 50 seconds. The electric power at stage 1 and stage 2 was 60 kVA to 100 kVA. The vacuum sensor located by the evacuation bore  134  was used to monitor pressure at the fusion front. The reading does not represent the pressure at the fusion front, but indicates relative values. The pressure reading at stage 3 was 150 Torr. The electric power was raised from 100 kVA to 600 kVA for stage 3 and stage 4. For stage 4, the evacuation system was tuned to 680 Torr. at the sensor position. 
     At the stage 5 the electric power was increased to 850 kVA and extensive fume generation was observed. 
     After cooling down, the fused part was pulled out from the mold and cut to the required height. Outer diameter was 457 mm with 11.5 mm wall thickness. 
     One crucible was cut to coupons for vacuum bake test. After performing a vacuum bake test in which the crucible was baked at approximately 1650° C. for approximately three hours at a pressure of approximately 0.1 Pa. argon, it was determined that the crucible had an inner crucible layer 3.5 mm thick, which was substantially free of bubbles and showed no significant bubble growth. 
     About 30 μm of the inner crucible layer was dissolved for analysis. Three consecutive layers were analyzed. The concentration profiles were given in the  FIG. 7 . 
     The crucible was used for CZ process without any problems associated with crystalline structures. 
     Test Example 2 
     A crucible was prepared according to the same procedure outlined for the Test Example 1, except that a 1.5 mm thick synthetic silica grain layer was formed on the natural quartz grain layer. 
     The five steps of fusion program were operated as shown in the Table 1, otherwise the same Test example 1. 
     Three innermost layers were analyzed to know the impurity profile, as shown in the  FIG. 6 . Purity of innermost layer, layer L1, is not as pure as Test Example1, but an order of magnitude better than the comparative examples below. 
     The crucible performed equally well concerning structure loss. The silicon melt should have more impurities than the case of example  1 , but was not critical with respect to structure loss. 
     Comparative Example 1 
     A crucible was prepared according to the same procedure outlined for the Test Example 1, except fusion stages were taken as shown in Table 3. The biggest difference is that the evacuation system has been activated from the beginning of stage 1. 
     A vacuum bake test results showed a layer of bubbles around the corner radius, indicating that the skin was not completely removed. 
     The crucible was used at CZ process, but three repeated trials were required before successful start of the pulling single crystal. 
     Comparative Example 2 
     A crucible was prepared according to the same procedure outlined for the Test Example 1, except that no synthetic grain silica layer was formed on the natural quartz grain layer. The fusion stages are as shown in the Table 4. There is no fifth stage for intentional sublimation. Fusion was stopped when required dimensions, diameters and wall thicknesses, were met. The impurity profiles are shown in  FIG. 8 . Even there was no sublimation stage at high powers, such as 850 kVA, a huge impurity pile up was observed. Aluminum content in the first layer L1 was 110 ppm, which is order 7.8 times larger than Example 2 and 55 times more than Example 1. 
     With this crucible, successful CZ pulls were less than 70%. Even for the successful pulls, several retrials were required for finishing the pull.