Abstract:
A fused glass crucible includes a collar of doped aluminum silica that defines uppermost and outermost surfaces of the crucible. The melt line that defines the surface of molten silicon in the crucible may be substantially at the lower end of the collar or slightly above it. Crystallization of the collar makes it hard and therefore supports the remaining uncrystallized portion of the crucible above the melt line. The melt line may also be below the lower end of the collar, especially if the melt is drawn down or poured early in the process. Because there is little or no overlap or because the overlap does not last long, the doped aluminum collar is not damaged by the heat of from the melt.

Description:
SUMMARY OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the field of silica crucibles and more particularly to a silica crucible having a doped layer formed in the wall. 
   2. Background of the Invention 
   The Czochralski (CZ) process is well-known in the art for production of ingots of single crystalline silicon, from which silicon wafers are made for use in the semiconductor industry. 
   In the CZ process, metallic silicon is charged in a silica glass crucible housed within a susceptor. The charge is then heated by a heater surrounding the susceptor to melt the charged silicon. A single silicon crystal is pulled from the silicon melt at or near the melting temperature of silicon. 
   In addition to the CZ process, fused silica crucibles are used to melt metallic silicon, which is then poured—from a nozzle formed into the crucible—into a mold to create a polycrystalline silicon ingot, which is used to make solar cells. As with the CZ crucible, a heater surrounds a susceptor, which holds the crucible. 
   When fused glass crucibles are so used, metallic silicon in the crucible melts—at least in part—as a result of radiant heat transmitted by the heater through the susceptor and crucible. The radiant heat melts the silicon in the crucible, which has a melting point of about 1410 degrees C., but not the crucible. Once the silicon in the crucible is melted, however, the inner surface of the crucible beneath the surface of the molten silicon is heated to the same temperature as the molten silicon by thermal conduction. This is hot enough to deform the crucible wall, which is pressed by the weight of the melt into the susceptor. 
   The melt line is the intersection of the surface of the molten silicon and the crucible wall. Because the wall above the melt line is not pressed into the susceptor by the weight of the melt, i.e., it is standing free, it may deform. It is difficult to control the heat to melt the silicon, and keep it molten, while preventing the wall above the melt line from sagging, buckling or otherwise deforming. Maintaining precise control over the heat slows down the CZ process and thus throughput of silicon ingots. 
   It is known in the art to form a fused crucible with doped silica in the outer layer. The element used to dope the silica is one that promotes crystallization, such as aluminum, when the crucible is heated. Crystallized silica is much stronger than fused glass and will not deform as a result of heat in furnaces of the type used in the CZ and similar processes. 
   One such known approach dopes the outer layer of a crucible with aluminum in the range of 50-120 ppm. Relatively early in the course of a long CZ process, the outer wall crystallizes as a result of the aluminum doping. The crystallized portion is more rigid than the remainder of the crucible and therefore supports the upper wall above the melt line. 
   This prior art approach produces at least two kinds of problems, depending on the level of doping. First, the doping level must be high enough to create a rigid outer wall that supports the upper wall above the melt line. If the doping level is too low, the wall is subject to deformation in a manner similar to an undoped crucible. But when the doping level is high enough to support the upper wall, that portion of the wall beneath the melt line is subject to very high heat during the CZ process. This forms a very thick crystalline layer below the melt line. As a result of the prolonged heat and thick crystalline layer, the wall beneath the melt line may crack. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1-3  are cross-sectional, highly schematic, side views of a mold showing sequential stages for forming a crucible of the type having a funnel at the lower end thereof. 
       FIG. 4  is a cross-sectional view of a crucible so formed. 
       FIG. 5  is a cross-sectional view of an alternative crucible formed according to the present invention in use during a CZ process. 
       FIG. 6  is a cross-sectional view of the crucible of  FIG. 4  in use during a process for making solar cells. 
   

   DETAILED DESCRIPTION 
   Indicated generally at  10  in  FIG. 1  is a system for making a fused glass crucible. The system includes a crucible mold  12  that is rotatable on a vertical axel  14 . Mold  12  includes a generally horizontal surface  14  on which a bottom portion of a crucible is formed, as well be seen. The mold also includes a generally upright surface  16  against which a wall portion of the crucible is formed. In  FIG. 1 , system  10  is configured to form a crucible of the type having a nozzle at a lower end thereof. To this end, a graphite plug  18  is positioned in a lower end of the mold to form a passageway that communicates with a nozzle (not shown) that is attached to the crucible after it is fused. For the details of manufacturing a crucible having such a nozzle, reference is made to U.S. patent application Ser. No. 11/271,491 for a Silica Vessel with Nozzle and Method of Making, filed Nov. 9, 2005, which is hereby incorporated herein by reference for all purposes. 
   System  10  includes a bulk grain hopper  20  and a doped grain hopper  22 . The flow of grain from each hopper is controlled by regulating valves  24 ,  26 , respectively. A feed tube  28  introduces flow of silica grain into mold  12  from either one of or both of the hoppers depending upon how valves  24 ,  26  are set. Feed tube  28  is vertically movable into and out of mold  12 . This facilitates selectively depositing grain on upright surface  16  and on generally horizontal surface  14 , as well be further explained. A spatula  30  is also vertically movable and in addition is horizontally movable to shape grain in mold  12  as it rotates. 
   Consideration will now be given to how system  12  is used to make a crucible. First, hopper  20  is loaded with bulk silica grain  32 . And hopper  22  is loaded with aluminum-doped silica grain  34 . Silica grain  34  may be doped with aluminum in the range of about 85-500 ppm. 
   Next, mold  12  is rotated at a rate of about 100 rpm, feed tube  28  is positioned as shown in  FIG. 1 , and valve  26  is opened to begin depositing doped grain  34  in a band or collar  36  about the perimeter of mold  12 . The feed tube is moved vertically to deposit doped grain as shown. The rotation rate is high to keep the doped grain in collar  36  above a predetermined level on generally upright surface  16 . If the rotation rate is too low, doped grain falls into lower portions of the mold, which is undesirable. In the present embodiment, the radially outer surface of collar  36  comprises the outermost portion of the uppermost part of the crucible wall. The doped grain that forms the collar is deposited in a layer that has a thickness (measured along a radial axis of mold  12 ) that is defined by the position of spatula  30 . This thickness may have a range of about 0.7-2.0 mm in the fully formed crucible. As will be seen, there is an outermost layer of silica grain that is not fused. This prevents burning of the mold and makes it easier to remove the crucible from the mold. The thickness of this unfused grain must be taken into account to provide the 0.7-2.0 mm thickness in the finished product. 
   After collar  36  is laid down as described above, valve  26  is closed, and valve  24  as opened, as shown in  FIG. 2 . In addition, the rate of rotation of mold  12  is reduced to 75 rpm. This permits some of the bulk grain  32  to fall to the lower portion of mold  12 . As bulk silica grain feeds from hopper  20  out of feed tube  28 , the feed tube moves vertically to coat the side and bottom of the mold with a layer  38  of bulk grain silica as shown. Spatula  30  shapes the bulk grain layer into the form of a crucible. As can be seen, layer  38  covers substantially all of collar  36 . Graphite plug  18  defines an opening through layer  38  in the shape of the plug. 
   With reference to  FIG. 3 , after the silica grain crucible is defined in mold  12  as shown in  FIG. 2 , spatula  30  and feed tube  28  are withdrawn. Electrodes  40 ,  42  are vertically movable into and out of the interior of mold  12 . The electrodes are attached to a DC power supply  46  that can apply power to the electrodes in a selectable range between about 300 KVA and 1200 KVA. When sufficient power is supplied to the electrodes, an extremely hot plasma ball forms around the electrodes. The heat so generated creates a fusion front that fuses the silica grain beginning at the inner surface of the formed crucible and proceeding to the outer surface. This fusion front fuses most of layer  38  and the collar  36  of doped silica grain but stops—as a result of stopping the application of power to electrodes  40 ,  42 —before it fuses an outermost unfused layer  49  of grain that includes both bulk silica grain  38  and doped silica grain  36 . As previously mentioned, the depth of the grain deposited into mold  12  must take into account this unfused layer  49  so that a depth of the fused doped grain  36 , as shown in  FIG. 4 , is in the range of 0.7-2.0 mm. A unitary fused glass crucible  50  is shown in  FIG. 4  after it is removed from mold  12  and graphite plug  18  has been removed. 
   It can be seen that an upper portion of crucible  50  has been cut off to produce a flat upper rim  52 . This provides a crucible of a predetermined height and also provides a flat upper rim. As can be seen, in  FIG. 4 , collar  36  provides an outermost and uppermost portion of crucible  50 . After the upper portion of the cut is made, collar  36 —in the present embodiment—extends about 50 mm downwardly from rim  52 . It should be appreciated, however, that collar  36  could be formed to extend much further down the crucible—as much as ⅔ or ⅓ of the way down thus providing a much taller collar. As will be described shortly, a shorter caller is preferred. 
   Turning now to  FIG. 5 , indicated generally at  54  is a crucible in use in a CZ process. Crucible  54  is made in substantially the same manner as crucible  50  except that it does not have an opening in a lower portion thereof. This is accomplished simply by using a mold having a continuous smooth lower surface and omitting use of a graphite plug, like plug  18 . Crucible  54  includes an aluminum doped collar  56 , which is formed as described above in connection with crucible  50 . Like crucible  50 , crucible  56  has been cut along a plane at right angles to its longitudinal axis. This produces a substantially flat rim  58 . 
   Crucible  54  is supported in a susceptor  60  that is inside a furnace (not shown). The susceptor is surrounded by a heater  62 . Crucible  54  has been charged with metallic silicon that has melted, which is now referred to as the melt  64 , in response to heat produced by heater  62  inside the furnace. A single silicon seed crystal  61  is held by a holder  63 , which slowly draws seed crystal  61  from the molten silicon in accordance with the CZ process. A crystalline ingot  65  forms, also in accordance with the CZ process, on the lower end of seed crystal  61 . Melt line  66  is defined about the perimeter of crucible  54 . The melt line progressively lowers as ingot  65  forms and is pulled from melt  64 . 
   The melt  64  is at a temperature of about 1400 degrees C. As a result, the surface of crucible  54  beneath the melt line is also at that temperature. Even though the heat from the melt makes the crucible below melt line  66  very soft, the weight of the melt presses the crucible into susceptor  60  thus preventing any deformation of crucible  54  below melt line  66 . As the metallic silicon melts, the heat begins to crystallize crucible  54  in collar  56  as a result of the aluminum doped silicon within the collar. The portion of the crucible that is crystallized is hardened. This creates a relatively rigid crystalline ring or collar around the crucible, which stabilizes the portion of the crucible wall that is not crystallized. In other words, the rigid collar prevents the softer uncrystallized wall above the melt line from collapsing or otherwise deforming even as melt line  66  lowers to the bottom of the crucible. 
   Finally, crucible  50  is shown in use in  FIG. 5 . It also is held in a susceptor  68 . Likewise a heater  70  surrounds the susceptor  68  with all of the structure shown in  FIG. 6  being contained within a furnace (not shown). Silicon melt  72  was formed by melting metallic silicon in crucible  50  by heating it with heater  70  in the furnace. A nozzle  74 , which was formed with graphite plug  18 , on the lower portion of crucible  50  is plugged during while the silicon is melted. Once fully molten, the plug is removed, and melt  72  pours through nozzle  74 —as shown in the drawing—into molds (not shown) that are used to make solar cells. 
   As with the crucible of  FIG. 5 , the  FIG. 6  crucible walls are supported as a result of the crystalline ring formed when collar  36  begins to crystallize early in the CZ process. As a result, the walls of the crucible are supported above the melt line. 
   It should be appreciated that the aluminum-doped collars, like collars  36 ,  58 , can be formed so that the lower portion thereof is substantially at or slightly above the melt line when the crucibles are used. Or they may be slightly below the melt line—at least at the beginning of the CZ process. A good position for the lower end of the collar is less than about 5% of the crucible height below the melt line. 
   The following examples demonstrate the advantages of the invention. 
   EXAMPLE A 
   A crucible like crucible  50  was formed that has a height of 400 mm, 270 mm inner diameter, and 10 mm wall thickness. In this example the crucible was doped with 100 ppm aluminum to form a collar, like collar  36  that extends 150 mm down from rim  52 . The collar is 1.4 mm thick and defines an outermost and uppermost surface of the crucible as shown in the drawing. A charge of 120 kg metallic silicon was charged and kept in the crucible for 120 hours without problems. 
   EXAMPLE B 
   A crucible like crucible  50  was formed that has a height of 400 mm, 270 mm inner diameter, and 10 mm wall thickness. In Example B the crucible was doped with 500 ppm aluminum to form a collar, like collar  36  that extends 50 mm down from rim  52 . The collar is 1.6 mm thick and defines an outermost and uppermost surface of the crucible as shown in the drawing. A charge of 120 kg metallic silicon was charged and kept in the crucible for 120 hours without problems. 
   EXAMPLE C 
   A crucible like crucible  50  was formed that has a height of 400 mm, 270 mm inner diameter, and 10 mm wall thickness. In this example the crucible was doped with 100 ppm aluminum to form a collar, like collar  36  that extends 310 mm down from rim  52 , which is substantially all of the generally upright outer wall of the crucible. The collar defines an outermost and uppermost surface of the crucible as shown in the drawing. A charge of 120 kg metallic silicon was charged and in the crucible. In this example, the melt overlaps substantially with the collar. Put differently, the melt line was substantially above the lower edge of the collar. After 50 hours of holding the melt, the crucible showed cracking between the substantially upright wall portion and the substantially horizontal bottom portion. This cracking results from the melt being in close proximity to the doped, and therefore crystallized, collar. 
   Although the examples each use aluminum as a dopant, it should be appreciated that the invention could be implemented with any dopant that promotes crystallization, e.g., Barium. 
   As can be seen, when the doped portion and the melt do not overlap, or overlap only slightly, the problems associated with the prior art fully doped outer crucible wall can be avoided. In addition, when the process use is known, i.e., how much silicon will be charged in the crucible and how quickly the melt will be drawn down, a crucible can be designed in which there is overlap between the collar and the melt, but only for a few hours, not enough to damage the crucible, during the early stages of the process. As a result, the problems associated with the prior art can be avoided even where there is overlap of the melt and the doped collar in the early stages of the process. 
   While the invention has been disclosed in its preferred form, the specific embodiments thereof as disclosed and illustrated herein are not to be considered in a limiting sense. Indeed, it should be readily apparent to those skilled in the art in view of the present description that the invention can be modified in numerous ways. The inventor regards the subject matter of the invention to include all combinations and subcombinations of the various elements, features, functions and/or properties disclosed herein.