Patent Publication Number: US-2022228292-A1

Title: Crucible molds

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. Non-Provisional patent application Ser. No. 16/796,482, filed Feb. 20, 2020 and is a continuation of U.S. Non-Provisional patent application Ser. No. 16/796,522, filed Feb. 20, 2020. Both of these applications are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The field of the disclosure relates to methods for forming a unitized crucible assembly for holding a melt of silicon for forming a silicon ingot and, in particular, methods for forming a unitized crucible assembly having central and inner weirs for use in continuous Czochralski silicon ingot growth. The field of the disclosure also relates to crucible molds and unitized crucibles. 
     BACKGROUND 
     Single crystal silicon ingots may be prepared by the so-called Czochralski method in which a single crystal silicon seed is contacted with a silicon melt held within a crucible. The single crystal silicon seed is withdrawn from the melt to pull the single crystal silicon ingot from the melt. The ingot may be prepared in a batch system in which a charge of polycrystalline silicon is initially melted within the crucible and the silicon ingot is withdrawn from the melt until the melted silicon within the crucible is depleted. Alternatively, the ingot may be withdrawn in a continuous Czochralski method in which polysilicon is intermittently or continuously added to the melt to replenish the silicon melt during ingot growth. 
     In a continuous Czochralski method, the crucible may be divided into separate melt zones. For example, the crucible assembly may include an outer melt zone in which polycrystalline silicon is added and melted to replenish the silicon melt as the silicon ingot grows. The silicon melt flows from the outer melt zone to a stabilization zone within the outer melt zone in which the melt thermally stabilizes. The silicon melt then flows from the stabilization zone to a growth zone from which the silicon ingot is pulled. 
     Conventionally, crucible assemblies for growing silicon ingots by a continuous Czochralski method may include one or more nested crucibles such as the crucible assembly shown in U.S. Pat. No. 10,450,670. Alternatively or in addition, the crucible assembly may include one or more weirs that are connected to and extend upward from the bottom of the crucible as disclosed in U.S. Pat. No. 8,262,797. Both types of crucible arrangements involve use of alignment tools when placed into the crystal puller hot zone to provide uniform gas flow over the crucible assembly. Both types of constructions require a connection be formed at the bottom of the crucible assembly (i.e., boding of weirs or crucibles to the bottom of the assembly). These connections may degrade which causes the crucible assembly to deform and lose one or more tolerances. 
     A need exists for crucible assemblies that resist deformation, that may be placed in the hot zone without or with less alignment tools, and which may be more simply fabricated, and for methods for preparing such crucible assemblies. 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     SUMMARY 
     An aspect of the present disclosure is directed to a method for forming a unitized crucible assembly for holding a melt of silicon for forming a silicon ingot by the Czochralski method. A crucible mold is provided. The mold has a channel network that includes a bottom channel and an outer sidewall channel that extends from the bottom channel. The channel network also includes a central weir channel that extends from the bottom channel and an inner weir channel that extends from the bottom channel. The central weir channel is disposed between the outer sidewall channel and the inner weir channel. A slip slurry is introduced into the channel network to fill the bottom channel, outer sidewall channel, central weir channel, and inner weir channel with the slip slurry. The slip slurry includes silica and a liquid carrier. The liquid carrier is at least partially removed from the channel network to form a green body. The green body is removed from the crucible mold. The green body is sintered to dry and densify the green body to form the unitized crucible assembly. 
     Another aspect of the present disclosure is directed to a method for forming a unitized crucible assembly for holding a melt of silicon for forming a silicon ingot by the Czochralski method. A crucible mold is provided. The mold includes a porous body and a channel network disposed within the porous body. The channel network includes a bottom channel, an outer sidewall channel that extends from the bottom channel, and an inner weir channel that extends from the bottom channel. The inner weir channel is disposed interior to the outer sidewall channel. A slip slurry is introduced into the channel network to fill the bottom channel, outer sidewall channel and inner weir channel with the slip slurry. The slip slurry includes silica and a liquid carrier. The liquid carrier is at least partially drawn into the mold by capillary action to form a green body. The green body is separated from the crucible mold. The green body is sintered to dry and densify the green body to form the unitized crucible assembly. 
     Yet another aspect of the present disclosure is directed to a crucible mold. The crucible mold includes a lower portion and an upper portion disposed above the lower portion. The upper portion forms a central weir channel and an inner weir channel. The upper portion and lower portion together form a bottom channel and a sidewall channel fluidly connected to the bottom channel. The central weir channel and inner weir channel extend from the bottom channel. 
     Yet a further aspect of the present disclosure is directed to a crucible assembly. The crucible assembly includes a bottom, an outer sidewall that extends upward from the bottom, a central weir that extends upward from the bottom and an inner weir that extends upward from the bottom. The central weir is disposed between the outer sidewall and the inner weir. The bottom, outer sidewall, central weir and inner weir are unitized and are free of seams at (1) a joint between the outer sidewall and the bottom, (2) a joint between the central weir and the bottom, (3) a joint between the inner weir and the bottom. 
     Various refinements exist of the features noted in relation to the above-mentioned aspects of the present disclosure. Further features may also be incorporated in the above-mentioned aspects of the present disclosure as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to any of the illustrated embodiments of the present disclosure may be incorporated into any of the above-described aspects of the present disclosure, alone or in any combination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-section of an embodiment of a unitized crucible assembly; 
         FIG. 2  is a perspective view of an embodiment of a crucible mold having a green body formed therein: 
         FIG. 3  is a perspective cross-section view of the crucible mold and green body taken along line  3 - 3  in  FIG. 2 ; 
         FIG. 4  is a top view of the crucible mold and green body; 
         FIG. 5  is a cross-section view of the crucible mold without the green body taken along line  5 - 5  in  FIG. 4 ; 
         FIG. 6  is another cross-section view of the crucible mold without the green body taken along line  6 - 6  in  FIG. 4 ; 
         FIG. 7  is a cross-section view of the crucible mold having a slip slurry poured therein; 
         FIG. 8  is another cross-section view of the crucible mold having a slip slurry poured therein; and 
         FIG. 9  is a detailed cross-section view of the crucible mold showing a central weir peg and an inner weir peg for forming openings in a central weir and an inner weir of the resulting crucible. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the drawings. 
     DETAILED DESCRIPTION 
     Provisions of the present disclosure relate to methods for forming a unitized crucible (e.g., one-piece construction) for holding a melt of silicon for forming a silicon ingot by the Czochralski method. An example unitized crucible assembly  5  that may be produced by embodiments of the present disclosure is shown in  FIG. 1 . The unitized crucible assembly  5  includes a bottom  17  and an outer sidewall  10  that extends upward from the bottom  17 . The crucible assembly  5  includes a central weir  24  and an inner weir  31  that both extend upward from the bottom  17 . The central weir  24  is disposed between the outer sidewall  10  and the inner weir  31 . 
     In accordance with embodiments of the present disclosure, a crucible mold  8  ( FIG. 2 ) having a channel network  20  ( FIGS. 5-6 ) formed therein is provided. The mold  8  includes an upper portion  12  and a lower portion  18  which, when assembled together, form the channel network  20 . As described further below, a slip is poured into the channel network  20  and a “green body”  40  ( FIG. 2 ) in the shape of the crucible is formed. The green body  40  is removed from the mold  8  and is sintered to form the unitized crucible assembly  5  ( FIG. 1 ). 
     Referring now to  FIGS. 2-4  in which the mold  8  is transparent for illustration and the channel network  20  is shown with a green body  40  formed therein, the channel network  20  includes a bottom channel  30  and an outer sidewall channel  32 . The channel network  20  also includes a central weir channel  34  and an inner weir channel  36  that are each disposed inward of the outer sidewall channel  32 . The central weir channel  34  is disposed between inner weir channel  36  and the outer sidewall channel  32 . As shown in  FIG. 3 , the outer sidewall channel  32 , central weir channel  34  and the inner weir channel  36  are fluidly connected to the bottom channel  30  (i.e., each of the channels of the channel network are fluidly connected which allow the resulting crucible to be unitized). 
     The upper portion  12  of the mold  8  includes a main body  49  ( FIGS. 5-6 ) and a flange  54  that extends radially outward from the main body  49 . The main body  49  has a lower surface  53  (i.e., the surface below the flange  54  that forms a surface of the bottom channel  30  and the outer sidewall channel  32 ). 
     The lower portion  18  of the mold  8  also includes a main body  56 . The main body  56  has an upper surface  59  (i.e., the surfaces that form a surface of the bottom channel  30  and the outer sidewall channel  32 ) and forms a recess  61  inward of the upper surface  59 . In the illustrated embodiment, at least a portion of the lower surface  53  of the main body  49  of the upper portion  12  of the mold  8  and at least a portion of the upper surface  59  of the lower portion  18  of the mold  8  are rounded to form a rounded crucible bottom  17  ( FIG. 1 ). To assemble the mold  8 , the main body  49  of the upper portion  12  is lowered into the recess  61  of the lower portion  18  until the flange  54  rests on the lower portion  18 . In this position, the upper and lower portions  12 ,  18  of the mold  8  together form the bottom channel  30  and the outer sidewall channel  32 . 
     In the illustrated embodiment, the bottom channel  30  is rounded (and the resulting crucible bottom  17  ( FIG. 1 ) is rounded). In other embodiments, the bottom channel  30  may be more or fully horizontal (i.e., flat) or may be conical or inverted (e.g., having a pedestal in the middle). As shown in  FIGS. 5-6 , a portion of the sidewall channel  32  is higher than the central weir channel  34  and a portion of the central weir channel  34  is higher than the inner weir channel  36 . In other embodiments, the channels may have different relative heights (e.g., the same height of sidewall channel  32 , central weir channel  34  and inner weir channel  36 ). 
     First and second outer sidewall channel risers  58 ,  60  ( FIG. 2 ) extend through the upper portion  12  and are in fluid communication with the outer sidewall channel  32 . First and second central weir channel risers  64 ,  66  also extend through the upper portion  12  and are in fluid communication with the central weir channel  34 . First and second inner weir channel risers  70 ,  72  also extend through the upper portion  12  and are in fluid communication with the inner weir channel  36 . The risers  58 ,  60 ,  64 ,  66 ,  70 ,  72  extend through the upper portion  12  to allow the channel network  20  to be filled with the slip slurry. For example, the slip slurry may be added to either the first and second sidewall riser  58 ,  60  to fill the outer sidewall channel  32  and the bottom channel  30  with slurry. As the channels  30 ,  32  fill with slurry, air escapes through the corresponding riser  58 ,  60  that is not being filled with slip slurry. Similarly, slip slurry may be added to either the first or second central weir channel risers  64 ,  66  and to the first or second inner weir channel risers  70 ,  72  until the channel network  20  is filled with slip slurry ( FIGS. 7-8 ). In the illustrated embodiment, each riser  58 ,  60 ,  64 ,  66 ,  70 ,  72  includes a cone portion  51  ( FIG. 5 ) that opens to the surface  57  of the upper portion  12  of the mold  8  to facilitate pouring the slip slurry into the riser. A duct  52  below the cone portion  51  fluidly connects the cone portion  51  to the respective channel. The slip slurry may be added until the ducts  52  and/or cone portion  51  begins to fill to ensure that the channel network  20  is filled with slip slurry. For example, as the liquid carrier is drawn into the mold  8 , the volume of the slip slurry shrinks. Slip slurry disposed within the ducts  52  and/or cone portion  51  of one or more of the risers  58 ,  60 ,  64 ,  66 ,  70 ,  72  may be drawn into the channel network  20  as the slip slurry shrinks. 
     The channels may generally be filled in any order that allows the channel network  20  to fill with slip slurry before parts of the slurry begin to solidify as a green body to allow the resulting crucible to be unitized. The arrangement of risers  58 ,  60 ,  64 ,  66 ,  70 ,  72  is exemplary and other arrangements may be used unless stated otherwise. The risers  58 ,  60 ,  64 ,  66 ,  70 ,  72  may be openings and/or chambers formed in the upper portion  12  of the mold  8 . In some embodiments, the risers  58 ,  60 ,  64 ,  66 ,  70 ,  72  include openings/chambers and a liner that is disposed on the surface of the openings/chambers. 
     In some embodiments, the slip slurry that is added to the channel network  20  to fill the bottom channel  30 , outer sidewall channel  32 , central weir channel  34  and inner weir channel  36  includes silica and a liquid carrier such as water. The slip slurry may also include other reagents such as suspending agents that keep the silica particles in suspension including any of the suspending agents known to those of skill in the art. Example suspending agents include polymers or organics that absorb onto the particles (e.g., long-chain organic molecules or other agents that allow a surface charge to build-up on the silica particles to reduce particle to particle contact). The slip slurry may also include one or more binding agents that may optionally burn off during sintering as described below. Optionally the slip slurry may include one or more release agents to promote separation of the crucible mold  8  from the resulting green body  40 . 
     The crucible mold  8  may be made of materials that allow the liquid carrier to be removed from the channel network  20  (e.g., such as by capillary action) to form the green body  40 . In some embodiments, the crucible mold  8  is made of a plaster such as gypsum plaster (e.g., CaSO 4 .nH 2 O which may also be referred to as plaster of Paris). In other embodiments, the crucible mold  8  is made of porous silica. The crucible mold  8  may be a generally porous body that draws the liquid carrier into the mold  8  by capillary action. In other embodiments, the liquid carrier may be drawn out by vacuum. 
     Once the liquid carrier is drawn out of the slip slurry and into the mold  8 , a “green body”  40  ( FIGS. 2-3 ) remains in the mold  8 . For example, the green body  40  may have sufficient structure to maintain its shape when separated from the mold. For example, the moisture content of the green body may be less than about 50%, less than about 45%, at least about 30 wt %, at least about 35 wt %, at least about 40 wt %, at least about 45 wt %, from about 30 wt % to about 50 wt % or from about 35 wt % to about 45 wt %. 
     The green body  40  may be further dried such as by exposing the green body  40  to a relatively low and/or controlled humidity ambient (e.g., after the green body  40  has sufficient strength, the mold  8  is removed and the green body  40  is exposed to the relatively low and/or controlled humidity ambient). The term “green body” or “green state” as used herein should not be considered in a limiting sense and generally refers to an intermediate state of the crucible after the liquid carrier has been partially drawn from the slip slurry and before sintering of the structure. 
     To separate the mold  8  from the green body  40 , the upper portion  12  of the mold  8  may be lifted from the green body  40  and from the lower portion  18 . The green body  40  may then be lifted from the lower portion  18 . The resulting green body  40  may have projections (not shown) that extend up from the sidewalls and weirs that correspond to the riser locations (e.g., the slip slurry was added to completely fill the channel network  20  such that an additional amount filled in the risers). These projections may be ground or cut from the green body  40  or the resulting crucible assembly  5  (FIG. 
     In some embodiments, the mold  8  may include pegs in one or more of the channels to form openings within the resulting crucible to allow molten silicon to move between the various sections of the crucible. For example and with reference to  FIG. 9 , the upper portion  12  includes a central weir peg  48  that extends laterally across the central weir channel  34 . The slip slurry flows around the central weir peg  48  which creates an opening  38  ( FIG. 1 ) through the central weir  24  of the resulting crucible assembly  5 . Alternatively or in addition, the upper portion  12  includes an inner weir peg  55  that extends laterally across the inner weir channel  36  to form an opening  41  ( FIG. 1 ) in the inner weir  31 . After the upper portion  12  of the mold  8  is removed from the green body  40  and the lower portion  18 , the central weir peg  48  and the inner weir peg  55  may be removed (e.g., drilled or chiseled out). The positon of the pegs  48 ,  55  (and resulting crucible openings  38 ,  41 ) is exemplary and other positions (e.g., heights, relative circumferential positions between pegs, and the like). The mold  8  may include additional or other arrangements of pegs to form openings in the central weir  24  and/or inner weir  31 . In some embodiments, the mold  8  does not include pegs. For example, the openings between weirs may be formed directly in the green body and/or resulting crucible (e.g., by drilling or chiseling) and/or the crucible assembly  5  is configured such that silicon pours over the weirs. 
     The crucible mold  8  is exemplary and other embodiments may contain different arrangements of weirs and the like unless stated otherwise (e.g., containing only a single weir (i.e., only an inner weir) or more than two weirs). In some embodiments, the crucible mold  8  may be re-used in additional cycles for forming a crucible assembly  5  (e.g., used in two, three, four, five or ten or more cycles). The mold  8  may be dried between cycles such as by placing the mold in a drying oven to evaporate the liquid that was pulled therein during formation of the green body  40 . 
     Once the green body  40  is removed from the mold  8 , the green body  40  may be sintered (e.g., in a drying furnace) to dry and densify the green body  40  and to form the unitized crucible assembly  5  ( FIG. 1 ). The green body  40  may be sintered at a temperature from about 1200° C. to about 1800° C., from about 1300° C. to about 1700° C., or from about 1300° C. to about 1650° C. In some embodiments, the crucible assembly  5  has a moisture content of less than 20 wt %, less than 15 wt % or less than 10 wt % after sintering. 
     The unitized crucible assembly  5  after sintering is shown in  FIG. 1 . In various embodiments of the present disclosure, the unitized crucible assembly  5  does not include seams at the joint  13  formed between the bottom  17  and the central weir  24  and the joint  15  formed at the bottom  17  and the inner weir  31  (i.e., without voids at these joints  13 ,  15 ). Alternatively or in addition, the unitized crucible assembly  5  does not include tacking at the joints  13 ,  15 . Such tacking may be used in conventional crucible assemblies to connect the weirs  24 ,  31  at the crucible bottom  17 . In the illustrated embodiment, the unitized crucible assembly  5  includes a single layer bottom  17  (i.e., the crucible assembly  5  does not include stacked crucibles each having its own bottom). The unitized crucible assembly  5  may be transparent. 
     In some embodiments, the slip slurry is selected such that the resulting crucible assembly has a desired purity threshold. For example and in accordance with some embodiments, the crucible assembly  5  includes calcium in a concentration less than about 1 ppmw, less than about 0.8 ppmw or less than about 0.7 ppmw. Alternatively or in addition, the crucible assembly  5  may include sodium at a concentration of less than about 0.5 ppmw, less than about 0.2 ppmw or less than about 0.1 ppmw. Alternatively or in addition, the crucible assembly  5  may include potassium at a concentration of less than about 0.5 ppmw, less than about 0.2 ppmw or less than about 0.1 ppmw. Alternatively or in addition, the crucible assembly  5  may include lithium at a concentration of less than about 0.5 ppmw, less than about 0.4 ppmw or less than 0.3 ppmw. Alternatively or in addition, the crucible assembly  5  may include iron at a concentration of less than about 0.5 ppmw, less than about 0.3 ppmw or less than about 0.15 ppmw. 
     In some embodiments, the mold  8  is selected (and/or processed) to achieve one or more of the purity amounts listed above. For example, the mold may be made of porous silica to reduce calcium content of the resulting crucible (e.g., relative to gypsum plaster molds). 
     The unitized crucible assembly  5  shown and described herein is an example assembly. The assembly  5  may have other dimensions (e.g., a shallower or deeper melt in one or more melt zones), purities, features and/or arrangements unless stated otherwise. 
     The methods of the present disclosure for forming a unitized crucible may be used to produce a single crystal silicon ingot. In such methods, a unitized crucible produced by an embodiment of the methods of the present disclosure is provided. In some embodiments, the unitized crucible assembly  5  includes a crucible melt zone  22  disposed between the outer sidewall  10  and the central weir  24 . The unitized crucible assembly  5  also contains a stabilization zone  26  disposed between the central weir  24  and the inner weir  31 . The unitized crucible assembly  5  also contains a growth zone  28  disposed within the inner weir  31 . 
     Polycrystalline silicon is added to the crucible melt zone  22  where the silicon melts and replenishes the silicon melt. Silicon melt flows through the central weir opening  38  and into the stabilization zone  26 . The silicon melt then flows through the inner weir opening  41  to the growth zone  28  disposed within the inner weir  31 . The silicon melt within the growth zone  28  is contacted with a single seed crystal and the seed crystal is withdrawn from the silicon melt to form a single crystal silicon ingot. The various silicon melt zones (e.g., melt zone  22 , stabilization zone  26  and growth zone  28 ) allow the ingot to be grown in accordance with continuous Czochralski methods in which polycrystalline silicon is continuously or semi-continuously added to the melt while an ingot is continuously pulled from the growth zone  28 . 
     Compared to conventional crucible assemblies for holding a silicon melt, the crucible assemblies and methods for producing such crucible assemblies of the present disclosure have several advantages. By producing a unitized (e.g., one-piece construction) crucible having a central weir and an inner weir, the crucible assembly allows for continuous ingot growth (continuous Czochralski) while allowing for the advantages of a unitized crucible. By unitizing the crucible bottom, sidewalls and weirs, the crucible assembly is better able to hold tolerances (e.g., height of sidewall and weirs, wall thickness, melt flow path, and the like). This allows the crucible assembly to more easily fit-up into the ingot puller system which may reduce or eliminate use of alignment tools (i.e., tools to fit the various parts of a non-unitized crucible together within the ingot puller system). The unitized crucible may undulate less which allows it to better fit into the susceptor of the ingot puller apparatus. The unitized crucible also enables more wall thickness control for improved thermal response, allows for more flexibility for doped chemistries, and reduces or eliminates pre-bond cycles (i.e., heating cycles used to connect a weir to the crucible bottom) which consume ingot puller assembly run times. Unitized crucible assemblies also involve less processing and supply logistics compared to crucible assemblies formed from multiple parts. In embodiments in which the crucible mold includes pegs that extend across the channels for forming weirs in the crucible, the resulting crucible has pre-formed openings which allow silicon melt to move to various melt zones within the resulting crucible during ingot growth. 
     In some embodiments, the slip and/or mold are selected such that the resulting crucible has a relatively low impurity content (e.g., less than about 1 ppmw calcium, less than about 0.5 ppmw sodium, less than about 0.5 ppmw potassium, less than 0.5 ppmw lithium, and/or less than 0.5 ppmw iron) to reduce likelihood of loss of zero dislocation during ingot growth. In embodiments in which the mold is made of porous silica, the crucible assembly may be more pure due to the lower calcium content of the porous silica relative to components cast from molds made of gypsum plaster. 
     As used herein, the terms “about,” “substantially,” “essentially” and “approximately” when used in conjunction with ranges of dimensions, concentrations, temperatures or other physical or chemical properties or characteristics is meant to cover variations that may exist in the upper and/or lower limits of the ranges of the properties or characteristics, including, for example, variations resulting from rounding, measurement methodology or other statistical variation. 
     When introducing elements of the present disclosure or the embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” “containing” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. The use of terms indicating a particular orientation (e.g., “top”, “bottom”, “side”, etc.) is for convenience of description and does not require any particular orientation of the item described. 
     As various changes could be made in the above constructions and methods without departing from the scope of the disclosure, it is intended that all matter contained in the above description and shown in the accompanying drawing[s] shall be interpreted as illustrative and not in a limiting sense.