Patent Publication Number: US-2018044815-A1

Title: Crystal growing systems and crucibles for enhancing heat transfer to a melt

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/087,604, filed Nov. 22, 2013, which is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The field of the disclosure relates generally to systems for producing ingots of semiconductor or solar material from a melt and, more particularly, to systems for reducing dislocations and impurity concentrations in the ingot, and enhancing heat transfer within the melt. 
     BACKGROUND 
     In the production of single silicon crystals grown by the Czochralski (CZ) method, polycrystalline silicon is first melted within a crucible, such as a quartz crucible, of a crystal pulling device to form a silicon melt. The puller then lowers a seed crystal into the melt and slowly raises the seed crystal out of the melt. To produce a single, high quality crystal using this method, the temperature and the stability of the surface of the melt immediately adjacent to the ingot must be maintained substantially constant. Further, the melt temperature adjacent to the ingot must be maintained at a sufficiently high temperature to prevent the melt from prematurely solidifying. Prior systems for accomplishing this goal have not been completely satisfactory. Thus, there exists a need for a system that not only limits temperature fluctuations and surface disruptions in the melt immediately adjacent to the ingot, but also provides sufficient heat transfer to the melt adjacent to the ingot for maintaining the temperature of the melt. 
     This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present 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. 
     BRIEF SUMMARY 
     A first aspect of the present disclosure is a system for growing an ingot from a melt. The system includes an outer crucible, an inner crucible, and a weir. The outer crucible includes a first sidewall and a first base. The first sidewall and the first base define an outer cavity for containing the melt. The inner crucible is located within the outer cavity, and has a central longitudinal axis. The inner crucible includes a second sidewall and a second base having an opening therein. The opening in the second base is concentric with the central longitudinal axis. The weir is disposed between the outer crucible and the inner crucible for supporting the inner crucible. 
     Another aspect of the present disclosure is a system for growing an ingot from a melt. The system includes an outer crucible, an inner crucible, and a weir. The outer crucible includes a first sidewall and a first base. The first sidewall and the first base define an outer cavity for containing the melt. The inner crucible is located within the outer cavity, and includes a second sidewall and a second base having an opening therein. The opening has a first cross-sectional area. The weir is disposed between the outer crucible and the inner crucible for supporting the inner crucible. The weir has a second cross-sectional area. The ratio between the first cross-sectional area and the second cross-sectional is at least about 0.25. 
     Another aspect of the present disclosure is a system for growing an ingot from a melt. The system includes an outer crucible, an inner crucible, a first weir, and a second weir. The outer crucible includes a first sidewall and a first base. The first sidewall and the first base define an outer cavity for containing the melt. The inner crucible is located within the outer cavity, and includes a second sidewall and a second base having an opening therein. The second sidewall and the second base define an inner cavity. The opening is sized to facilitate the transfer of heat between the outer cavity and the inner cavity. The first weir is disposed between the outer crucible and the inner crucible. The second weir is positioned radially outward from the first weir for separating the melt into multiple melt zones. 
     Various refinements exist of the features noted in relation to the above-mentioned aspects. Further features may also be incorporated in the above-mentioned aspects 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 may be incorporated into any of the above-described aspects, alone or in any combination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is cross-section of a crystal growing system including a crucible assembly; 
         FIG. 2  is an enlarged cross-section of the crucible assembly of  FIG. 1 ; 
         FIG. 3  is an exploded view of a plurality of weirs used in the crucible assembly of  FIG. 2 ; 
         FIG. 4  is a side elevation of the plurality of weirs of  FIG. 3  in an assembled configuration; 
         FIG. 5  is a cross-section of the plurality of weirs of  FIGS. 3-5  taken along line  5 - 5  of  FIG. 4 ; 
         FIG. 6  is a partial cross-section of the plurality of weirs of  FIGS. 3-5  taken along line  6 - 6  of  FIG. 5 ; 
         FIG. 7  is a top perspective of a second crucible used in the crucible assembly of  FIG. 2 ; 
         FIG. 8  is a top elevation of the second crucible of  FIG. 7 ; 
         FIG. 9  is a cross-section of the second crucible of  FIGS. 7-8  taken along line  9 - 9  of  FIG. 8 ; and 
         FIG. 10  is a partial cross-section of the crystal growing system of  FIG. 1  illustrating the temperature field and streamlines of a melt. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION 
     In a crystal growing system using a continuous Czochralski process, one or more silica weirs are located between an outer or first crucible and an inner or second crucible to form a crucible assembly. The second crucible may be supported by the one or more weir(s) that are submerged within the melt. These weir(s) create multiple zones within the crucible assembly to limit the melt within one zone from passing into another zone to specific locations. One example of such a crystal growing system is disclosed in U.S. patent application Ser. No. 13/804,585 (the “&#39;585 Application”) filed Mar. 14, 2013, the entirety of which is hereby incorporated by reference. 
     Referring to  FIG. 1 , a crystal growing system is shown schematically and is indicated generally at  100 . The crystal growing system  100  is used to produce a single crystal ingot by a Czochralski method. As discussed herein, the system is described in relation to the continuous Czochralski method of producing single crystal ingots, though a batch process may be used. For example, the process may be used in a “recharge” CZ process. 
     The crystal growing system  100  includes a susceptor  150  supported by a rotatable shaft  152 , and a crucible assembly  200  that contains a silicon melt  112  from which an ingot  114  is being pulled by a puller  134 . During the crystal pulling process, a seed crystal  132  is lowered by the puller  134  into the melt  112  and then slowly raised from the melt  112 . As the seed crystal  132  is slowly raised from the melt  112 , silicon atoms from the melt  112  align themselves with and attach to the seed crystal  132  to form the ingot  114 . 
     The system  100  also includes a feed system  115  for feeding solid feedstock material  116  into the crucible assembly  200  and/or the melt  112 , a heat reflector  160 , and a heat system  123  for providing heat to the crucible assembly  200  and maintaining the melt  112 . 
     With additional reference to  FIG. 2 , the crucible assembly  200  includes a first crucible  210  having a first base  212  and a first sidewall  214 , a second crucible  230  having a second base  232  and a second sidewall  234 , and a plurality of concentrically arranged weirs  260 ,  280 ,  300 . 
     The first base  212  has a top surface  218  and the second base  232  has a bottom surface  238  and a top surface  240 . Each sidewall  214 ,  234  extends around the circumference of the respective base  212 ,  232 , and defines a diameter  220 ,  242  of the respective crucible  210 ,  230 . The first sidewall  214  and the first base  212  form an outer cavity  216 . The second sidewall  234  and the second base  232  form an inner cavity  236 . The second crucible  230  is sized and shaped to allow placement of the second crucible  230  within the outer cavity  216  of the first crucible  210 . In some embodiments, the first crucible may have an internal diameter of about 32 inches and the second crucible may have an internal diameter of about 24 inches. In other embodiments, the first crucible may have an internal diameter of about 24 inches and the second crucible may have an internal diameter of about 16 inches. In yet other embodiments, the first and second crucibles may have any suitable internal diameter that enables the crucible assembly  200  to function as described herein. 
     With additional reference to  FIGS. 3-6 , the plurality of concentrically arranged weirs includes a first weir  260 , a second weir  280 , and a third weir  300 . While the illustrated embodiment is shown and described as including three weirs, the system  100  may include more or fewer than three weirs, such as one weir, two weirs, or any other suitable number of weirs that enables the system  100  to function as described herein. 
     The weirs  260 ,  280 ,  300  each have a cylindrical body with an open top and bottom. Each weir  260 ,  280 ,  300  also has a top weir surface  262 ,  282 ,  302  and a bottom weir surface  264 ,  284 ,  304 , respectively. 
     The weirs  260 ,  280 ,  300  support the second crucible  230  within the outer cavity  216 . More specifically, the bottom weir surfaces  264 ,  284 ,  304  rest against the top surface  218  of first base  212 , and the bottom surface  238  of the second base  232  rests against the top weir surfaces  262 ,  282 ,  302 . In the illustrated embodiment, each bottom weir surface  264 ,  284 ,  304  is shaped to conform to a respective contact point of the first crucible  210 . Similarly, each top weir surface  262 ,  282 ,  302  is shaped to conform to a respective contact point of the second crucible  230 . In alternative embodiments, one or more of the top and bottom weir surfaces may have a shape other than a shape that conforms to a respective contact point of the first or second crucible. 
     Each weir  260 ,  280 ,  300  has a respective diameter  266 ,  286 ,  306  ( FIG. 5 ) defined by the cylindrical body of the weir. In the illustrated embodiment, the diameter  306  of the third weir  300  is greater than the diameter  286  of the second weir  280 , and the diameter  286  of the second weir  280  is greater than the diameter  266  of the first weir  260 . The weirs  260 ,  280 ,  300  are concentrically aligned with one another such that the third weir  300  is positioned radially outward from the second weir  280 , and the second weir  280  is positioned radially outward from the first weir  260 . 
     In some embodiments, one or more of the weirs  260 ,  280 ,  300  are bonded to the first base  212 . In other embodiments, one or more the weirs  260 ,  280 ,  300  are bonded to the second base  232 , while in others, one or more of the weirs  260 ,  280 ,  300  are bonded to both the first and second bases  212 ,  232 . The first crucible  210  and the second crucible  230  may be fire polished to improve the bond, e.g., the durability and reliability of the bond. 
     The weirs  260 ,  280 ,  300  and the second crucible  230  are arranged within the outer cavity  216  to separate the melt  112  into a plurality of melt zones. More specifically, the second crucible  230  and the weirs  260 ,  280 ,  300  separate the melt  112  into an outer melt zone  170  and an inner melt zone  172  ( FIG. 1 ). In the illustrated embodiment, the outer melt zone  170  is formed between the first sidewall  214  and the second sidewall  234 , and the inner melt zone  172  is formed within the inner cavity  236  of the second crucible  230 . 
     Each weir  260 ,  280 ,  300  is disposed between the first crucible  210  and the second crucible  230 , and is located along the first base  212  at a location inward from the first sidewall  214  to inhibit movement of the melt  112  from the outer melt zone  170  to the inner melt zone  172 . In the illustrated embodiment, the weirs  260 ,  280 ,  300  are arranged to further separate the melt  112  into a first intermediate melt zone  174  ( FIG. 1 ), formed between the third weir  300  and the second weir  280 , and a second intermediate melt zone  176  ( FIG. 1 ), formed between the second weir  280  and the first weir  260 . 
     Each weir  260 ,  280 ,  300  includes at least one weir passageway  268 ,  288 ,  308 , respectively, extending therethrough to permit the melt to flow between the outer melt zone  170  and the inner melt zone  172 . The weir passageways  268 ,  288 ,  308  may be positioned along the respective weir  260 ,  280 ,  300  to increase the path of travel for the melt  112  between the outer melt zone  170  and the inner melt zone  172 . In the illustrated embodiment, the weir passageways of adjacent weirs are diametrically opposed from one another to provide a circuitous path for the melt  112  between the outer melt zone  170  and the inner melt zone  172 , although in other embodiments the weir passageways may be positioned at any suitable location along the respective weir. In the illustrated embodiment, each weir  260 ,  280 ,  300  includes two weir passageways  268 ,  288 ,  308 , although the weirs may include more or fewer than two weir passageways, such as one passageway, three passageways, or any other suitable number of passageways that enables the system  100  to function as described herein. 
     In other embodiments, one or more weirs do not include passageways. In these embodiments, movement of the melt  112  from the outer melt zone  170  to the inner melt zone  172  is limited to movement above or below the weirs. 
     With further reference to  FIG. 1 , the feed system  115  includes a feeder  118  and a feed tube  120 . Solid feedstock material  116  may be placed into the outer melt zone  170  from feeder  118  through feed tube  120 . The amount of feedstock material  116  added to the melt  112  may be controlled by a controller  122  based on a temperature reduction in the melt resulting from the cooler feedstock material  116  being added to melt  112 . 
     As solid feedstock material  116  is added to melt  112 , the surface of the melt may be disturbed where the solid feedstock material  116  is introduced. This disturbance, if allowed to propagate through the melt  112 , also affects the ability of the silicon atoms of the melt  112  to properly align with the silicon atoms of the seed crystal  132 . The weirs  260 ,  280 ,  300  and the second sidewall  234  of the second crucible  230  inhibit inward propagation of the disturbances in the melt  112 . 
     The heat reflector  160  is positioned adjacent the crucible assembly  200 , and covers a portion of the inner cavity  236  and all of the outer cavity  216 . The heat reflector  160  inhibits line-of-sight polysilicon projectiles from reaching the inner melt zone  172  during the addition of the solid feedstock material  116 , and prevents gas from the outer melt zone  170  from entering the inner melt zone  172 . The heat reflector  160  also shields the ingot  114  from radiant heat from the melt  112  to allow the ingot  114  to solidify. 
     The heat system  123  provides heat to crucible assembly  200  by heaters  124 ,  126 , and  128  arranged at suitable positions about the crucible assembly  200 . Heat from heaters  124 ,  126 , and  128  initially melt the solid feedstock material  116  and then maintains melt  112  in a liquefied state. Heater  124  is generally cylindrical in shape and provides heat to the sides of the crucible assembly  200 , and heaters  126  and  128  provide heat to the bottom of the crucible assembly. In some embodiments, heaters  126  and  128  are generally annular in shape, and are positioned around and radially outward from the shaft  152 . 
     Heaters  124 ,  126 , and  128  are resistive heaters coupled to controller  122 , which controllably applies electric current to the heaters to alter their temperature. A sensor  130 , such as a pyrometer or like temperature sensor, provides a continuous measurement of the temperature of melt  112  at the crystal/melt interface of the growing single crystal ingot  114 . Sensor  130  also may be directed to measure the temperature of the growing ingot. Sensor  130  is communicatively coupled with controller  122 . Additional temperature sensors may be used to measure and provide temperature feedback to the controller with respect to points that are critical to the growing ingot. While a single communication lead is shown for clarity, one or more temperature sensor(s) may be linked to the controller by multiple leads or a wireless connection, such as by an infra-red data link or another suitable means. 
     The amount of current supplied to each of the heaters  124 ,  126 , and  128  by controller  122  may be separately and independently selected to optimize the thermal characteristics of melt  112 . In some embodiments, one or more heaters may be disposed around the crucible to provide heat. 
     As described above, the weirs and the second crucible separate the melt into multiple melt zones. Separating the melt into multiple melt zones and inhibiting the melt movement between the various zones facilitates heating and melting silicon material (e.g., silicon feedstock) added in the outer melt zone as the silicon material passes through the multiple zones to the inner melt zone, and thus prevents un-liquefied feedstock material from passing into the inner melt zone and disturbing the structural integrity of the ingot being formed therefrom. 
     Further, inhibiting movement of the melt between the zones allows the surface of the inner zone to remain relatively undisturbed. The weirs and the second crucible substantially prevent disturbances in the outer melt zone or intermediate melt zones from disrupting the surface of the melt in the inner melt zone by substantially containing the energy waves produced by the disturbances in the outer melt zone and intermediate melt zones. 
     The transfer of heat to the inner melt zone may, however, be adversely affected by the addition of too many weirs. For example, quartz weirs can act as a thermal barrier to heat provided by heaters  124 ,  126 ,  128 , which may prevent a sufficient amount of heat from being transferred to the inner melt zone  172  to maintain the liquid melt  112 . The second crucible  230  is therefore configured to facilitate the transfer of heat to the inner melt zone  172 . 
     More specifically, with additional reference to  FIGS. 7-9 , the second base  232  of the second crucible  230  has an opening  244  defined by an annular rim  246  extending from the top surface  240  of the second base  232  to the bottom surface  238  of the second base  232 . While the illustrated embodiment is shown and described as including one opening, alternative embodiments may have more than one opening formed in the second crucible  230 . 
     The rim  246  is substantially parallel to the second sidewall  234  of the second crucible, although the rim  246  may be tapered inward or outward with respect to a central longitudinal axis  248  of the second crucible  230 . 
     The opening  244  extends through the second crucible  230 , and is sized and shaped to facilitate the transfer of heat from the outer cavity  216  to the inner cavity  236 . More specifically, the opening is sized based on the size of the first weir  260 . In one suitable embodiment, for example, the opening has a diameter  250  that is sized based on the diameter  266  of the first weir  260 . More specifically, the ratio between the diameter  250  of the opening  244  and the diameter  266  of the first weir  260  is at least about 0.5, more suitably at least about 0.7, and, even more suitably, at least about 0.95. In the illustrated embodiment, for example, the ratio between the diameter  250  of the opening  244  and the diameter  266  of the first weir  260  is about 1.0. 
     In another suitable embodiment, the opening  244  is sized based on a cross-sectional area enclosed by the first weir  260  taken perpendicular to the central longitudinal axis  248  of the second crucible  230 . In one suitable embodiment, for example, the ratio between the cross-sectional area of the opening  244  and the cross-sectional area of the first weir  260  is at least about 0.25, more suitably at least about 0.5, and even more suitably, at least about 0.8. In the illustrated embodiment, for example, the ratio between the cross-sectional area of the opening  244  and the cross-sectional area of the first weir  260  is about 1.0. 
     In the illustrated embodiment, the opening  244  has a substantially circular shape, although in other embodiments, the opening may have any suitable shape that enables the system  100  to function as described herein. 
     The opening  244  is positioned radially inward from the innermost weir (i.e., the first weir  260 ) such that separation between the multiple melt zones is maintained. In the illustrated embodiment, the opening  244  is concentric with the central longitudinal axis  248  of the second crucible  230 , although the opening  244  may be offset from the central longitudinal axis  248  of the second crucible  230 . Also, in the illustrated embodiment, the opening  244  is sized and positioned such that the rim  246  is substantially aligned with the radially inner wall of the first weir  260 . 
     The opening  244  also provides fluid communication between the outer melt zone  170  and the inner melt zone  172 , and allows the melt  112  to flow between the inner cavity  236  and the outer cavity  216 . 
     The opening  244  enables heat to be transferred directly from the first crucible  210  to the inner melt zone  172 . Further, the temperature gradient across the melt  112  from the first base  212  of the first crucible  210  to the surface of the melt  112  causes the melt  112  to recirculate within the inner melt zone  172 , thereby enhancing the transfer of heat from the first crucible  210  to the melt  112  within the inner melt zone  172 . 
     Specifically, referring to  FIG. 10 , streamlines and temperature fields of the melt  112  are shown within the inner melt zone  172 . The melt  112  is hotter near the first base  212  of the first crucible  210  and the first weir  260  than it is near the surface of the melt  112 . As a result, the melt  112  recirculates between the hotter and cooler portions, thereby enhancing the transfer of heat from the first crucible  210  to the melt  112  within the inner melt zone  172 . Further, recirculation of the melt  112  within the inner melt zone  172  provides a more uniform distribution of impurities within the melt  112  (e.g., by carrying high concentrations of impurities away from the ingot-melt interface), thereby reducing the level of impurity concentrations within the melt  112  and enhancing the quality of the ingot  114  grown from the melt  112 . 
     As described above, the crystal growing systems of the present disclosure provide an improvement over known crystal growing systems. The crystal growing systems of the present disclosure enable separation of a silicon melt into multiple melt zones, while at the same time enhancing heat transfer to an inner melt zone of the melt. 
     Separating the melt into multiple melt zones and inhibiting the melt movement between the various zones facilitates heating and melting silicon material (e.g., silicon feedstock) added in the outer melt zone as the silicon material passes through the multiple zones to the inner melt zone, and thus prevents un-liquefied feedstock material from passing into the inner melt zone and disturbing the structural integrity of the ingot being formed therefrom. Further, inhibiting movement of the melt between the zones allows the surface of the inner zone to remain relatively undisturbed. The weirs and the second crucible substantially prevent disturbances in the outer melt zones or intermediate melt zones from disrupting the surface of the melt in the inner melt zone by substantially containing the surface vibrations produced by the disturbances in the outer melt zone and intermediate melt zones. 
     Embodiments of this disclosure may also reduce the amount of oxygen in the ingot, lower the consumption rates of the weir and second crucible providing a longer run life, and provide better system performance, as described in the co-pending &#39;585 Application. 
     Another benefit is that the volume and liquid-quartz surface area of the outer melt zone is increased compared to known crystal growing systems. The increase in volume and liquid-quartz surface area of the outer melt zone enhances heat transfer to the outer melt zone increasing the rate that solid feedstock material is liquefied. The increase in the conversion rate is particularly beneficial when the rate of adding solid feedstock material is high and a large amount of heat is needed to continuously liquefy solid feedstock material. 
     The above embodiments also provide improved impurity characteristics while reducing incidents of loss of crystal structure due to solid particles impacting the crystal. 
     Additionally, the above embodiments enhance the transfer of heat to an inner melt zone of the melt. Enhancing the transfer of heat to the inner melt zone substantially prevents the melt from solidifying within the inner melt at locations other than the melt-ingot interface. Additionally, enhancing the transfer of heat to the inner melt zone causes the melt to recirculate within the inner melt zone, thereby further enhancing the transfer of heat to the inner melt zone and providing a more uniform distribution of impurities within the melt. 
     When introducing elements of the present invention 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” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     As various changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.