Patent Publication Number: US-2023151510-A1

Title: Crystal pulling systems having a cover member for covering the silicon charge

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional of U.S. Non-Provisional patent application Ser. No. 17/396,370, filed Aug. 6, 2021, which claims the benefit of U.S. Provisional Patent Application No. 63/073,180, filed Sep. 1, 2020. Both applications are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The field of the disclosure relates to crystal pulling systems for growing a monocrystalline ingot from a silicon melt and, in particular, crystal pulling systems that include a cover member for use in continuous Czochralski silicon ingot growth. 
     BACKGROUND 
     Silicon crystal silicon ingots may be prepared by the 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 The single crystal silicon seed is withdrawn from the melt to pull a single crystal silicon ingot from the melt.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system in which a charge of polycrystalline silicon is initially melted within the crucible and the silicon ingot is withdrawn from the melt until the melted silicon within the crucible is depleted. Alternatively, the ingot may be withdrawn in a continuous Czochralski method in which polysilicon is intermittently or continuously added to the melt to replenish the silicon melt during ingot growth.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 an intermediate zone within the outer melt zone in which the melt thermally stabilizes. The silicon melt then flows from the intermediate zone to a growth zone from which the silicon ingot is pulled. 
     Crystal pulling systems may include a heat shield disposed above the crucible and the silicon melt. The heat shield includes a passage through which the silicon ingot passes as it is drawn vertically from the silicon melt. The heat shield protects and shields the drawn ingot from radiant heat from the melt. 
     During the melting phase, a temperature gradient may be created within the crystal pulling system. The temperature gradient creates thermal stress in the crucible resulting in damage, and in some cases, destruction of the crucible. 
     A need exists for crystal pulling systems that maintain a more uniform temperature gradient during meltdown to reduce crucible damage during meltdown. 
     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 
     One aspect of the present disclosure is directed to a crystal pulling system for growing a monocrystalline ingot from a silicon melt. The system includes a pull axis and a housing defining a growth chamber. A crucible assembly is disposed within the growth chamber for containing the silicon melt. A heat shield defines a central passage through which an ingot passes during ingot growth. The system includes a cover member that is moveable within the heat shield along the pull axis. The cover member includes one or more insulation layers. 
     Another aspect of the present disclosure is directed to a method for preparing a melt of silicon in a crucible of a crystal pulling system. The crystal pulling system includes a housing defining a growth chamber, a crucible assembly disposed within the growth chamber for containing the silicon melt and a heat shield that defines a central passage through which an ingot passes during ingot growth. A charge of solid polycrystalline silicon is added to the crucible assembly. A cover member is lowered through the central passage defined by the heat shield to cover at least a portion of the charge. The silicon charge is heated to produce a silicon melt in the crucible assembly while the cover member covers a portion of the charge. The cover member is raised after the melt has been formed. 
     Yet another aspect of the present disclosure is directed to a crystal pulling system for growing a monocrystalline ingot from a silicon melt. The system has a pull axis and includes a housing defining a growth chamber. A crucible assembly is disposed within the growth chamber for containing the silicon melt. The system includes a heat shield that defines a central passage through which an ingot passes during ingot growth. A cover member is moveable within the heat shield along the pull axis. The cover member includes a first plate having a first plate axis that is parallel to the pull axis. The cover member includes a second plate having a second plate axis that is parallel to the pull axis. The second plate is disposed above the first plate. 
     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 view of a crystal pulling system for growing a monocrystalline ingot from a silicon melt; 
         FIG.  2    is a cross-section view of a portion of the crystal pulling system, including a cover member disposed within a central passage of a heat shield; 
         FIG.  3    is a perspective view of a cover member of the crystal pulling system; 
         FIG.  4    is a cross-section view of the cover member; 
         FIG.  5    is an assembly view of the cover member; 
         FIG.  6    is a perspective view of a first plate of the cover member; 
         FIG.  7    is a bottom view of the first plate; 
         FIG.  8    is a cross-section view of the first plate; 
         FIG.  9    is a perspective view of a second plate of the cover member; 
         FIG.  10    is a top view of the second plate; 
         FIG.  11    is a cross-section view of the second plate; 
         FIG.  12    is a perspective view of an insulating layer of the cover member; 
         FIG.  13    is a cross-section view of a shaft of the cover member; 
         FIG.  14    is top view of the shaft; 
         FIG.  15    is a perspective view of a chuck of the crystal pulling system; 
         FIG.  16    is a perspective view of the chuck engaged with the shaft of the cover member; 
         FIG.  17    is a graph of the internal temperature of an outer crucible of a nested crucible assembly when a cover member with and without insulation is used during meltdown; 
         FIG.  18    is a graph of the internal temperature of a middle crucible of a nested crucible assembly when a cover member with and without insulation is used during meltdown; 
         FIG.  19    is a graph of the internal temperature of an innermost crucible of a nested crucible assembly when a cover member with and without insulation is used during meltdown; and 
         FIG.  20    is a graph of a power profile when a cover member with and without insulation is used during meltdown. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the drawings. 
     DETAILED DESCRIPTION 
     Provisions of the present disclosure relate to a crystal pulling system for producing monocrystalline (i.e., single crystal) silicon ingots (e.g., semiconductor or solar-grade material) from a silicon melt by the continuous Czochralski (CZ) method. The systems and methods disclosed herein may also be used to grow monocrystalline ingots by a batch or recharge CZ method. With reference to  FIG.  1   , the crystal pulling system is shown schematically and is indicated generally at  10 . The crystal pulling system  10  includes a pull axis Y 10  and a housing  12  defining a growth chamber  14 . A crucible assembly  16  is disposed within the growth chamber  14 . The crucible assembly  16  contains the silicon melt  18  (e.g., semiconductor or solar-grade material) from which a monocrystalline ingot  20  is pulled by a pulling mechanism  22  as discussed further below. 
     The crystal pulling system  10  includes a heat shield  24  (sometimes referred to as a “reflector”) that defines a central passage  26  through which the ingot  20  passes during ingot growth. In accordance with embodiments of the present disclosure, prior to the ingot  20  being drawn from the melt  18 , during an initial melting phase, a cover member  100  ( FIG.  2   ) is lowered to at least partially cover the solid charge of polycrystalline silicon to reduce heat that radiates through the central passage  26  during meltdown. The cover member  100  is moveable within the heat shield  24  along the pull axis Y 10 . 
       FIG.  2    shows a portion of the crystal pulling system  10  with the cover member  100  arranged within the central passage  26  during an initial phase in which the charge is melted (i.e., meltdown phase), prior to the ingot  20  being drawn. The crucible assembly  16  includes a bottom  30  and an outer sidewall  32  that extends upwards from the bottom  30 . The crucible as ably  16  includes a central weir  34  and an inner weir  36  that extends upward from the bottom  30 . The central weir  34  is disposed between the outer sidewall  32  and the inner weir  36 . The crucible assembly  16  includes a crucible melt zone  38  disposed between the outer sidewall  32  and the central weir  34 . The crucible assembly  16  also contains an intermediate zone  40  disposed between the central weir  34  and the inner weir  36 . The crucible assembly  16  also contains a growth zone  42  disposed within the inner weir  36 . The crucible assembly  16  may be made of, for example, quartz or any other suitable material that enables the crystal pulling system  10  to function as described herein. Further, the crucible assembly  16  may have any suitable size that enables the crystal pulling system  10  to function as described herein. The crucible assembly  16  may also include three “nested” crucibles which have separate bottoms that together make a bottom and in which the sidewalls of the crucibles are the weirs  34 ,  36  described above. 
     During ingot growth, polycrystalline silicon is added to the crucible melt zone  38  where the silicon melts and replenishes the silicon melt. Silicon melt flows through a central weir opening  44  and into the intermediate zone  40 . The silicon melt then flows through an inner weir opening  41  to the growth zone  42  disposed within the inner weir  36 . The various silicon melt zones (e.g., melt zone  38 , intermediate zone  40  and growth zone  42 ) 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  20  is continuously pulled from the growth zone  42 . The silicon melt  18  within the growth zone  42  is contacted with a single seed crystal  75  ( FIG.  1   ). As the seed crystal  75  is slowly raised from the melt  18 , atoms from the melt  18  align themselves with and attach to the seed to form the ingot  20 . 
     The crucible assembly  16  is supported by a susceptor  50  ( FIG.  1   ). The susceptor  50  is supported by a rotatable shaft  51 . A side heater  52  surrounds the susceptor  50  and crucible assembly  16  for supplying thermal energy to the system  10 . One or more bottom heaters  62  are disposed below the crucible assembly  16  and susceptor  50 . The heaters  52 ,  62  operate to melt an initial charge of solid polycrystalline silicon feedstock, and maintain the melt  18  in a liquefied state after the initial charge is melted. The heaters  52 ,  62  also act to melt solid polycrystalline silicon added through feed tube  54  ( FIG.  1   ) during growth of the ingot. The heaters  52 ,  62  may be any suitable heaters that enable to system  10  to function as described herein (e.g., resistance heaters). 
     The crystal pulling system  10  includes a gas inlet (not shown) for introducing an inert gas into the growth chamber  14 , and one or more exhaust outlets (not shown) for discharging the inert gas and other gaseous and airborne particles from the growth chamber  14 . The gas inlet supplies suitable inert gases such as argon. 
     The system  10  includes a cylindrical jacket  57  disposed with the heat shield  24 . The jacket  57  is fluid-cooled and includes a jacket chamber  60  that is aligned with the central passage  26 . The ingot  20  is drawn along the pull axis Y 10 , through the central passage  26  and into the jacket chamber  60 . The jacket  57  cools the drawn ingot  20 . 
     The heat shield  24  is generally frustoconical in shape. The heat shield  24  includes an outer surface  61  which faces the crucible assembly  16  and the melt  18 . The heat shield  24  may be coated to prevent contamination of the melt. In some embodiments, the heat shield  24  is made of two graphite shells that include molybdenum sheets therein. The surface  61  may be coated (e.g., SiC) to reduce contamination of the melt. 
     The heat  24  includes a bottom  58  ( FIG.  2   ). The central passage  26  of the heat shield  24  has a diameter D 26  at the bottom  58  of the heat shield  24 . The heat shield  24  is disposed above the crucible assembly  16 , such that the central passage  26  is arranged directly above the growth zone  42  so that the ingot drawn from the melt  18  may be pulled through the central passage  26 . The passage diameter D 26  is sized to accommodate the diameter of the ingot  20  (e.g., 200 mm or 300 mm or other diameter ingots). 
     The outer surface  61  may be coated with a reflective coating which reflects radiant heat back towards the melt  18  and the crucible assembly  16 . As such, the heat shield  24  assists in retaining heat within the crucible assembly  16  and the melt  18 . In addition, the heat shield  24  aids in maintaining a generally uniform temperature gradient along the pull axis Y 10 . 
     During the initial melting phase, an initial amount of solid polycrystalline silicon is loaded to a crucible melt zone  38 , intermediate zone  40  and growth zone  42 . In other embodiments, solid polycrystalline silicon is added to only one or two of the zones selected between the crucible melt zone  38 , intermediate zone  40  and growth zone  42 . During meltdown, the cover member  100  is lowered to cover at least a portion of the silicon charge while the initial charge is melted (i.e., by occluding the central passage  26  of the heat shield  24 ). The pulling mechanism  22  raises and lowers the cover member  100 . 
     In accordance with embodiments of the present disclosures the cover member  100  is lowered to within less than 30 mm from the bottom  58  of the heat shield  24  (i.e., from below or above the bottom  58 ), or less than 20 mm, less than 10 mm, or less than 5 mm from the bottom  58  of the heat shield  24 . In some embodiments, the cover member  100  is lowered such that it is aligned with the bottom  58  of the heat shield  24 . In some embodiments, the cover member  100  is lowered to within 80 to 100 mm of the surface of the charge during melt down. 
     After the initial amount of silicon charge has been melted, a secondary amount of polycrystalline silicon may be added to the crucible melt zone  38  (e.g., continuously added until the entire secondary amount is added). In accordance with some embodiments of the present disclosure, the cover member  100  covers the central passage  26  while this secondary amount of polycrystalline silicon is added to the melt zone  38  and melted down. After the secondary charge has melted, the cover member  100  is raised by the pulling mechanism  22 . In other embodiments, the cover member  100  is not used while the secondary amount of polycrystalline silicon is added. 
     An embodiment of the cover member  100  shown in  FIG.  3   . The cover member  100  includes a first plate  102  and a second plate  104  (which may also be referred to herein as “lower plate  102 ” and “upper plate  104 ”, respectively). Each plate  102 ,  104  has a central axis that is generally parallel to the pull axis Y 10  ( FIG.  1   ). The first plate  102  and the second plate  104  are generally parallel. The second plate  104  is disposed above the first plate  102 . 
     The first plate  102  includes a first annular wall  106  ( FIGS.  5 - 6   ) and the second plate  104  includes a second annular wall  108  ( FIG.  9   ). Referring now to  FIG.  5   , the first wall  106  includes a first shoulder  110  and a first lip  111 . The second wall  108  includes a second shoulder  112  and second lip  113 . When assembled, the second shoulder  112  rests on the first lip  111  and the second lip  113  rests on the first shoulder  110 . A cover member chamber  116  ( FIGS.  8  and  11   ) is disposed between the first and second plates  102 ,  104 . 
     An insulation layer  130  ( FIG.  4   ) is disposed within the chamber  116  formed between the first and second plates  102 ,  104 . The insulation layer  130  has a thickness of T 130  (e.g., 10 mm to about 50 mm). The insulation layer  130  may be compressed between the first plate  102  and the second plate  104 . The insulation layer  130  may include several stacked layers of insulation or may be a single layer. The insulation layer  130  may include an opening  132  formed therein. 
     The insulation layer  130  may be made of felt. The felt may be composed of natural or synthetic fibers. The felt may be purified to (e.g., with a max ash of 30 ppm). The insulation layer  130  may generally be composed of any material that includes suitable insulating properties. 
     The first plate  102  includes a hub  145  ( FIG.  4   ) that protrudes upward for connecting a shaft  150 . The second plate includes a second plate opening  128  ( FIG.  9   ). The hub  145  extends through the opening  128  of the second plate  104  and through the insulation opening  132  ( FIG.  12   ). The hub  145  includes a ledge  149  ( FIG.  6   ) with the second plate  104  being seated on the ledge  149 . The hub  145  includes a hub opening  153  ( FIG.  8   ) through which the shaft  150  extends. The hub opening  153  has a profile that matches the profile of the shaft  150  (e.g., square or rectangular as in the illustrated embodiment or other shape such as circular). The hub  145  includes a hub chamber  126  having a top wall  157 . 
     The cover member  100  is generally in the shape of a circular segment having a circular portion  120  ( FIG.  7   ) including a center X and a circumference  122  and having a linear edge  124 . Specifically, the first and second plates  102 ,  104  have the shape of a circular portion with a segment along a chord that has been removed. The first and second plates  102 ,  104  include a major length L 1 , and a minor length L 2 . The major length L 1  a diameter of the circular portion  120  and the minor length L 2  extends from the circumference  122 , through the center X to the circumference  122  of the circular portion  120 . The first and second plates  102 ,  104  are shaped as a circular segment to allow the charge/melt to be viewed. In other embodiments, the cover member  100  is full circular. 
     In some embodiments, the diameter of the cover member  100  is at least 0.75 times the diameter of the central passage  26  at the bottom  58  of the heat shield  24  or, as in other embodiments, at least 0.8 times, at least 0.9 times, at least 0.95 times, or at least 0.99 times the diameter of the central passage  26  at the bottom  58  of the heat shield  24 . 
     In some embodiments, the first plate  102  and second plates  104  are made of graphite. The graphite may be coated with silicon carbide (SiC). The first and second plate  102 ,  104  may be composed of other suitable materials. The first and second plates  102 ,  104  have any suitable thickness T 102 , T 104  ( FIGS.  8  and  11   ) that prevents thermal stresses which result in cracking or damage of the first and second plates  102 ,  104  (e.g., thickness between 3 mm and 50 mm). 
     With reference to  FIGS.  13 - 14   , the cover member  100  includes a shaft  150  that supports the cover member  100 . The shaft  150  may be connected to the first and/or second plates  102 ,  104  in any suitable coupling arrangement. In the illustrated embodiment, the shaft  150  includes an elongated rectangular portion  154  and a collar  156 . The collar  156  has a diameter D 156  less than a diameter of the hub chamber  126  ( FIG.  8   ) and greater than the width of the hub opening  153 . The first plate  102  rests on the collar  156 . The insulation layer  130  and second plate  104  ( FIG.  4   ) are supported by the first plate  102 . Alternatively, the shaft  150  may be formed integrally with either or both of the first plate  102  and/or second plate  104 . 
     With reference to  FIGS.  15 - 16   , the pulling mechanism  22  includes a chuck  70  that is raised and lowered along the pull axis Y 10 . The chuck  70  may be connected to a pull wire or cable  37  that is raised and lowered by a drive motor (i.e., the pull wire or cable and motor are part of the pulling mechanism  22 ). The cover member  100  removably connectable to the chuck  70 . For example, the shaft  150  and the chuck  70  may be connected using a pin lock. The shaft  150  includes a recess  158  ( FIG.  13   ). The shaft  150  is inserted into a bore  72  within the chuck  70 , such that the recess  156  is contained within the bore  72 . The chuck  70  includes an opening  74  that extends generally perpendicularly to the bore  72 , passing through the chuck  70  and opening into the bore  72 . A pin  76  is inserted through the opening  74  and into the bore  72  such that the pin  76  becomes engaged with the recess  150  of the shaft  150  that is disposed within the bore  72 . In this manner the shaft  150  and the chuck  70  are coupled. The shaft  150  and chuck  70  may include any alternative and/or additional features to couple the cover member  100  to the chuck  70 . 
     After meltdown, the cover member  100  disconnected from the chuck  70  and the seed crystal  75   FIG.  1   ) is connected to the chuck  70 . The seed crystal  75  may include a similar recess, not shown, such shown, such that the seed  75  may also be coupled and/or uncoupled to the chuck  70 . During the ingot growth process, the seed  75  is lowered by the pulling mechanism  22  into contact with the melt  18  and then slowly raised from the melt  18 . The cover member  100  and/or the seed crystal are selectively coupled and uncoupled to the chuck  70  so that the pull mechanism  22  may be used to raise and lower either the cover member  100  and/or the seed crystal  75 . 
     Compared to conventional crystal pull systems, the crystal pull systems of embodiments of the present disclosure have several advantages. Use of a cover member that at least partially covers the charge during meltdown acts to reduce radiant heat loss in the vertical direction which reduces thermal stress in the crucible assembly. In embodiments of the present disclosure in which the cover member includes insulation disposed therein, heat loss through the cover member may be reduced. In embodiments in which the cover member includes insulation, heater power may be reduced and the lifetime of the crucible can be further increased. 
     EXAMPLES 
     The processes of the present disclosure are further illustrated by the following Examples. These Examples should not be viewed in a limiting sense. 
     Example 1: Comparison of Crucible Temperature When a Cover Member With and Without Insulation is Used During Meltdown 
     Internal temperatures of the crucible assembly were modeled during the initial meltdown phase when a cover member similar to that shown in  FIG.  4    was positioned at the bottom of the heat shield. Another cover member similar to that of  FIG.  4    was used but the cover member did not include insulation (i.e., felt). A nested crucible assembly made of three crucibles was used. The cover member with insulation resulted in a lower temperature profile compared to the temperature profile of the crucible assembly when a cover member without insulation was used when the temperature was determined at the outer crucible/sidewall ( FIG.  17   ), the central weir/middle crucible ( FIG.  18   ), and the inner weir/innermost crucible ( FIG.  19   ). The maximum decrease in temperature was 20° C. which occurred in the inner weir ( FIG.  19   ). This decrease in temperature reduces damage to the crucible assembly. 
     Example 2: Comparison of the Power Profiles When a Cover Member With and Without Insulation is Used During Meltdown 
     The power supplied to the crucible assembly during the initial melt phase (i.e., the power supplied to the heaters of the crystal pulling system) was determined when a cover member similar to that shown in  FIG.  4    was positioned at the bottom of the heat shield and when another cover member similar to that of  FIG.  4    was used but the cover member did not include insulation. The power supplied using the cover member with insulation was less than the power supplied using the cover member without insulation ( FIG.  20   ). The maximum power supplied for the cover member without insulation was 5 kW greater than the maximum power supplied using the cover member with insulation. 
     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.