Crystal pulling system having a cover member for covering a silicon charge and method for growing a silicon melt in a crucible assembly - Patents.com

Abstract

A crystal pulling system is disclosed having a housing and a crucible assembly. The system includes a heat shield defining a central passage through which an ingot passes during growth. A cover member is movable within the heat shield along a pulling axis. The cover member may include a thermal insulating layer. The cover member covers at least a portion of the charge during melting.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 073,180, filed September 1, 2020, which is incorporated herein by reference in its entirety. [Technical Field]

[0002] The field of the disclosure relates to crystal pulling systems for growing single crystal ingots from a silicon melt, and in particular to crystal pulling systems including cover members for use in continuous Czochralski silicon ingot growth. [Background technology]

[0003] Silicon ingots of silicon crystals can be prepared by the Czochralski method, in which a single crystal silicon seed is contacted with a silicon melt held in a crucible. The single crystal silicon seed is withdrawn from the melt, and a single crystal silicon ingot is pulled from the melt. Ingots can be prepared in a batch system, in which a charge of polycrystalline silicon is first melted in a crucible, and silicon ingots are withdrawn from the melt until the molten silicon in the crucible is depleted. Alternatively, ingots can be withdrawn in a continuous Czochralski method, in which polysilicon is intermittently or continuously added to the melt to replenish the silicon melt during ingot growth.

[0004] In the continuous Czochralski process, the crucible can be divided into separate melting zones. For example, the crucible assembly can include an outer melting zone where polycrystalline silicon is added and melted to replenish the silicon melt during silicon ingot growth. The silicon melt flows from the outer melting zone to an intermediate zone within the outer melting zone where the melt is thermally stable. The silicon melt then flows from the intermediate zone to a growth zone from which a silicon ingot is pulled.

[0005] The crystal pulling system can include a heat shield positioned above the crucible and silicon melt. The heat shield includes a passageway through which the silicon ingot passes as it is pulled vertically from the silicon melt. The heat shield protects and shields the pulled ingot from radiant heat from the melt.

[0006] During the melting stage, temperature gradients can be created within the crystal pulling system that can cause thermal stresses in the crucible, damaging or even destroying it.

[0007] What is needed is a crystal pulling system that maintains a more uniform temperature gradient during melting to reduce crucible damage during melting.

[0008] This section is intended to introduce the reader to various aspects of technology 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. As such, it should be understood that these statements are to be read in this light, and not as admissions of prior art. Summary of the Invention

[0009] One aspect of the present disclosure relates to a crystal pulling system for growing a single crystal ingot from a silicon melt. The system includes a pulling shaft and a housing defining a growth chamber. A crucible assembly is disposed within the growth chamber to contain the silicon melt. A heat shield defines a central passage through which an ingot passes during growth. The system includes a cover member movable within the heat shield along the pulling shaft. The cover member includes one or more thermal insulation layers.

[0010] Another aspect of the present disclosure relates to a method for preparing a silicon melt 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 to contain a silicon melt, and a heat shield defining a central passage through which an ingot passes during 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. While the cover member covers a portion of the charge, the silicon charge is heated to produce a silicon melt in the crucible assembly. The cover member is raised after the melt is formed.

[0011] Yet another aspect of the present disclosure relates to a crystal pulling system for growing a single crystal ingot from a silicon melt. The system includes a housing having a pulling axis and defining a growth chamber. A crucible assembly is disposed within the growth chamber to contain the silicon melt. The system includes a heat shield defining a central passage through which an ingot passes during growth. A cover member is movable within the heat shield along the pulling axis. The cover member includes a first plate having a first plate axis parallel to the pulling axis. The cover member includes a second plate having a second plate axis parallel to the pulling axis. The second plate is disposed above the first plate.

[0012] Various refinements of the features described in connection with the above-described aspects of the present disclosure exist. Additionally, additional features may be incorporated into the above-described aspects of the present disclosure. These refinements and additional features may exist individually or in any combination. For example, various features described below in connection with any of the illustrated embodiments of the present disclosure may be incorporated, alone or in any combination, into any of the above-described aspects of the present disclosure. [Brief explanation of the drawings]

[0013] [Figure 1]1 is a cross-sectional view of a crystal pulling system for growing a single crystal ingot from a silicon melt. [Figure 2] 1 is a cross-sectional view of a portion of a crystal pulling system including a cover member disposed within a central passage of a heat shield. [Figure 3] FIG. 2 is a perspective view of a cover member of a crystal pulling system. [Figure 4] FIG. [Figure 5] FIG. [Figure 6] FIG. 4 is a perspective view of a first plate of the cover member. [Figure 7] FIG. 10 is a bottom view of the first plate. [Figure 8] FIG. 2 is a cross-sectional view of a first plate. [Figure 9] FIG. 10 is a perspective view of a second plate of the cover member. [Figure 10] FIG. 10 is a top view of the second plate. [Figure 11] FIG. 2 is a cross-sectional view of a second plate. [Figure 12] FIG. 2 is a perspective view of a heat insulating layer of the cover member. [Figure 13] FIG. 4 is a cross-sectional view of the shaft of the cover member. [Figure 14] FIG. [Figure 15] FIG. 1 is a perspective view of a chuck of a crystal pulling system. [Figure 16] FIG. 10 is a perspective view of a chuck engaged with a shaft of a cover member. [Figure 17] 10 is a graph of the internal temperature of the outer crucible of a nested crucible assembly when using a cover member with and without insulation during melting. [Figure 18] 10 is a graph of the internal temperature of the central crucible of a nested crucible assembly when using a cover member with and without insulation during melting. [Figure 19] 1 is a graph of the internal temperature of the innermost crucible of a nested crucible assembly when using a cover member with and without insulation during melting. [Figure 20] 10 is a graph of the power profile using a cover member with and without insulation during melting.

[0014] Like reference numbers refer to like parts throughout the drawings. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present disclosure provides a crystal pulling system for producing monocrystalline (i.e., single crystal) silicon ingots (e.g., semiconductor-grade or solar-grade material) from a silicon melt by the continuous Czochralski (CZ) process. The systems and methods disclosed herein may also be used to grow single crystal ingots by the batch or recharge CZ process. Referring to FIG. 1 , a crystal pulling system is shown generally and generally designated 10. The crystal pulling system 10 is configured with a pulling axis Y 10 and a housing 12 that defines a growth chamber 14. A crucible assembly 16 is disposed within the growth chamber 14. The crucible assembly 16 contains a silicon melt 18 (e.g., semiconductor-grade or solar-grade material) from which a single crystal ingot 20 is pulled by a pulling mechanism 22, as described further below.

[0016] The crystal pulling system 10 includes a heat shield 24 (sometimes referred to as a "reflector") that defines a central passageway 26 through which the ingot 20 passes during ingot growth. According to an embodiment of the present disclosure, during the initial melting stage, before the ingot 20 is pulled from the melt 18, a cover member 100 (FIG. 2) is lowered to at least partially cover the solid charge of polycrystalline silicon to reduce heat radiating through the central passageway 26 during melting. The cover member 100 is positioned about the pulling axis Y. 10 , and is movable within the heat shield 24 along the axis .

[0017] FIG. 2 illustrates a portion of the crystal pulling system 10 with the cover member 100 positioned within the central passage 26 during the initial phase (i.e., melting phase) in which the charge is melted before the ingot 20 is pulled. The crucible assembly 16 includes a bottom 30 and an outer sidewall 32 extending upwardly from the bottom 30. The crucible assembly 16 includes a central dam 34 and an inner dam 36 extending upwardly from the bottom 30. The central dam 34 is positioned between the outer sidewall 32 and the inner dam 36. The crucible assembly 16 includes a crucible melting zone 38 positioned between the outer sidewall 32 and the central dam 34. The crucible assembly 16 also includes an intermediate zone 40 positioned between the central dam 34 and the inner dam 36. The crucible assembly 16 also includes a growth zone 42 positioned within the inner dam 36. Crucible assembly 16 may be made, for example, from quartz or any other suitable material that enables crystal pulling system 10 to function as described herein. Furthermore, crucible assembly 16 may be of any suitable size that enables crystal pulling system 10 to function as described herein. Crucible assembly 16 may also include three "nested" crucibles having separate bases that together form a bottom, with the side walls of the crucibles being the weirs 34, 36 described above.

[0018] During ingot growth, polycrystalline silicon is added to the crucible melt zone 38, where it melts and replenishes the silicon melt. The silicon melt flows into the intermediate zone 40 through the central weir opening 44. The silicon melt then flows through the inner weir opening 41 into the growth zone 42, located within the inner weir 36. The various silicon melt zones (e.g., melt zone 38, intermediate zone 40, growth zone 42) enable the ingot to be grown according to the continuous Czochralski method, in which polycrystalline silicon is added to the melt continuously or semi-continuously and the ingot 20 is continuously raised from the growth zone 42. The silicon melt 18 in the growth zone 42 contacts 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 with and attach to the seed, forming the ingot 20.

[0019] 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 the crucible assembly 16 to supply thermal energy to the system 10. One or more bottom heaters 62 are positioned below the crucible assembly 16 and the susceptor 50. The heaters 52, 62 operate to melt the initial charge of solid polycrystalline silicon feedstock and to maintain the melt 18 in a liquefied state after the initial charge has melted. The heaters 52, 62 also act to melt solid polycrystalline silicon added through a feed tube 54 ( FIG. 1 ) during ingot growth. The heaters 52, 62 may be any suitable heaters (e.g., resistance heaters) that enable the system 10 to function as described herein.

[0020] 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 exhausting the inert gas and other gaseous and airborne particles from the growth chamber 14. The gas inlet provides a suitable inert gas, such as argon.

[0021] 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 aligned with the central passage 26. The ingot 20 is aligned with the pulling axis Y. 10 The ingot 20 is pulled along the central passage 26 into a jacket chamber 60. The jacket 57 cools the pulled ingot 20.

[0022] The heat shield 24 is generally frustoconical in shape. The heat shield 24 includes an outer surface 61 that 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 with molybdenum sheets inside. The surface 61 may be coated (e.g., with SiC) to reduce contamination of the melt.

[0023] The heat shield 24 includes a bottom 58 (FIG. 2). The central passage 26 of the heat shield 24 has a diameter D 26 The heat shield 24 is positioned above the crucible assembly 16 such that the central passage 26 is positioned directly above the growth zone 42 so that the ingot drawn from the melt 18 can be drawn through the central passage 26. The passage diameter D 26 is sized to accommodate the diameter of the ingot 20 (eg, a 200 mm or 300 mm or other diameter ingot).

[0024] The outer surface 61 may be coated with a reflective coating that reflects radiant heat toward the melt 18 and crucible assembly 16. In this manner, the heat shield 24 helps to retain heat within the crucible assembly 16 and the melt 18. Additionally, the heat shield 24 is oriented in a direction away from the pulling axis Y. 10 This helps maintain a nearly uniform temperature gradient along the

[0025] During the initial melting stage, an initial amount of solid polycrystalline silicon is provided to the crucible melting zone 38, the intermediate zone 40, and the growth zone 42. In other embodiments, solid polycrystalline silicon is added to only one or two zones selected from the crucible melting zone 38, the intermediate zone 40, and the growth zone 42. During melting, the cover member 100 is lowered to cover at least a portion of the silicon charge (i.e., to block the central passage 26 of the heat shield 24) while the initial charge is melted. The lifting mechanism 22 raises and lowers the cover member 100.

[0026] According to embodiments of the present disclosure, cover member 100 is lowered less than 30 mm from bottom 58 of heat shield 24 (i.e., from below or above bottom 58), or less than 20 mm, less than 10 mm, or less than 5 mm from bottom 58 of heat shield 24. In some embodiments, cover member 100 is lowered to be aligned with bottom 58 of heat shield 24. In some embodiments, cover member 100 is lowered to within 80-100 mm of the surface of the charge during melting.

[0027] After the initial amount of silicon charge is melted, a second amount of polycrystalline silicon may be added to the crucible melting zone 38 (e.g., added sequentially until the entire second amount is added). According to some embodiments of the present disclosure, a cover member 100 covers the central passageway 26 while this second amount of polycrystalline silicon is added and melted in the melting zone 38. After the second charge is melted, the cover member 100 is lifted by the lifting mechanism 22. In other embodiments, the cover member 100 is not used while the second amount of polycrystalline silicon is added.

[0028] One embodiment of the cover member 100 is shown in Figure 3. The cover member 100 has a first plate 102 and a second plate 104 (sometimes referred to herein as the "lower plate 102" and the "upper plate 104", respectively). Each plate 102, 104 is aligned with a lift axis Y. 10 The first plate 102 and the second plate 104 are substantially parallel to each other. The second plate 104 is disposed above the first plate 102. The first plate 102 and the second plate 104 are substantially parallel to each other. The second plate 104 is disposed above the first plate 102.

[0029] First plate 102 includes a first annular wall 106 (FIGS. 5-6), and second plate 104 includes a second annular wall 108 (FIG. 9). Referring now to FIG. 5, first wall 106 includes a first shoulder 110 and a first lip 111. Second wall 108 includes a second shoulder 112 and a second lip 113. When assembled, second shoulder 112 rests on first lip 111, and second lip 113 rests on first shoulder 110. A cover member chamber 116 (FIGS. 8 and 11) is disposed between first plate 102 and second plate 104.

[0030] The insulating layer 130 (FIG. 4) is disposed within the chamber 116 formed between the first plate 102 and the second plate 104. The insulating layer 130 is 130 The insulating layer 130 may have a thickness of about 10 mm to about 50 mm. The insulating layer 130 may be compressed between the first plate 102 and the second plate 104. The insulating layer 130 may include several layers of insulating material or may be a single layer. The insulating layer 130 may include openings 132 formed therein.

[0031] The insulating layer 130 may be made of felt. The felt may be composed of natural or synthetic fibers. The felt may be purified felt (e.g., 30 ppm maximum ash content). In general, the insulating layer 130 may be composed of any material that contains suitable insulating properties.

[0032] The first plate 102 includes an upwardly protruding hub 145 (FIG. 4) for coupling the shaft 150. The second plate includes a second plate opening 128 (FIG. 9). The hub 145 extends through the opening 128 and the insulating opening 132 (FIG. 12) in the second plate 104. The hub 145 includes a ledge 149 (FIG. 6), and the second plate 104 is 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 contour that matches the contour 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.

[0033] The cover member 100 has a generally arcuate shape including a circular portion 120 (FIG. 7) with a center X and a perimeter 122, and has a straight edge 124. Specifically, the first and second plates 102, 104 have the shape of the circular portion with a segment removed along a chord. The first and second plates 102, 104 have a long dimension L1 and a short dimension L2. The long dimension L1 is the diameter of the circular portion 120, and the short dimension L2 is the Straight edge 124 Extending from the center X through the center to the outer periphery 122 of the circular portion 120. The first and second plates 102, 104 are shaped as an arc to allow for viewing of the fill / melt. In other embodiments, the cover member 100 is completely circular.

[0034] In some embodiments, the diameter of the cover member 100 is at least 0.75 times the diameter of the central passageway 26 in the bottom 58 of the heat shield 24, or 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 passageway 26 in the bottom 58 of the heat shield 24.

[0035] In some embodiments, the first plate 102 and the second plate 104 are made of graphite. The graphite may be coated with silicon carbide (SiC). The first and second plates 102, 104 may be constructed of other suitable materials. The first and second plates 102, 104 may have any suitable thickness T that prevents thermal stresses that could result in cracking or damage to the first and second plates 102, 104. 102 , T 104 (for example, a thickness between 3 mm and 50 mm) (FIGS. 8 and 11).

[0036] 13-14, cover member 100 includes a shaft 150 that supports cover member 100. Shaft 150 may be coupled to first and / or second plates 102, 104 in any suitable coupling configuration. In the illustrated embodiment, shaft 150 includes an elongated rectangular portion 154 and a collar 156. Collar 156 has a diameter D 156 that is smaller than the diameter of hub chamber 126 (FIG. 8) and larger than the width of hub opening 153. 156 The first plate 102 rests on a collar 156. The insulating layer 130 and the second plate 104 (FIG. 4) are supported by the first plate 102. Alternatively, the shaft 150 may be integrally formed with either or both of the first plate 102 and / or the second plate 104. Shaft 150 is shaft axis A 150 Includes:

[0037] 15 and 16, the lifting mechanism 22 is configured to have a lifting axis Y 10The cover member 100 includes a chuck 70 that is raised and lowered along a pull-up wire or cable 37 that is raised and lowered by a drive motor (i.e., the pull-up wire or cable and motor are part of the pull-up mechanism 22). The cover member 100 is removably connectable to the chuck 70. For example, the shaft 150 and chuck 70 may be connected using a pin lock. The shaft 150 has a recess 158 ( FIG. 13 ). The shaft 150 is inserted into a hole 72 in the chuck 70 so that the recess 158 is received within the hole 72. The chuck 70 includes an opening 74 that extends approximately perpendicular to the hole 72, passes through the chuck 70, and opens into the hole 72. A pin 76 is inserted through the opening 74 into the hole 72 so that the pin 76 engages with the recess 158 of the shaft 150 disposed within the hole 72. In this manner, the shaft 150 and the chuck 70 are coupled together. The shaft 150 and chuck 70 may include any alternative and / or additional features for coupling the cover member 100 to the chuck 70 .

[0038] After melting, the cover member 100 is removed from the chuck 70 and a seed crystal 75 ( FIG. 1 ) is coupled to the chuck 70. The seed crystal 75 may include a similar recess, not shown, to allow the seed 75 to be coupled and / or uncoupled from the chuck 70. During the ingot growth process, the seed 75 is lowered into contact with the melt 18 by the lifting mechanism 22 and then slowly lifted out of the melt 18. The cover member 100 and / or the seed crystal are selectively coupled and uncoupled from the chuck 70 such that the lifting mechanism 22 can be used to raise and lower either the cover member 100 and / or the seed crystal 75.

[0039] Compared to conventional crystal pulling systems, the crystal pulling systems of the present disclosure have several advantages. The use of a cover member that at least partially covers the charge during melting acts to reduce vertical radiative heat loss, 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 can be reduced. In embodiments in which the cover member includes insulation, heater power can be reduced, further extending the life of the crucible.

[0040] [Example] The processes of the present disclosure are further illustrated by the following examples, which should not be viewed in a limiting sense.

[0041] Example 1: Comparison of crucible temperatures using insulated and uninsulated cover members during melting The internal temperature of a crucible assembly during the initial melting stage was modeled when a cover element similar to that shown in Figure 4 was placed at the bottom of the heat shield. A different cover element similar to that shown in Figure 4 was used, but the cover element did not include insulation (i.e., felt). A nested crucible assembly consisting of three crucibles was used. When temperatures were measured at the outer crucible / sidewall (Figure 17), center weir / center crucible (Figure 18), and inner weir / innermost crucible (Figure 19), the cover element with insulation resulted in a lower temperature profile compared to the temperature profile of the crucible assembly when a cover element without insulation was used. The maximum temperature drop of 20°C occurred at the inner weir (Figure 19). This temperature drop reduces damage to the crucible assembly.

[0042] Example 2: Comparison of power profiles using insulated and uninsulated cover members during melting The power delivered to the crucible assembly (i.e., the power delivered to the heater of the crystal pulling system) during the initial melting stage was measured when a cover similar to that shown in Figure 4 was placed on the bottom of the heat shield and when another cover similar to that of Figure 4 was used, but the cover did not include insulation. The power delivered using the cover with insulation was less than the power delivered using the cover without insulation (Figure 20). The maximum power delivered for the cover without insulation was 5 kW greater than the maximum power delivered using the cover with insulation.

[0043] As used herein, the terms "about," "substantially," "essentially," and "approximately," when used in conjunction with a range of dimensions, concentrations, temperatures, or other physical or chemical properties or characteristics, are meant to encompass variations that may exist at the upper and / or lower limits of the range of the property or characteristic, including, for example, variations resulting from rounding, measurement method, or other statistical variations.

[0044] When introducing elements of the present disclosure or embodiment(s) thereof, the articles "a," "an," "the," and "said" are intended to mean that there are one or more of the element. The terms "comprising," "including," and "having" are intended to be inclusive and mean that additional elements may be present other than the listed elements. The use of specific orientational terms (e.g., "top," "bottom," "side," etc.) is for convenience of description and does not require a particular orientation of the parts being described.

[0045] Since various changes can be made in the above-described structures and methods without departing from the scope of the present disclosure, 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.

Claims

1. 1. A crystal pulling system for growing a single crystal ingot from a silicon melt, said system having a pulling shaft, said system comprising: a housing defining a growth chamber; a crucible assembly disposed within the growth chamber for containing the silicon melt; a heat shield defining a central passageway through which the ingot passes during growth of the ingot; a cover member movable within the heat shield along the lifting axis, a first plate having a first plate axis parallel to the pull axis, the first plate including a hub, the hub having a hub opening; a second plate having a second plate axis parallel to the pulling axis, the second plate being positioned above the first plate; one or more thermal insulation layers disposed between the first plate and the second plate; a shaft having an elongated portion and a collar extending radially outward from the elongated portion, the shaft extending through the hub opening, the first plate resting on the collar; a cover member; A crystal pulling system comprising:

2. 10. The crystal pulling system of claim 1, wherein said first plate and said second plate are both made of graphite.

3. 3. The crystal pulling system of claim 2, wherein said first plate and said second plate are coated with silicon carbide.

4. 2. The crystal pulling system of claim 1, wherein said insulating layer is made of felt.

5. 2. The crystal pulling system of claim 1, wherein the heat shield has a bottom, the central passageway of the heat shield has a diameter at the bottom of the heat shield, and the cover member has a diameter, the diameter of the cover member being at least 0.75 times the diameter of the central passageway at the bottom of the heat shield.

6. 10. The crystal pulling system of claim 1, wherein the crystal pulling system comprises a pulling mechanism having a chuck, the pulling mechanism being capable of raising and lowering the chuck along the pulling axis, and the cover member being removably connectable to the chuck.

7. 7. The crystal pulling system of claim 6, wherein the chuck is capable of being connected to a seed crystal for initiating ingot growth.

8. 10. The crystal pulling system of claim 1, wherein the crucible assembly comprises a bottom, an outer sidewall, and an inner dam extending upwardly from the bottom.

9. 9. The crystal pulling system of claim 8, wherein the crucible assembly comprises a central dam disposed between the outer sidewall and the inner dam.

10. 10. The crystal pulling system of claim 9, wherein the crucible assembly comprises three nested crucibles.

11. 10. The crystal pulling system of claim 1, further comprising a fluid-cooled cylindrical jacket having a jacket chamber through which the ingot passes during growth.

12. 1. A method for preparing a silicon melt in a crucible assembly of a crystal pulling system, the crystal pulling system comprising: a housing defining a growth chamber; a crucible assembly disposed within the growth chamber for containing the silicon melt; and a heat shield defining a central passageway through which an ingot passes during growth of the ingot; the crucible assembly comprising a bottom, an outer sidewall, an inner dam extending upwardly from the bottom, and a central dam disposed between the outer sidewall and the inner dam; a crucible melting zone disposed between the outer sidewall and the central dam; an intermediate zone disposed between the central dam and the inner dam; and a growth zone disposed within the inner dam; the method comprising: adding an initial charge of polycrystalline silicon to said crucible assembly; lowering a cover member through the central passage defined by the heat shield to cover at least a portion of the initial charge; heating the initial charge to produce a silicon melt within the crucible assembly while the cover member covers at least a portion of the initial charge; adding a second quantity of polycrystalline silicon to the crucible melting zone after the initial charge has melted; melting the second quantity of polycrystalline silicon, causing the silicon melt to flow through a central weir opening into the intermediate zone disposed between the central weir and the inner weir, and causing the silicon melt to flow through an inner weir opening into the growth zone disposed within the inner weir, and the cover member covering at least a portion of the silicon melt while the second quantity of polycrystalline silicon is being added to the crucible melting zone; raising the cover member after the second amount of polycrystalline silicon has melted. A method comprising:

13. 13. The method of claim 12, wherein the heat shield has a bottom, the central passage of the heat shield has a diameter at the bottom of the heat shield, and the cover member has a diameter, the diameter of the cover member being at least 0.9 times the diameter of the central passage at the bottom of the heat shield.

14. The method of claim 12 , wherein the cover member is lowered to less than 30 mm from the bottom of the heat shield.

15. The method of claim 12 , wherein the cover member is lowered to the bottom of the heat shield.

16. The method of claim 12 , wherein the cover member includes a thermal insulating layer.

17. The method of claim 12 , wherein the cover member comprises a silicon carbide coated graphite plate.

18. the silicon carbide-coated graphite plate is a first silicon carbide-coated graphite plate, and the cover member is a second silicon carbide-coated graphite plate disposed above the first silicon carbide-coated graphite plate; a thermal insulation layer disposed between the first silicon carbide-coated graphite plate and the second silicon carbide-coated graphite plate; 20. The method of claim 17, comprising:

19. 1. A method for forming a single crystal silicon ingot, comprising: Preparing a melt of silicon in a crucible of a crystal pulling system according to the method of claim 12; after raising the cover member, lowering a seed crystal into contact with the melt; A method comprising:

20. The cover member and the seed crystal are raised and lowered by a lifting mechanism having a chuck, and the method further comprises: removing the cover member from the chuck after the cover member is raised; After removing the cover member from the chuck, connecting the seed crystal to the chuck.

20. The method of claim 19, comprising:

21. 1. A crystal pulling system for growing a single crystal ingot from a silicon melt, said system having a pulling shaft, said system comprising: a housing defining a growth chamber; a crucible assembly disposed within the growth chamber for containing the silicon melt; a heat shield defining a central passageway through which the ingot passes during growth of the ingot; a cover member movable within the heat shield along the lifting axis, a first plate having a first plate axis parallel to the pull axis, the first plate including a hub, the hub having a hub opening; a second plate having a second plate axis parallel to the pull axis, the second plate being disposed above the first plate, the hub opening extending through the first plate and the second plate; a shaft having a collar and an elongated portion extending from the collar, the collar extending radially outward from the elongated portion, the shaft extending through the hub opening, the first plate resting on the collar; a cover member comprising: A crystal pulling system comprising:

22. A lifting mechanism having a chuck, the lifting mechanism being capable of raising and lowering the chuck along the lifting axis, and the cover member being removably connectable to the chuck.

22. The crystal pulling system of claim 21.

23. The method of claim 22, wherein the chuck is capable of being connected to a seed crystal for initiating ingot growth.

23. The crystal pulling system of claim 22.

24. The crucible assembly comprising a bottom, an outer sidewall, and an inner weir extending upwardly from the bottom.

22. The crystal pulling system of claim 21.

25. The crucible assembly comprising a central weir disposed between the outer sidewall and the inner weir.

25. The crystal pulling system of claim 24.

26. The crucible assembly comprising three nested crucibles.

26. A crystal pulling system according to claim 25.

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