Use of quartz plates during the growth of monocrystalline silicon ingots

Abstract

A method for producing single crystal silicon ingots by continuous Czochralski (CCz) is disclosed. One or more plates are added to the outer melt zone of a crucible assembly such that the plates are positioned above an initial charge of solid silicon. The silicon melts and the plates float above the silicon melt. When silicon is added to the outer melt zone to replenish the melt during ingot growth, the silicon contacts the plates rather than falling directly into the melt in the outer melt zone. The silicon melts and falls through an opening that penetrates the thickness of the plate.

Description

[Technical Field]

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 197,726, filed June 7, 2021, which is incorporated herein by reference in its entirety.

[0002] The field of the disclosure relates to continuous Czochralski (CCz) methods of producing single crystal silicon ingots, and in particular to methods of adding a quartz plate to the outer melt zone of a crucible assembly. [Background technology]

[0003] Continuous Czochralski (CCz) is suitable for forming 300mm or 200mm diameter monocrystalline silicon ingots, including ingots with relatively high arsenic or phosphorus doping concentrations. The continuous Czochralski method forms monocrystalline silicon ingots from a silicon melt, with the melt replenished by continuous or intermittent addition of solid silicon during ingot growth. This method may involve forming multiple ingots from the same melt while maintaining a constant hot zone temperature (i.e., the melt resides continuously in the crucible assembly while multiple ingots are grown).

[0004] Wafers sliced ​​from ingots grown by the continuous Czochralski process are increasingly specified by customers to have low void counts (e.g., fewer than 30 defects per wafer) for both 200 mm and 300 mm ingots. The continuous Czochralski process can include a crucible assembly that includes at least two, and often three, melt zones separated by physical barriers: an outer melt zone where solid polycrystalline silicon is introduced, an intermediate melt zone where the melt stabilizes, and an inner melt zone where the silicon ingot is pulled. When solid polycrystalline silicon is added to the melt, inert gas bubbles (e.g., argon bubbles) form in the melt, affecting the void count.

[0005] In some conventional methods, buffer materials such as quartz cullets are added to the melt to reduce the formation of inert gas bubbles. The quartz cullets cushion the interference of polysilicon falling into the melt. The cullets also facilitate the dissipation of inert gas bubbles. However, adding quartz cullets complicates the crystal growth process. Cullets also melt relatively quickly. Gaps that can form between cullet groups limit their effectiveness.

[0006] A need exists for alternative methods for forming silicon ingots that reduce the number of defects in silicon wafers sliced ​​from the ingot and / or reduce the formation of inert gas bubbles in the melt or promote the dissipation of inert gas bubbles.

[0007] This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, as described and / or claimed below. This discussion is believed to be helpful in providing the reader with background information to better understand 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

[0008] One aspect of the present disclosure is directed to a method for growing a single crystal silicon ingot in a continuous Czochralski process. A charge of polycrystalline silicon is added to a crucible assembly. The crucible assembly includes a weir and a sidewall, defining an outer melt zone between the weir and the sidewall. One or more plates are added to the outer melt zone. A silicon melt is formed in the crucible assembly. A surface of the melt is contacted with a seed crystal. A single crystal silicon ingot is pulled from the melt. While the single crystal silicon ingot is pulled to replenish the melt, solid polycrystalline silicon feedstock is added to the outer melt zone. The one or more plates at least partially cover the melt in the outer melt zone.

[0009] Various refinements exist in the features described in connection with the above aspects of the present disclosure. Additional features may be incorporated into the above aspects of the present disclosure as well. These refinements and additional features may exist individually or in any combination. For example, the various features discussed below in connection with any of the illustrated embodiments of the present disclosure may be incorporated into any of the above aspects of the present disclosure, alone or in any combination. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional view of an example ingot pulling apparatus having a solid charge of silicon disposed therein. [Figure 2] FIG. 2 is a cross-sectional view of the ingot pulling apparatus after the plate has been placed on the surface of the silicon charge. [Figure 3] FIG. 3 is a top view of the crucible assembly of the ingot puller with the plates in place. [Figure 4] FIG. 4 is a top view of the plate. [Figure 5] FIG. 5 is a cross-sectional view of an ingot pulling apparatus having a melt with a plate floating above the melt. [Figure 6] FIG. 6 is a cross-sectional view of an ingot pulling apparatus showing how a silicon ingot is pulled up from a silicon melt. [Figure 7] FIG. 7 is a box plot showing the number of voids in wafers sliced ​​from the first ingot (first batch) grown in a continuous Czochralski process. [Figure 8] FIG. 8 is a box plot showing the number of voids in wafers sliced ​​from a first ingot grown in a continuous Czochralski process in which a plate covered the melt during addition of solid silicon to the outer melt zone ("TEST") and a continuous Czochralski process in which a cullet covered the melt during addition of solid silicon to the outer melt zone ("POR").

[0011] Corresponding reference characters indicate corresponding parts throughout the drawings. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present disclosure provides a method for growing monocrystalline silicon ingots using a continuous Czochralski (CCz) process. Prior to ingot formation, one or more plates (e.g., quartz plates) are added to the outer melt zone of a crucible assembly. During ingot growth, solid silicon (e.g., polycrystalline silicon) is added to the outer melt zone. The polycrystalline silicon falls onto the plates. The solid polycrystalline silicon melts and falls through openings formed in the plates into the silicon melt.

[0013] An example of an ingot pulling apparatus 5 for producing an ingot 60 in a continuous Czochralski process is shown in Figures 1-6. As shown in Figure 6, the ingot pulling apparatus 5 includes a crucible assembly 10 containing a melt 6 of semiconductor or solar-grade silicon material. A susceptor 13 supports the crucible assembly 10. The crucible assembly 10 has a sidewall 40 and one or more fluid barriers 20, 30 or "weirs" that separate the melt into different melt zones. In the illustrated embodiment, the crucible assembly 10 includes a first weir 20. The first weir 20 and the sidewall 40 define an outer melt zone 42 of the silicon melt (and crucible assembly 10). The crucible assembly 10 includes a second weir 30 radially inward of the first weir 20, which defines an inner melt zone 22 of the silicon melt. The inner melt zone 22 is a growth region in which the single crystal silicon ingot 60 grows. The first weir 20 and the second weir 30 define an intermediate melt zone 32 of the silicon melt where the melt 60 may be stabilized as it moves toward the inner melt zone 22. The first and second weirs 20, 30 each have at least one opening defined therein to allow the silicon melt to flow radially inward toward the growth region of the inner melt zone 22.

[0014] In the illustrated embodiment, the first weir 20, the second weir 30, and the sidewall 40 each have a generally annular shape. The first weir 20, the second weir 30, and the sidewall 40 may be part of three nested crucibles joined at the bottom or floor 45 of the crucible assembly 10 (i.e., the first and second weirs 20, 30 are sidewalls of two crucibles nested within a larger crucible). The crucible assembly configurations shown in FIGS. 1-6 are exemplary. In other embodiments, the crucible assembly 10 has a single-layer floor with weirs extending upward from the floor 45 (i.e., no nested crucibles). Optionally, the floor 45 may be flat rather than curved, and / or the weirs 20, 30 and / or the sidewall 40 may be straight. Additionally, although the illustrated crucible assembly 10 is shown with two dams, in other embodiments the crucible assembly may have one dam or no dams.

[0015] A feed tube 46 supplies solid silicon, which may be, for example, polysilicon chips, granular polysilicon, chunk polysilicon, or a combination thereof, to the outer melt zone 42 . chunk Polysilicon is generally larger in size than polysilicon chips, which in turn are larger in size than granular polysilicon. chunk The polysilicon generally may have an average nominal size of at least 15 mm (e.g., in the range of 5 mm to 110 mm), while the polysilicon chips may have an average nominal size of 1 to 15 mm. The solid silicon is added at a rate sufficient to maintain a substantially constant melt temperature rise level and volume during growth of ingot 60.

[0016] Typically, the melt 6 from which the ingot 60 is pulled is formed by filling a crucible with polycrystalline silicon to form an initial silicon charge 27 (FIG. 1). Typically, the initial charge is between about 10 kilograms and about 200 kilograms of polycrystalline silicon and may be chips, granules, chunks, or a combination thereof. The mass of the initial charge varies depending on the desired crystal diameter and hot zone design. The initial charge does not reflect the length of the ingot crystal because it is continuously fed during polycrystalline silicon growth.

[0017] Various sources of polycrystalline silicon can be used, including, for example, granular polycrystalline silicon produced by the thermal decomposition of silane or halosilane in a fluidized bed reactor, or polycrystalline silicon produced in a Siemens reactor. The solid silicon is typically polycrystalline silicon, although single crystal silicon (e.g., discarded portions from cut ingots) can also be used.

[0018] Once polycrystalline silicon is added to crucible assembly 10 to form charge 27, one or more plates 31 (FIG. 2) are added to charge 27 in outer melt zone 42. In the illustrated embodiment, multiple plates 31 (FIG. 3) are added to outer melt zone 42 (e.g., at least two, at least three, at least four, at least five, at least eight, at least ten, or at least twelve or more, or more). In other embodiments, only one plate 31 is added (e.g., a plate surrounding the entire perimeter of outer melt zone 42).

[0019] In embodiments where multiple plates 31 are added to the outer melt zone 42, the plates may be free-floating and not connected to one another. In other embodiments, the plates 31 may be connected. The plates 31 may be sized to minimize gaps between adjacent plates 31.

[0020] Plate 31 may be made of quartz or other material that enables plate 31 to operate as described herein. Plate 31 generally has a lower density than silicon melt 6, such that the plate floats on melt 6 after melt 6 is formed.

[0021] Referring now to FIG. 4 , each plate 31 has one or more openings or slots 49 extending through the thickness of the plate 31. As solid silicon is discharged through the feed tube 46 into the outer melt zone 42 of the crucible assembly 10 and onto one or more plates 31, the silicon melts and falls through the openings 49 into the melt 6. In the illustrated embodiment, the openings 49 are slots with major axes generally parallel to the longitudinal axis A of the plate 31 (i.e., the openings 49 are radially spaced apart). In general, the openings 49 can have any shape that enables the plate 31 to operate as described herein. In general, the openings 49 can be sized to be smaller than the size of the type of polycrystalline silicon (e.g., granular, chip, or chunk) introduced into the outer melt zone.

[0022] Plate 31 has an inner edge 51 and an outer edge 53. Edges 51, 53 are curved to fit the contour of outer melt zone 42 (i.e., the area bounded by first weir 20 and sidewall 40). Outer edge 53 is longer than inner edge 51. First and second side surfaces 57, 59 extend between inner edge 51 and outer edge 53.

[0023] Each plate 31 has a width W 31 Each plate 31 has a width W 31 is the width W of the outer melt zone 42 to allow the plate 31 to be positioned within the outer melt zone 42 without contacting the first weir 20 or the sidewall 40 (e.g., during meltdown and / or ingot growth). 42 (Figure 3) is smaller.

[0024] Once polycrystalline silicon is added to crucible assembly 10 to form charge 27 and plate 31 is added to outer melt zone 42, charge 27 is heated to a temperature above the melting temperature of silicon (e.g., about 1412°C) to melt the charge, thereby forming silicon melt 6 (FIG. 6) comprised of molten silicon. Silicon melt 6 has an initial silicon melt volume and an initial melt temperature rise level, with these parameters determined by the size of initial charge 27. In some embodiments, crucible assembly 10 with silicon melt 6 is heated to a temperature of at least about 1425°C, at least about 1450°C, or at least about 1500°C. As initial melt 6 is formed, plate 31 floats on melt 6 in outer melt zone 42.

[0025] The ingot pulling apparatus 5 includes a pulling mechanism 114 ( FIG. 6 ) for growing and pulling an ingot 60 from the melt 6 in the inner melt zone 22. The pulling mechanism 114 includes a pulling cable 118, a seed holder or chuck 120 coupled to one end of the pulling cable 118, and a seed crystal 122 coupled to the seed holder or chuck 120 for initiating crystal growth. One end of the pulling cable 118 is connected to a lifting mechanism (e.g., a driven pulley or drum or other suitable type of lifting mechanism), and the other end is connected to the chuck 120, which holds the seed crystal 122. In operation, the seed crystal 122 is lowered into contact with the melt 6 in the inner melt zone 22. The pulling mechanism 114 is actuated to raise the seed crystal 122 along a pulling axis A, thereby pulling a single crystal ingot 60 from the melt 6.

[0026] Once the polycrystalline silicon charge 27 (FIG. 1) is liquefied to form a silicon melt 6 (FIG. 5) of molten silicon with the plate 31 floating above the melt 6, the silicon seed crystal 122 (FIG. 6) is lowered into contact with the melt 6 in the inner melt zone 22. The silicon seed crystal 122, with the silicon attached to it, is then pulled out of the melt 6, forming a neck 52, thereby forming a melt-solid interface near or at the surface of the melt 6.

[0027] The pulling mechanism 114 can rotate the seed crystal 122 and the ingot 60 coupled thereto. The crucible drive 44 can rotate the susceptor 13 and the crucible assembly 10. In some embodiments, the silicon seed crystal 122 and the crucible assembly 10 rotate in opposite directions, i.e., counter-rotated. The counter-rotation achieves convection within the silicon melt 6. The rotation of the seed crystal 122 is primarily used to provide a symmetrical temperature profile, reduce angular variations in impurities, and control the shape of the crystal-melt interface.

[0028] After the formation of the neck 52, an outwardly flaring seed cone portion 54 (or "crown") adjacent the neck 52 is grown. Typically, the pull rate is reduced from the neck portion pull rate to a rate suitable for growing the outwardly flaring seed cone portion 54. Once the seed cone portion reaches a target diameter, the body 56 or "constant diameter portion" of the ingot 60 is grown. In some embodiments, the body 56 of the ingot 60 has a diameter of about 150 mm, at least about 150 mm, about 200 mm, at least about 200 mm, about 300 mm, at least about 300 mm, about 450 mm, or at least about 450 mm.

[0029] As the ingot 60 is pulled from the melt 6, solid polycrystalline silicon feedstock is added to the outer melt zone 42 through a pipe 46 or other channel to replenish the melt 6 within the ingot growth apparatus 5. The solid polycrystalline silicon can be added from a polycrystalline silicon supply system 66 and can be added continuously or intermittently to the ingot puller to maintain the melt level. Generally, polycrystalline silicon can be metered into the ingot puller 5 by any method available to those skilled in the art. The solid polycrystalline silicon added to the outer melt zone 42 can be in the form of silicon chips, chunks, or granules.

[0030] In some embodiments, dopants are also added to the melt 6 during ingot growth. The dopants may be introduced from a dopant supply system 72. The dopants may be added as a gas or a solid and may be added to the outer melt zone 42.

[0031] The apparatus 5 may include a heat shield 116 disposed around the growing ingot 60 to allow the growing ingot 60 to radiate its latent heat of solidification and heat flux from the melt 6. The heat shield 116 may be at least partially conical and sloped diagonally downward to form an annular opening in which the ingot 60 is placed. A flow of inert gas, such as argon, is typically provided along the length of the growing crystal. The ingot 60 is pulled through a growth chamber 78 that is sealed from the surrounding atmosphere.

[0032] Multiple independently controlled annular bottom heaters 70 may be radially positioned below the crucible assembly 10. The annular bottom heaters 70 apply heat in a relatively controlled distribution across the entire bottom area of ​​the crucible assembly 10. The annular bottom heaters 70 may also be individually controlled planar resistive heating elements as described in U.S. Pat. No. 7,635,414, which is incorporated herein by reference for all relevant consistent purposes. The apparatus 5 may include one or more side heaters 74 positioned radially outward of the crucible assembly 10 to control the temperature distribution through the melt 6.

[0033] The ingot growth apparatus 5 shown in Figures 1-6 and described herein is exemplary, and unless otherwise specified, generally any system for producing crystalline ingots by the continuous Czochralski process can be used.

[0034] As the ingot 60 is pulled from the melt 6, solid polycrystalline silicon feedstock is added to the crucible assembly 10 while the single crystal silicon ingot 60 is being pulled to replenish the melt 6. The solid silicon falls onto the plate 31, which at least partially covers the melt 6 in the outer melt zone 42. The heat of the melt 6 heats the solid polycrystalline silicon disposed on the plate 31, causing the silicon to melt and fall through the openings 49 extending through the plate 31 or over the edges 51, 53 and / or sides 57, 59 of the plate 31.

[0035] In some continuous Czochralski processes, multiple ingots are grown while the hot zone (i.e., the lower portion of the apparatus 5, such as the crucible assembly 10 and susceptor 13) is heated with the silicon melt 6 continuously present within the crucible assembly 10. In such methods, a first ingot is grown to a target length, growth is terminated, the ingot is removed from the ingot puller, and a seed crystal is then lowered into the melt to begin growth of a second single crystal silicon ingot (i.e., using the same melt from which the first ingot was pulled). The plates 31 remain in the melt 6 while the second and subsequent ingots are grown and while polycrystalline silicon is added to the outer melt zone 42 to replenish the melt. The thickness of the plates 31 can be selected so that the plates 31 do not completely melt and remain in the melt 6 after the first ingot is formed. In other embodiments, a new set of plates 31 is added before the growth of each subsequent ingot.

[0036] Subsequent ingots may be grown at a temperature where the hot zone remains intact and a continuous melt of silicon is present within the crucible assembly 10 (e.g., until one or more components of the hot zone deteriorate such that the crucible assembly requires cooling and replacement of the deteriorated components). For example, at least 1, 2, 3, 4, 5, 6, 10, or 20 or more ingots may be grown.

[0037] The disclosed method offers several advantages over conventional methods for growing single-crystal silicon ingots using the continuous Czochralski (CCz) process. Without being bound by theory, it is believed that adding polycrystalline silicon to the outer melt zone of the crucible assembly creates relatively small bubbles (e.g., less than 10 μm) of inert gas (e.g., argon) that can be carried by the melt through openings in each weir, allowing the bubbles to reach the solid-melt interface. The plates may serve to prevent inert gas entrapment in the melt by preventing the polycrystalline feedstock from being discharged directly into the melt. The plates also provide surface area and nucleation points for inert gas bubbles to coalesce, thereby increasing bubble size and buoyancy. The plates provide a monolithic layer of quartz on the surface of the melt (e.g., reducing porosity compared to quartz cullet). A certain amount of the plates dissolve after melt formation, and the dissolved quartz also aids in removing inert gas from the melt. The plates melt at a slower rate than quartz cullet, improving the durability of the plates compared to cullet. The plates can be placed relatively easily into the crucible assembly before the hot zone reaches a predetermined temperature (e.g., for placement in the initial charge of polycrystalline silicon). In embodiments where multiple plates are used, the plates have low rigidity and can move with the silicon as it moves relative to the crucible, thereby preventing the plates from submerging in the melt. In embodiments where the plates have a width narrower than the width of the outer melt zone, the plates are less likely to sinter to the sides / weirs of the crucible assembly during meltdown. [Example]

[0038] The processes of the present disclosure are further illustrated by the following examples, which should not be construed in a limiting sense. Example 1: Void counts in wafers grown from ingots with quartz plates added to the outer melt zone

[0039] As shown in Figure 7, the first ingot (Batch A) grown by the continuous Czochralski process (200 mm) typically contains more microvoids (detected by laser light scattering of at least 0.12 μm in size) compared to the subsequently grown ingots (Ingot BG).

[0040] Figure 8 shows the void count for the first grown ingot (Batch A) during a number of consecutive Czochralski runs. The "test" runs (far right boxplots) include plates on the initial charge of silicon (Figure 2) and on the melt after polycrystalline silicon has been added to the outer melt zone (Figure 6). Other runs include quartz cullets instead of quartz plates. As can be seen in Figure 8, runs using plates did not increase void counts to unacceptable levels.

[0041] When introducing elements of the disclosure or embodiments thereof, the articles "a," "an," "the," and "said" are intended to mean that there are one or more 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 specific directional terms (e.g., "top," "bottom," "side," etc.) is for convenience of description and does not require a specific orientation of the items described.

[0042] Because 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 be interpreted as illustrative and not in a limiting sense.

Claims

1. 1. A method for growing a single crystal silicon ingot in a continuous Czochralski process, comprising: The method comprises: adding a charge of polycrystalline silicon to a crucible assembly, the crucible assembly including a weir and a sidewall defining an outer melt zone between the weir and the sidewall; adding one or more plates to said outer melt zone; forming a melt of silicon in the crucible assembly, the one or more plates floating above the melt, the one or more plates each including one or more openings extending therethrough to allow silicon to enter the melt, the one or more plates floating above the melt each including one or more openings extending therethrough to allow silicon to enter the melt; contacting the surface of the melt with a seed crystal; Pulling the single crystal silicon ingot from the melt; adding a solid polycrystalline silicon feedstock to the outer melt zone to replenish the melt while the single crystal silicon ingot is being pulled, and one or more plates at least partially covering the melt in the outer melt zone during the addition of the solid polycrystalline silicon feedstock to the outer melt zone, and silicon entering the melt through the one or more openings extending through the one or more plates. method.

2. The weir is a first weir, and the crucible assembly includes a second weir radially inward from the first weir, the first weir and the second weir defining an intermediate melt zone between the first weir and the second weir, and the second weir defining an inner melt zone within the second weir. The method of claim 1.

3. The outer melt zone has a width, and one or more plates each have a width, the width of each of the one or more plates being less than the width of the outer melt zone. The method of claim 1.

4. the plate is positioned on solid polycrystalline silicon in the outer melt zone before the melt of silicon is formed in the crucible assembly. The method of claim 1.

5. The one or more plates are made of quartz. The method of claim 1.

6. The single crystal silicon ingot is the first ingot pulled from the melt of silicon after the melt is formed in the crucible assembly. The method of claim 1.

7. contacting the surface of the melt with the seed crystal; pulling a second single crystal silicon ingot from the melt; and adding solid polycrystalline silicon feedstock to the outer melt zone to replenish the melt while pulling the second single crystal silicon ingot, the one or more plates at least partially covering the melt in the outer melt zone, and adding the solid polycrystalline silicon feedstock to the outer melt zone. The method of claim 6.

8. The single crystal silicon ingot is a single crystal silicon ingot grown subsequent to the first single crystal silicon ingot pulled from the melt. The method of claim 1.

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