Use of quartz plates during the growth of single-crystal silicon ingots

Abstract

Methods for producing single-crystal silicon ingots by continuous Czochralski (CCz) are disclosed. One or more plates are added to the outer melting zone of a crucible assembly so that the plates are placed on an initial fill of solid-state silicon. The silicon is melted and the plates float on the silicon melt. When silicon is added to the outer melting zone to replenish the melt during ingot growth, the silicon does not fall directly into the melt of the outer melting zone but comes into contact with the plates. The silicon is melted and falls through openings extending through the thickness of the plates.

Description

Technology Field

[0001] Cross-reference regarding related applications

[0002] This application claims priority to U.S. provisional patent application No. 63 / 197,726 filed June 7, 2021, the entirety of which is incorporated herein by reference.

[0003] The field of the present disclosure is to methods for producing single-crystal silicon ingots by continuous Czochralski (CCz), and in particular, to methods in which quartz plates are added to an outer melting zone of a crucible assembly. Background Technology

[0004] Continuous Czochralski (CCz) is very suitable for forming single-crystal silicon ingots with a diameter of 300 mm or 200 mm, such as ingots that are relatively highly doped with arsenic or phosphorus. Continuous Czochralski methods involve forming a single-crystal silicon ingot from a melt of silicon while continuously or intermittently adding solid-state silicon to the melt to replenish the melt while the ingot is growing. The methods may involve forming multiple ingots from the same melt while the hot zone remains at a certain temperature (i.e., the melt is continuously present in the crucible assembly while multiple ingots are growing).

[0005] Customers are increasingly specifying that wafers sliced ​​from ingots grown by continuous Czochralski processes have low void counts (e.g., fewer than 30 defects per wafer) for both 200 mm and 300 mm ingots. Continuous Czochralski processes may involve a crucible assembly comprising at least two, and often three, melting zones separated by physical barriers—an outer melting zone where solid polycrystalline silicon is fed, an intermediate melting zone where the melt is stabilized, and an inner melting zone where the silicon ingot is raised. The addition of solid polycrystalline silicon to the melt causes inert gas bubbles (e.g., argon bubbles) to form within the melt, which affects the void count.

[0006] In some existing methods, buffer elements such as quartz cullets have been added to the melt to reduce the formation of inert gas bubbles. Quartz cullets buffer the polysilicon falling into the melt. The cullets also promote the dissipation of inert gas bubbles. However, adding quartz cullets adds complexity to the crystal growth process. The cullets also dissolve relatively quickly. Gaps may form between groups of cullets that limit their effects.

[0007] There is a need for alternative methods for forming silicon ingots that reduce the defect count in silicon wafers sliced ​​from ingots and / or reduce the formation of inert gas bubbles in the melt or promote the disappearance of inert gas bubbles.

[0008] This section is intended to introduce to the reader various technical aspects that may be related to the various aspects of the present disclosure described and / or claimed below. This discussion is intended 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 descriptions should be read in this context and not in the context of acknowledging prior art.

[0009] One aspect of the present disclosure relates to a method for growing a single-crystal silicon ingot in a continuous Czochralski process. A filler of polycrystalline silicon is added to a crucible assembly. The crucible assembly includes a weir and a sidewall that defines an outer melting zone between the weir and the sidewall. One or more plates are added to the outer melting zone. A molten silicon is formed within the crucible assembly. The surface of the molten silicon comes into contact with a seed crystal. A single-crystal silicon ingot is withdrawn from the molten silicon. A solid polycrystalline silicon feedstock is added to the outer melting zone while the single-crystal silicon ingot is being withdrawn to replenish the molten silicon. One or more plates cover at least partially the molten silicon in the outer melting zone.

[0010] Various refinements of the features discussed with respect to the embodiments of the present disclosure exist. Additional features may also be incorporated into the embodiments of the present disclosure. These refinements and additional features may exist individually or in any combination. For example, various features discussed below with respect to any of the exemplified embodiments of the present disclosure may be included in any of the embodiments described above, either alone or in any combination. Brief explanation of the drawing

[0011] FIG. 1 is a cross-sectional view of an exemplary ingot fuller device in which a solid silicon filler is disposed inside; FIG. 2 is a cross-sectional view of an ingot puller device after plates are positioned on the surface of a silicon filler; FIG. 3 is a plan view of a crucible assembly of an ingot fuller device in which plates are arranged internally; FIG. 4 is a plan view of the plate; FIG. 5 is a cross-sectional view of an ingot fuller apparatus having plates floating on a molten material; FIG. 6 is a cross-sectional view of an ingot puller apparatus illustrating the lifting of a silicon ingot from a silicon melt; FIG. 7 is a box figure illustrating the number of void counts in wafers sliced ​​from the first ingots (first batch) grown in a continuous Czochralski process; and FIG. 8 is a box figure illustrating the number of void counts in wafers sliced ​​from a first ingot grown in a continuous Czochralski process, wherein plates cover the melt during addition to the outer melting zone ("TEST") of solid-state silicon and collets cover the melt during addition to the outer melting zone ("POR") of solid-state silicon. The corresponding reference numerals indicate the corresponding parts throughout the drawings. Specific details for implementing the invention

[0012] The offerings of the present disclosure relate to methods for growing a single-crystal silicon ingot in a continuous Czochralski (CCz) process. One or more plates (e.g., quartz plates) are added to the outer melting zone of a crucible assembly prior to the formation of the ingot. During ingot growth, solid-state silicon (e.g., polycrystalline silicon) is added to the outer melting 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 exemplary ingot puller device (5) for producing an ingot (60) by a continuous Czochralski process is illustrated in FIGS. 1 through 6. As illustrated in FIG. 6, the ingot puller device (5) comprises 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 side wall (40) and one or more fluid barriers (20, 30) or "dikes" that separate the melt into different melting zones. In the illustrated embodiment, the crucible assembly (10) comprises a first dike (20). The first dike (20) and the side wall (40) define an outer melting zone (42) of the silicon melt (and of the crucible assembly (10). The crucible assembly (10) includes a second bank (30) radially inward with respect to a first bank (20), and the second bank defines an internal melting zone (22) of the silicon melt. The internal melting zone (22) is a growth zone where a single-crystal silicon ingot (60) is grown. The first bank (20) and the second bank (30) define an intermediate melting zone (32) of the silicon melt that can be stabilized as the melt (6) moves toward the internal melting zone (22). Each of the first and second banks (20, 30) has at least one opening defined internally to allow the molten silicon to flow radially inward toward the growth zone of the internal melting zone (22).

[0014] In the illustrated embodiment, the first dam (20), the second dam (30), and the side wall (40) each generally have an annular shape. The first dam (20), the second dam (30), and the side wall (40) may be part of three nested crucibles joined at the bottom or bottom (45) of the crucible assembly (10) (i.e., the first and second dams (20, 30) are the side walls of two crucibles nested within a larger crucible). The crucible assembly configurations depicted in FIGS. 1 through 6 are exemplary. In other embodiments, the crucible assembly (10) has a single bottom where the dams extend upward from the bottom (45) (i.e., does not have nested crucibles). Optionally, the bottom (45) may be flat rather than curved, and / or the dams (20, 30) and / or the side wall (40) may be straight. In addition, although the illustrated crucible assembly (10) is shown to have two dams, in other embodiments the crucible assembly may have a single dam or even not have dams.

[0015] A feeding tube (46) feeds solid-state silicon, which may be, for example, polysilicon chips, granular polysilicon, or chunk polysilicon, or a combination thereof, to an outer melting zone (42). Chunk polysilicon is generally larger in size than chip polysilicon, which is larger in size than granular polysilicon. For example, chunk polysilicon may generally have an average nominal size of at least 15 mm (e.g., in the range of 5 mm to 110 mm), while chip polysilicon may have an average nominal size of 1 to 15 mm. Solid-state silicon is added at a rate sufficient to maintain a substantially constant melting height level and volume during the growth of the ingot (60).

[0016] Generally, the molten material (6) from which the ingot (60) is drawn is formed by loading polycrystalline silicon into a crucible to form an initial silicon filler (27) (Fig. 1). Generally, the initial filler is between about 10 kilograms and about 200 kilograms of polycrystalline silicon, which may be chips, chunks, granules, or a combination thereof. The mass of the initial filler depends on the desired crystal diameter and hot zone design. The initial filler does not reflect the length of the ingot crystal because the polycrystalline silicon is continuously fed during crystal growth.

[0017] For example, various sources of polycrystalline silicon may be used, including granular polycrystalline silicon produced by the thermal decomposition of silanes or halosilanes in a fluidized bed reactor or polycrystalline silicon produced in a Siemens reactor. While solid-state silicon is typically polysilicon, a certain amount of single-crystal silicon (e.g., discarded portions from cut ingots) may also be used.

[0018] Once polycrystalline silicon is added to the crucible assembly (10) to form the filler (27), one or more plates (31) (Fig. 2) are added to the filler (27) of the outer melting zone (42). In the illustrated embodiment, a plurality of plates (31) (Fig. 3) are added to the outer melting 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 plates are added to the outer melting zone (42)). In other embodiments, a single plate (31) (e.g., a plate surrounding the entire perimeter of the outer melting zone (42)) is added.

[0019] In embodiments where a plurality of plates (31) are added to an outer melting zone (42), the plates may be free-floating and not connected to each other. In other embodiments, the plates (31) may be connected. The plates (31) may be sized to minimize gaps between adjacent plates (31).

[0020] The plates (31) may be made of quartz or other materials that allow the plates (31) to operate as described in the present disclosure. The plates (31) are generally less dense than the molten silicon (6) so that the plates float within the molten silicon (6) after the formation of the molten silicon (6).

[0021] Now, referring to FIG. 4, each plate (31) has one or more openings or slots (49) extending through the thickness of the plate (31). Once solid silicon is discharged through the feed tube (46) into the outer melting 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 molten material (6). In the illustrated embodiment, the openings (49) are slots having a main axis generally parallel to the longitudinal axis (A) of the plate (31) (i.e., the openings (49) are radially spaced). Generally, the openings (49) may have any shape that allows the plates (31) to operate as described in this disclosure. Generally, the openings (49) can be smaller than the size of the polysilicon type (e.g., chunks, chips, or granules) introduced into the outer melting zone.

[0022] The plates (31) have an inner edge (51) and an outer edge (53). The edges (51, 53) are rounded to match the contours of the outer melting zone (42) (i.e., the area bounded by the first dike (20) and the side wall (40). The outer edge (53) is longer than the inner edge (51). First and second sides (57, 59) extend between the inner and outer edges (51, 53).

[0023] Each plate (31) has a width (W 31 It has the width (W) of each plate (31). 31 ) is the width (W) of the outer melting zone (42) to allow the plates (31) to be placed within the outer melting zone (42) without contacting the first bank (20) or side wall (40) (e.g., during meltdown and / or ingot growth). 42 It is less than )(Fig. 3).

[0024] Once polycrystalline silicon is added to a crucible assembly (10) to form a filler (27) and plates (31) are added to an outer melting zone (42), the filler (27) is heated to a temperature above the melting temperature of silicon (e.g., about 1412°C) to melt the filler, thereby forming a silicon melt (6) (Fig. 6) containing molten silicon. The silicon melt (6) has an initial volume of molten silicon and an initial melt elevation level, and these parameters are determined by the size of the initial filler (27). In some embodiments, the crucible assembly (10) containing the silicon melt (6) is heated to a temperature of at least about 1425°C, at least about 1450°C, or even at least about 1500°C. Once the initial melt (6) is formed, the plates (31) float on the melt (6) in the outer melting zone (42).

[0025] The ingot lifting device (5) includes a lifting mechanism (114) (Fig. 6) for growing and lifting an ingot (60) from a molten material (6) within an internal melting zone (22). The lifting mechanism (114) includes a lifting cable (118), a seed holder or chuck (120) coupled to one end of the lifting cable (118), and a seed crystal (122) coupled to the seed holder or chuck (120) to initiate crystal growth. One end of the lifting cable (118) is connected to a lifting mechanism (e.g., a drive pulley or drum, or any other suitable type of lifting mechanism), and the other end is connected to the chuck (120) holding the seed crystal (122). When operated, the seed crystal (122) is lowered to come into contact with the molten material (6) in the internal melting zone (22). The lifting mechanism (114) is operated to cause the seed crystal (122) to rise along the lifting axis (A). This causes the single crystal ingot (60) to be lifted from the melt (6).

[0026] Once the polycrystalline silicon filler (27) (Fig. 1) is liquefied to form a silicon melt (6) (Fig. 5) containing molten silicon and the plates (31) are floating over the melt (6), the silicon seed crystal (122) (Fig. 6) is lowered to come into contact with the melt (6) within the inner melt zone (22). The silicon seed crystal (122) is then drawn out from the silicon-attached melt (6) to form a neck (52), thereby forming a melt-solid interface near or on the surface of the melt (6).

[0027] The lifting mechanism (114) can rotate the seed crystal (122) and the ingot (60) connected thereto. The crucible drive unit (44) can rotate the susceptor (13) and the crucible assembly (10). In some embodiments, the silicon seed crystal (122) and the crucible assembly (10) are rotated in opposite directions, that is, in opposite rotation. The opposite rotation achieves convection within the silicon melt (6). The rotation of the seed crystal (122) is primarily used to provide a symmetric temperature profile, to suppress angular changes of impurities, and also to control the shape of the crystal melt interface.

[0028] After the neck (52) is formed, an outwardly flaring seed-cone portion (54) (or "crown") adjacent to the neck (52) is grown. Generally, the impression rate is reduced from the neck portion impression rate to a rate suitable for growing the outwardly flaring seed-cone portion (54). Once the seed-cone portion reaches a target diameter, the main body (56) or "constant diameter portion" of the ingot (60) is grown. In some embodiments, the main 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 even at least about 450 mm.

[0029] While the ingot (60) is being pulled up from the melt (6), a solid polysilicon feedstock is added to the outer melting zone (42) through a tube (46) or other channel to replenish the melt (6) in the ingot growth device (5). Solid polycrystalline silicon can be added from a polycrystalline silicon feed system (66) and can be added continuously or intermittently to the ingot puller device (5) to maintain the melt level. Generally, polycrystalline silicon can be metered into the ingot puller device (5) by any method available to a person skilled in the art. The solid polysilicon added to the outer melting zone (42) can be silicon chips, chunks, or granules.

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

[0031] The device (5) may include a heat shield (116) placed around the ingot (60) to allow the growing ingot (60) to release its latent heat of solidification and heat flux from the melt (6). The heat shield (116) may be at least partially conical in shape and is angled downward to create an annular opening into which the ingot (60) is placed. A flow of an inert gas, such as argon, is typically provided along the length of the growing crystal. The ingot (60) is raised through a growing chamber (78) that is sealed from the surrounding atmosphere.

[0032] A plurality of independently controlled annular bottom heaters (70) may be arranged in a radial pattern under the crucible assembly (10). The annular bottom heaters (70) apply heat with a relatively controlled distribution over the entire base surface area of ​​the crucible assembly (10). The annular bottom heaters (70) may be individually controlled planar resistive heating elements as described in U.S. Patent No. 7,635,414, which is incorporated herein by reference for all relevant and consistent purposes. The device (5) may include one or more side heaters (74) arranged radially outward with respect to the crucible assembly (10) to control the temperature distribution through the melt (6).

[0033] The ingot growth apparatus (5) illustrated in FIGS. 1 to 6 and described in this disclosure is exemplary, and any system in which a crystalline ingot is prepared by the continuous Czochralski method may be used unless otherwise noted.

[0034] As the ingot (60) is withdrawn from the melt (6), a solid polycrystalline silicon feedstock is added to the crucible assembly (10) while the single-crystal silicon ingot (60) is withdrawn to replenish the melt (6). The solid silicon falls onto plates (31) that at least partially cover the melt (6) in the outer melting zone (42). The heat of the melt (6) heats the solid polycrystalline silicon placed on the plates (31) so that the silicon melts and passes through the openings (49) extending through the plates (31) or falls onto the edges (51, 53) and / or sides (57, 59) of the plates (31).

[0035] In some continuous Czochralski processes, more than one ingot is grown while the hot zone (i.e., the lower part of the device (5), such as the crucible assembly (10) and the susceptor (13)) remains heated to the silicon melt (6) that continues to exist within the crucible assembly (10). In these methods, the first ingot is grown to a target length and growth is terminated, the ingot is removed from the ingot puller, and a seed crystal is then lowered into the melt to initiate the growth of a second single-crystal silicon ingot (i.e., using the same melt from which the first ingot was withdrawn). 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) remain in the melt (6) without being completely melted 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 can be grown into an intact hot zone (e.g., until one or more components of the hot zone degrade, such as when the crucible assembly requires cooling and replacement of the degraded component) and at a temperature in which a continuous melt of silicon is present within the crucible assembly (10). For example, at least 1, 2, 3, 4, 5, 6, 10, or 20 or more ingots can be grown.

[0037] Compared to conventional methods for growing single-crystal silicon ingots in a continuous Czochralski (CCz) process, the methods of the present disclosure have several advantages. Without being bound by any particular theory, the addition of a polycrystalline silicon crucible assembly to the outer melting zone is believed to generate relatively small bubbles (e.g., less than 10 µm) of inert gas (e.g., argon) that can be transported by the melt through openings within each bank allowing the bubbles to reach the solid-melt interface. The plates can serve to prevent the trapping of inert gas into the melt by preventing the polycrystalline feedstock from being discharged directly into the melt. The plates can also increase the size of the bubbles to allow them to become more buoyant by providing surface areas and nucleation sites where the inert gas bubbles will aggregate. The plates provide a monolithic layer of quartz on the surface of the melt (e.g., having smaller gaps compared to quartz collets). The plates dissolve a certain amount after the formation of the melt, and the dissolved quartz also helps to remove inert gases from the melt. The melting rate of the plates is lower than that of quartz collets, which increases the durability of the plates compared to collets. The plates can be placed relatively easily in the crucible assembly (e.g., on an initial fill of polycrystalline silicon) before the hot zone reaches a temperature. In embodiments where multiple plates are used, the plates are less rigid and are allowed to move with the silicon as the silicon moves relative to the crucible, which helps ensure that the plates do not become submerged in the melt. In embodiments where the plates have a width less than the width of the outer melt zone, the plates are less likely to sinter into the sides / banks of the crucible assembly during melting.

[0038] Examples

[0039] The processes of the present disclosure are further illustrated by the following examples. These examples should not be construed as limiting.

[0040] Example 1: Number of voids in wafers grown from ingots where quartz plates were added to the outer melting zone

[0041] As shown in FIG. 7, the first ingot (batch A) grown during a continuous Czochralski process (200 mm) typically contains more microvoids (detected by laser light scattering with a size of at least 0.12 μm) compared to the subsequently grown ingots (ingots B to G).

[0042] Figure 8 illustrates the void count for the first growth ingots (Batch A) during multiple consecutive Czochralski runs. The "test" run (the figure in the far right box) included plates on top of the initial silicon filler (Figure 2) and on the subsequent melt (Figure 6) as polycrystalline silicon was added to the outer melt zone. Other runs included quartz collets instead of quartz plates. As shown in Figure 8, the runs using plates did not increase the void count to unacceptable levels.

[0043] When introducing elements of the present disclosure or its embodiments, expressions corresponding to the use of "a," "an," and "the," and "the above," are intended to imply that one or more elements exist. The terms "comprising," "comprising," "containing," and "having" are intended to imply that there may be additional elements other than those listed. The use of terms indicating specific orientations (e.g., "top," "bottom," "side," etc.) is for convenience of description and does not require any specific orientation of the item being described.

[0044] Since various modifications may be made to the above configurations and methods without departing from the scope of the present disclosure, all matters included in the above description and illustrated in the accompanying drawings(s) are intended to be interpreted as illustrative but not limiting.

Claims

Claim 1 A method for growing a single-crystal silicon ingot in a continuous Czochralski process comprises the steps of: adding a filler of polycrystalline silicon to a crucible assembly—said that the crucible assembly comprises a dike and a sidewall defining an outer melting zone between the dike and the sidewall—; adding one or more plates to the outer melting zone; forming a molten silicon within the crucible assembly—said that the one or more plates float on the molten silicon, and each of the one or more plates floating on the molten silicon comprises one or more openings extending through the plates to allow silicon to enter the molten silicon—; bringing the surface of the molten silicon into contact with a seed crystal; and withdrawing the single-crystal silicon ingot from the molten silicon. A method comprising the step of adding a solid polycrystalline silicon feedstock to the outer melting zone while withdrawing the single-crystal silicon ingot to replenish the melt—wherein one or more plates at least partially cover the melt in the outer melting zone while adding the solid polycrystalline silicon feedstock to the outer melting zone, and silicon enters the melt through one or more openings extending through the one or more plates. Claim 2 A method according to claim 1, wherein the dike is a first dike, and the crucible assembly includes a second dike radially inward with respect to the first dike, wherein the first dike and the second dike define an intermediate melting zone between the first dike and the second dike, and the second dike defines an internal melting zone within the second dike. Claim 3 A method according to claim 1, wherein the outer melting zone has a width and each of the one or more plates has a width, and the width of each of the one or more plates is less than the width of the outer melting zone. Claim 4 A method according to claim 1, wherein the plates are placed on solid polycrystalline silicon in the outer melting zone before forming a molten silicon within the crucible assembly. Claim 5 A method according to claim 1, wherein one or more plates are made of quartz. Claim 6 A method according to claim 1, wherein the single-crystal silicon ingot is the first ingot withdrawn from the melt after the silicon melt is formed in the crucible assembly. Claim 7 A method according to claim 6, further comprising: a step of contacting the surface of the melt with the seed crystal; a step of withdrawing a second single-crystal silicon ingot from the melt; and a step of adding a solid polycrystalline silicon feedstock to the outer melt zone while withdrawing the second single-crystal silicon ingot to replenish the melt — wherein one or more plates at least partially cover the melt in the outer melt zone while adding the solid polycrystalline silicon feedstock to the outer melt zone —. Claim 8 A method according to claim 1, wherein the single-crystal silicon ingot is a single-crystal silicon ingot grown subsequently on a first single-crystal silicon ingot withdrawn from the melt. Claim 9 delete Claim 10 delete

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