Crystal pulling system having a composite polycrystalline silicon feed tube, method for preparing such a tube, and method for forming a monocrystalline silicon ingot - Patents.com

Abstract

A crystal pulling system having a composite polycrystalline silicon supply tube, a method for forming such a tube, and a method for forming a single crystal silicon ingot using such a tube. The composite polycrystalline silicon supply tube includes quartz and at least one dopant. The composite polycrystalline silicon supply tube may be fabricated by a slip casting process.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 244,047, filed September 14, 2021, which is incorporated by reference herein in its entirety. [Technical field]

[0002] The field of the disclosure relates to crystal pulling systems having composite polycrystalline silicon supply tubes, methods of forming such tubes, and methods of forming single crystal silicon ingots using such tubes. [Background technology]

[0003] In the continuous Czochralski process for forming single crystal silicon ingots, polycrystalline silicon is added continuously or intermittently to the melt to replenish the melt as the silicon ingot is pulled from the melt. In some conventional processes, solid polycrystalline silicon is added to the melt through a feed tube that extends through the ingot puller housing. In some batch modes for growing single crystal silicon ingots, the crystal puller may remain at temperature and polycrystalline silicon may be fed to a crucible to prepare a second silicon melt from which a second ingot may be grown.

[0004] Polycrystalline silicon can wear down the feed tube causing impurities to get into the melt. Feed tubes are traditionally manufactured by a fused silica process, which results in the tube having uniform properties over its length. There is a need for a crystal pulling system that reduces the amount of impurities that get into the melt during addition of solid polycrystalline silicon, and / or a method of manufacturing the feed tube that allows the tube to have variable properties over its length.

[0005] This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described below and / or claimed. 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 descriptions are read in this light, and not as admissions of prior art. Summary of the Invention

[0006] One aspect of the disclosure relates to a crystal pulling system for growing a single crystal ingot from a silicon melt. The system includes a housing including a pulling shaft and defining a growth chamber. A crucible assembly is disposed within the growth chamber for containing the silicon melt. A compound polycrystalline silicon supply tube extends through the housing and into the growth chamber for supplying polycrystalline silicon to the crucible assembly. The compound polycrystalline silicon supply tube is comprised of quartz and at least one dopant.

[0007] Another aspect of the present disclosure relates to a method of preparing a polycrystalline silicon supply tube, comprising: introducing a slip slurry into a mold; the slip slurry including silica, a dopant, and a liquid carrier; removing at least a portion of the liquid carrier from the mold to form a polycrystalline silicon supply tube green body; separating the polycrystalline silicon supply tube green body from the mold; sintering the polycrystalline silicon supply tube green body to dry and densify the polycrystalline silicon supply tube green body to form a polycrystalline silicon supply tube.

[0008] Yet another aspect of the present disclosure relates to a method of forming a single crystal silicon ingot, comprising: forming a silicon melt in a crucible assembly; contacting the silicon melt with a seed crystal; withdrawing the seed crystal from the melt to form a single crystal silicon ingot; adding polycrystalline silicon to the melt through a composite polycrystalline silicon supply tube to replenish the melt; the composite polycrystalline silicon supply tube comprising quartz and a dopant;

[0009] There are various refinements of the features noted in relation to the above-mentioned aspects of the present disclosure. Additional features may also be incorporated into the above-mentioned aspects of the present disclosure as well. These refinements and additional features may exist individually or in any combination. For example, the various features described below in relation to any of the illustrated embodiments of the present disclosure may be incorporated alone or in any combination into any of the above-mentioned aspects of the present disclosure. [Brief description of the drawings]

[0010] [Figure 1] 1 is a cross-sectional view of a crystal pulling system for growing a single crystal ingot from a silicon melt. [Diagram 2] FIG. 2 is a detailed cross-sectional view of a crystal pulling system. [Diagram 3] 1 is a graph showing the change in thermal conductivity with increasing amounts of Si or AlN dopants in the silicon supply tube.

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

[0012] The present disclosure provides 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) process. The systems and methods disclosed herein may also be used to grow monocrystalline ingots by the batch or recharge CZ process. Referring to FIG. 1, an exemplary crystal pulling system is shown generally at 10. The crystal pulling system 10 is configured with a pulling axis Y 10and 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 or solar grade material) from which a single crystal ingot 20 is pulled by a pulling mechanism 22, described 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.

[0013] FIG. 2 illustrates a portion of the crystal pulling system 10 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 center dam 34 and an inner dam 36 extending upwardly from the bottom 30. The center dam 34 is disposed between the outer sidewall 32 and the inner dam 36. The crucible assembly 16 includes a crucible melting zone 38 disposed between the outer sidewall 32 and the center dam 34. The crucible assembly 16 also includes an intermediate zone 40 disposed between the center dam 34 and the inner dam 36. The crucible assembly 16 also includes a growth zone 42 disposed within the inner dam 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. Additionally, the crucible assembly 16 can have any suitable size that enables the crystal pulling system 10 to function as described herein. The crucible assembly 16 can 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. In other embodiments (e.g., a batch refill system), the crucibles do not include weirs in the outer crucible side wall 32.

[0014] 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., the outer melt zone 38, the intermediate zone 40 and the growth zone 42) allow the ingot to be grown according to the continuous Czochralski process where polycrystalline silicon is added continuously or semi-continuously to the melt while the ingot 20 is continuously pulled from the growth zone 42. The silicon melt 18 in the growth zone 42 contacts a single seed crystal 75 (FIG. 1). The seed crystal 75 is held by a chuck 70 connected to a pulling wire or cable 37. The pulling wire 37, chuck 70, and seed crystal 75 are raised and lowered by a pulling mechanism 22 (e.g., powered rollers, pulleys, or spools). As the seed crystal 75 is slowly pulled out of the melt 18, atoms from the melt 18 align and attach to the seed crystal 75 to form the ingot 20.

[0015] 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 provide thermal energy to the system 10. One or more bottom heaters 62 are disposed below the crucible assembly 16 and the susceptor 50. The heaters 52, 62 operate to melt an initial charge of solid polycrystalline silicon feedstock and to 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 that is added through a polycrystalline silicon feed tube 54 (FIG. 1) during ingot growth. The heaters 52, 62 may be any suitable heater (e.g., resistive heaters) that enable the system 10 to function as described herein.

[0016] 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 ports (not shown) for exhausting the inert gas and other gases and airborne particles from the growth chamber 14. The gas inlet provides a suitable inert gas, such as argon.

[0017] 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 along the pull axis Y. 10 The ingot 20 is then pulled along the central passage 26 and into the jacket chamber 60. The jacket 57 cools the pulled ingot 20.

[0018] The heat shield 24 is generally frustum shaped. 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 therein. The surface 61 may be coated (e.g., SiC) to reduce contamination of the melt.

[0019] The heat shield 24 includes a bottom 58 (FIG. 2). The heat shield 24 is positioned above the crucible assembly 16 such that the central passage 26 is located directly above the growth zone 42 such that an ingot pulled from the melt 18 is pulled through the central passage 26. The exterior surface 61 may be coated with a reflective coating that reflects radiant heat toward the melt 18 and the 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 substantially axial direction relative to the pull axis Y. 10 This helps maintain a generally uniform temperature gradient along the

[0020] During the initial melting stage, an initial amount of solid polycrystalline silicon is loaded into the crucible melting zone 38, the intermediate zone 40, and the growth zone 42. The initial charge may be from about 10 kilograms to about 200 kilograms of silicon. The mass of the initial charge depends on the desired crystal diameter and the hot zone design.

[0021] An initial charge of solid state silicon is melted and an ingot 20 is pulled from the melt 18. During ingot growth (or after growth as in a batch recharge system), solid polycrystalline silicon is added to the crucible assembly 16 through a polycrystalline silicon supply tube 54 (or simply a "feed tube") that extends through the crystal puller housing 12 and into the growth chamber 14. The polycrystalline silicon supply tube 54 includes an inlet 71 that is external to the crystal puller housing 12. Solid polycrystalline silicon can be added to the tube 54 through the inlet 71 by a dopant supply system 77. Generally, any suitable dopant supply system 77 that operates the crystal puller 10 as described herein is suitable unless otherwise noted. An exemplary dopant supply system 77 can include a storage container and a vibrating chute (e.g., a chute having a vibrator connected to the chute). The vibrating chute moves the solid polycrystalline silicon from the storage container to the inlet of the tube 54.

[0022] The polycrystalline feed tube 54 includes an outlet 73 within the growth chamber 14. The solid silicon falls through the tube 54 and exits the tube 54 through the outlet 73. The outlet 73 may be located within the crucible assembly 16 (e.g., below the top of the crucible assembly 16), such as in the outer melt zone 38.

[0023] The polysilicon supply pipe 54 is disposed within a polysilicon supply pipe port 59 formed within the housing 12 of the crystal pulling system 10 (i.e., the polysilicon supply pipe 54 is formed by a method such as the slip casting method described below).

[0024] The solid silicon supplied through supply tube 54 may be, for example, polycrystalline silicon chips, granular polycrystalline silicon, or chunk polycrystalline silicon, or a combination thereof. Chunk polycrystalline silicon is generally larger in size than chip polycrystalline silicon, which is larger than granular polycrystalline silicon. For example, chunk polycrystalline silicon generally has an average nominal size of at least 15 mm (e.g., in the range of 5 mm to 110 mm), while chip polycrystalline silicon has an average nominal size of 1 mm to 15 mm. The solid silicon is added at a rate sufficient to maintain a substantially constant melt height level and volume during growth of ingot 20.

[0025] In an embodiment of the present disclosure, at least a portion of the polysilicon supply tube 54 is a composite material. The composite tube 54 includes a base material (e.g., quartz) and at least one dopant (sometimes referred to herein as a "second phase"). In general, any suitable dopant can be used (e.g., a dopant that modifies or enhances the properties of the supply tube 54).

[0026] Suitable dopants include, for example, SiC, Si3N4, AlN, Si, ZrO2 or Y2O3, and combinations thereof. The concentration of the dopant in the composite feed tube 54 may be in any suitable range that enables the feed tube 54 and the crystal pulling system 10 to operate as described herein. In some embodiments, the concentration of the dopant in the tube 54 is at least 20 ppm, at least 50 ppm, or at least 100 ppm (e.g., 20 ppm to 10,000 ppm, or 100 ppm to 10,000 ppm). Some mixed phases may have even higher concentrations of the second phase dopant (e.g., at least 30 vol.% second phase, at least 40 vol.% second phase, at least 50 vol.% second phase, or at least 60 vol.% or more second phase).

[0027] In some embodiments, the entire tube 54 is formed of a composite material (e.g., quartz and at least one second phase dispersed through the quartz). In other embodiments, only a portion of the tube 54 is formed of a composite material. For example, an upper segment 63 of the tube 54 that extends through the housing 12 may be made of a composite material, or a lower segment 65 that extends vertically downward from the upper segment 63 and into the crucible assembly 16 may be made of a composite material. Various segments of the tube 54 may be made of a composite material, although one or more of the segments may include different concentrations or types of dopants. The tube 54 illustrated in FIG. 1 is an exemplary tube, and the tube 54 may have different configurations (e.g., more or fewer sections).

[0028] The composite tube 54 may be formed by a slip casting process. As described further below, a slip slurry (or simply "slip") is poured into a mold to form a "green body" in the shape of the tube 54. The green body is removed from the mold and sintered to form the tube 54. The mold is typically formed in the negative form of the tube (e.g., having an outer portion and an inner core that form a cylindrical gap into which the slip slurry can be filled). The mold may be made in two separable segments.

[0029] In some embodiments, the slip slurry added to the channels in the mold includes silica (SiO2), at least one dopant, and a liquid carrier such as water. The slip slurry can also include other reagents such as suspending agents that keep the silica particles in suspension, including any of the suspending agents known to those skilled in the art. Exemplary suspending agents include polymers or organics that adsorb to the particles (e.g., long-chain organic molecules or other agents that cause the silica particles to accumulate a surface charge, reducing particle-to-particle contact). The slip slurry can also include one or more binders that can optionally be combusted during sintering, as described below. Optionally, the slip slurry can include one or more release agents to facilitate separation of the tube mold from the resulting green body.

[0030] The tube mold can be made of a material that allows the liquid carrier to be removed from the mold (e.g., by capillary action) to form the green body. In some embodiments, the tube mold is made of gypsum, such as gypsum (e.g., CaSO4-nH20, which may also be called plaster of Paris). In other embodiments, the tube mold is made of porous silica. The tube mold may be a generally porous body that draws the liquid carrier into the mold by capillary action. In other embodiments, the liquid carrier may be drawn by a vacuum.

[0031] When the liquid carrier is drawn from the slip slurry into the mold, a "green body" remains in the mold. For example, the green body may have sufficient structure to maintain its shape when separated from the mold. For example, the moisture content of the green body may be less than about 50%, less than about 45%, at least about 30 wt%, at least about 35 wt%, at least about 40 wt%, at least about 45 wt%, between about 30 wt% and about 50 wt%, or between about 35 wt% and about 45 wt%.

[0032] The green body may be further dried, such as by exposing the green body to a relatively low and / or controlled humidity environment (e.g., after the green body has sufficient strength, removing the mold and exposing the green body to a relatively low and / or controlled humidity environment). As used herein, the terms "green body" or "green state" should not be considered in a limiting sense and generally refer to the intermediate state of the tube after the liquid carrier has been partially withdrawn from the slip slurry and prior to sintering of the structure.

[0033] The green body may include protrusions (ie, protrusions used to form the shape of the mold parts) that may be ground or cut from the green body or the resulting polycrystalline silicon supply tube 54 (FIG. 1).

[0034] Once the green body of the polycrystalline silicon tube is removed from the mold, the green body can be sintered (e.g., in a drying oven) to dry and densify the green body to form the polycrystalline feed tube 54 (FIG. 1). The green body can be sintered at a temperature of about 1200° C. to about 1800° C., about 1300° C. to about 1700° C., or about 1300° C. to about 1650° C. In some embodiments, the polycrystalline tube has a moisture content of less than 20 wt%, less than 15 wt%, less than 10 wt%, or less than 5 wt% after sintering.

[0035] Other methods for forming the tube 54 may also be used. For example, in other embodiments, the tube 54 may be fabricated by 3D printing. In other embodiments, tape casting or extrusion is used. Although the tube 54 has been shown and described as a cylinder, the tube 54 may have other symmetrical or asymmetrical shapes. For example, the tube 54 may have a trough shape.

[0036] The disclosed method for forming a composite polycrystalline feed tube of quartz and at least one dopant may be used to produce a monocrystalline silicon ingot by adding polycrystalline silicon to the crucible assembly 16 by adding the polycrystalline silicon to a composite tube 54, the tube being made of quartz and at least one dopant.

[0037] Compared to conventional polysilicon feed tubes, the feed tubes of the present disclosure have several advantages. The use of a second phase (i.e., one or more dopants) in the tube (e.g., quartz tube) reduces wear and abrasion caused by contact with the tube as the solid silicon travels down the tube to the crucible assembly. This reduces the amount of impurities (e.g., oxygen and other impurities present in the silica used to form the tube). The use of dopants also reduces the incidence of blockage of the polysilicon formed in the tube. The use of dopants also allows for the thermal conductivity of the tube to be changed and / or the opacity of the tube to be changed. Changing the thermal conductivity and / or opacity can reduce melting of the polysilicon at the tube wall surface and / or reduce dust collection and clogging. By using a slip cast method to form the tube (as opposed to, e.g., molten silicon methods), dopants can be incorporated into the tube and the tube can be formed into an asymmetric shape. The asymmetric design can reduce wear and rebound effects, thereby reducing the impurity concentration in the feed system. The slip casting process also allows for tailoring specific portions of the tube to specific dopant levels, such as changing the thermal conductivity or opacity of specific regions of the tube. In embodiments where the tube is manufactured by slip casting, the process may result in a net shape close to the final dimensions of the tube and / or a ready-to-use tube (with reduced or eliminated machining).

[0038] Working Example The processes of the present disclosure are further illustrated by the following examples, which should not be construed in a limiting sense.

[0039] Example 1: Variation of thermal conductivity in slip-cast tubes with different dopants The amount of dopant (silicon or AlN) in the quartz polycrystalline tube can be varied to change the thermal conductivity of the tube. Figure 3 shows the change in thermal conductivity as a function of dopant concentration. For example, 20-30% by volume of dopant can be added to change the thermal conductivity from that of quartz (approximately 1.8 W / m*K) to approximately 3 W / m*K.

[0040] As used herein, when used in conjunction with a range of dimensions, concentrations, temperatures, or other physical or chemical properties or characteristics, the terms "about," "substantially," "essentially," and "approximately" are meant to cover variations that may exist at the upper and / or lower limits of the range of the property or characteristic, including variations that are due, for example, to rounding, measurement method, or other statistical variations.

[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 particular orientation of the items described.

[0042] Since various changes may be made in the above 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 crystal pulling system for growing a single crystal ingot from a silicon melt, comprising: a pulling shaft; a housing defining a growth chamber; a crucible assembly disposed within the growth chamber for containing a silicon melt; and a composite polycrystalline silicon supply tube extending through the housing into the growth chamber for supplying polycrystalline silicon to the crucible assembly, the composite polycrystalline silicon supply tube being comprised of quartz and at least one dopant, the dopant having a concentration of at least 20 ppm.

2. The dopants are SiC, Si ３ N ４ , AlN, Si, ZrO ２ and Y ２ O ３ 2. The crystal pulling system of claim 1, wherein the crystal pulling system is selected from the group consisting of:

3. 10. The crystal pulling system of claim 1, wherein the concentration of the dopant is at least 50 ppm.

4. 10. The crystal pulling system of claim 1, wherein the concentration of the dopant is at least 100 ppm.

5. 10. The crystal pulling system of claim 1, wherein the concentration of the dopant is between 100 ppm and 10,000 ppm.

6. The composite polycrystalline silicon supply pipe is formed by the following method: introducing a slip slurry into the mold, the slip slurry comprising silica, a dopant, and a liquid carrier; removing at least a portion of the liquid carrier from the mold to form a polycrystalline silicon delivery tube green body; Separating the polycrystalline silicon supply tube green body from the mold; and sintering the polycrystalline silicon supply pipe green body, and drying and densifying the polycrystalline silicon supply pipe green body to form a composite polycrystalline silicon supply pipe; 10. The crystal pulling system of claim 1, wherein the crystal pulling system is formed of:

7. 1. A method of preparing a polycrystalline silicon supply tube, comprising the steps of: introducing a slip slurry into the mold, the slip slurry comprising silica, a dopant, and a liquid carrier; removing at least a portion of the liquid carrier from the mold to form a polycrystalline silicon delivery tube green body; Separating the polycrystalline silicon supply tube green body from the mold; and sintering the polycrystalline silicon supply pipe green body, and drying and densifying the polycrystalline silicon supply pipe green body to form a polycrystalline silicon supply pipe; 10. A method for preparing a polycrystalline silicon supply pipe comprising:

8. 8. The method of claim 7, including the step of placing the polysilicon feed tube into a polysilicon feed tube port formed in a crystal puller housing.

9. 1. A method of forming a single crystal silicon ingot, comprising the steps of: forming a silicon melt in a crucible assembly; contacting a silicon melt with a seed crystal; Pulling the seed crystal from the melt to form a single crystal silicon ingot; and adding polycrystalline silicon to the melt through a composite polycrystalline silicon supply tube to replenish the melt, the composite polycrystalline silicon supply tube comprising quartz and a dopant, the dopant having a concentration of at least 20 ppm; 1. A method for forming a single crystal silicon ingot, comprising:

10. The dopants are SiC, Si ３ N ４ , AlN, Si, ZrO ２ and Y ２ O ３ The method of claim 9, wherein the compound is selected from the group consisting of:

11. 10. The method of claim 9, wherein the concentration of the dopant is at least 50 ppm.

12. 10. The method of claim 9, wherein the concentration of the dopant is at least 100 ppm.

13. 10. The method of claim 9, wherein the concentration of the dopant is from 100 ppm to 10,000 ppm.

14. The polycrystalline silicon supply pipe is subjected to the following steps: introducing a slip slurry into the mold, the slip slurry comprising silica, a dopant, and a liquid carrier; removing at least a portion of the liquid carrier from the mold to form a polycrystalline silicon delivery tube green body; Separating the polycrystalline silicon supply tube green body from the mold; and sintering the polycrystalline silicon supply pipe green body, and drying and densifying the polycrystalline silicon supply pipe green body to form a composite polycrystalline silicon supply pipe; The method of claim 9, wherein the compound is prepared by

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