Method of forming a single crystal silicon ingot with reduced crucible corrosion

Abstract

A method of forming a single crystal silicon ingot with reduced crucible corrosion is disclosed. Solid phase quartz is added to the melt to reduce corrosion at the crucible-melt surface interface. The quartz can be synthetic quartz, such as a synthetic quartz rod. The quartz can be positioned near the crucible-melt surface interface. The quartz dissolves and inhibits the amount of quartz dissolved from the crucible at the crucible-melt surface interface.

Description

[0001] Cross-citation of related applications

[0002] This application claims the rights of U.S. Provisional Patent Application No. 63 / 112,431, filed November 11, 2020, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The field of this disclosure relates to methods for forming single-crystal silicon ingots with reduced crucible corrosion, and more particularly, to adding quartz to a melt to reduce corrosion at the interface between the crucible and the surface of the melt. Background Technology

[0004] Single-crystal silicon ingots can be prepared using the so-called Czochralski method, in which a single-crystal seed crystal is brought into contact with molten silicon held within a crucible. The seed crystal is then extracted from the melt to pull the single-crystal silicon ingot from the melt. High or ultra-high resistivity applications may use crucibles containing a synthetic quartz liner to reduce the introduction of impurities into the melt. A characteristic of high or ultra-high resistivity applications is the extended cycle time to allow for dopant compensation. This can cause corrosion through the synthetic liner, exposing the quartz substrate and allowing impurities to enter the melt, thereby reducing the ingot's resistivity.

[0005] There is a need for a method for preparing a single-crystal silicon ingot in which the corrosion rate of the crucible (e.g., a synthetic liner or a natural quartz crucible) is reduced.

[0006] This paragraph is intended to introduce the reader to various aspects of the field to which this disclosure pertains, which may be related to the various aspects of this disclosure described and / or claimed below. It is believed that this discussion will help provide the reader with background information to facilitate a better understanding of the various aspects of this disclosure. Therefore, it should be understood that these statements should be read in this sense and not as an endorsement of prior art. Summary of the Invention

[0007] One aspect of this disclosure relates to a method for forming a single-crystal silicon ingot. Solid polycrystalline silicon is added to a crucible having sidewalls and a bottom. The polycrystalline silicon is heated to form a silicon melt having a surface. The silicon melt is brought into contact with a seed crystal. The seed crystal is extracted from the silicon melt to form a silicon ingot. Synthetic quartz is added to the melt. The synthetic quartz at least partially dissolves. As it dissolves, the synthetic quartz adheres to the crucible sidewalls at the surface of the melt.

[0008] Another aspect of this disclosure relates to a method for forming a single-crystal silicon ingot. Solid polycrystalline silicon is added to a crucible having sidewalls and a bottom. The polycrystalline silicon is heated to form a silicon melt. The silicon melt is brought into contact with a seed crystal. The seed crystal is extracted from the silicon melt to form a silicon ingot. Solid quartz is added to the melt. The quartz is at least partially dissolved to reduce the amount of dissolution in the crucible. The quartz, when dissolved, adheres to the entire circumference of the crucible at the surface of the melt.

[0009] Various modifications exist to the features described above regarding the aspects of this disclosure. Similarly, other features may be incorporated into the aspects mentioned above in this disclosure. These modifications and additional features may exist individually or in any combination. For example, the various features discussed below with respect to any illustrated embodiment of this disclosure may be incorporated individually or in any combination into any of the foregoing aspects of this disclosure. Attached Figure Description

[0010] Figure 1 This is the cross-section of the ingot pulling equipment before silicon ingot growth;

[0011] Figure 2 It is during the silicon ingot growth process. Figure 1 The cross-section of the ingot pulling equipment;

[0012] Figure 3 It is a detailed cross-section of the casting machine showing the crucible-molten surface interface; and

[0013] Figure 4 It is a synthetic quartz rod with a crucible-melt surface interface. Figure 3 Detailed cross-section of the ingot pulling equipment.

[0014] Throughout the accompanying drawings, corresponding reference symbols indicate the corresponding parts. Detailed Implementation

[0015] This disclosure relates to a method for forming a single-crystal silicon ingot in a puller apparatus. As further discussed below, solid-phase quartz (e.g., synthetic quartz (e.g., a quartz rod)) may be added to the melt to reduce corrosion at the interface between the crucible and the surface of the melt.

[0016] The method disclosed herein can be typically implemented in any ingot puller equipment configured to pull single-crystal silicon ingots. Figure 1 The exemplary ingot puller apparatus (or more simply, "ingot puller") is generally designated as "100". Ingot puller apparatus 100 includes a crucible 102 supported by a bearing 106 for holding a molten material 104 of a semiconductor or solar-grade material (e.g., silicon). Ingot puller apparatus 100 includes a section defining a section for pulling a silicon ingot 113 from the molten material 104 along a pulling axis A. Figure 2The growth chamber 152 and the crystal puller shell 108.

[0017] The crucible 102 includes a bottom surface 129 and a sidewall 131 extending upward from the bottom surface 129. The sidewall 131 is generally vertical. The bottom surface 129 includes a curved portion of the crucible 102 extending below the sidewall 131. Inside the crucible 102 is a silicon melt 104 having a melt surface 111 (i.e., a melt-ingot interface).

[0018] In some embodiments, crucible 102 is layered. For example, crucible 102 may be made of a quartz base layer and a synthetic quartz liner disposed on the quartz base layer.

[0019] The bearing 106 is supported by the shaft 105. The bearing 106, crucible 102, shaft 105, and ingot 113 ( Figure 2 They have a common longitudinal axis A or "lifting axis" A.

[0020] A lifting mechanism 114 is disposed within the ingot pulling device 100 for growing and pulling ingot 113 from melt 104. 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 122 coupled to the seed holder or chuck 120 for initiating crystal growth. One end of the lifting cable 118 is connected to a pulley (not shown) or roller (not shown) or any other suitable type of lifting mechanism (e.g., a shaft), and the other end is connected to the chuck 120 holding the seed 122. In operation, the seed 122 is lowered to contact melt 104. The lifting mechanism 114 is operated to raise the seed 122. This allows the single crystal ingot 113 ( Figure 2 Pull it out from the melt 104.

[0021] During heating and crystal pulling, the crucible drive unit 107 (e.g., an electric motor) rotates the crucible 102 and the support 106. The lifting mechanism 112 raises and lowers the crucible 102 along the pulling axis A during the growth process. For example, as... Figure 1 As shown, crucible 102 can be located at the lowest point (near the bottom heater 126) where the charge of solid polycrystalline silicon previously added to crucible 102 has melted. Crystal growth is initiated by contacting the melt 104 with the seed crystal 122 and lifting the seed crystal 122 by means of the pulling mechanism 114. As the ingot grows, the silicon melt 104 is consumed and the height of the melt in crucible 102 decreases. The crucible 102 and the support 106 can be raised to maintain the surface 111 of the melt relative to the ingot pulling device 100. Figure 2 At or near the same location.

[0022] The crystal driving unit (not shown) can also rotate the lifting cable 118 and the ingot 113 in the opposite direction to the direction in which the crucible driving unit 107 rotates the crucible 102. Figure 2 (e.g., reverse rotation). In embodiments using isotropic rotation, the crystal driving unit may rotate the lifting cable 118 in the same direction as the crucible driving unit 107 rotates the crucible 102. Additionally, the crystal driving unit may raise and lower the ingot 113 relative to the molten surface 111 as needed during the growth process.

[0023] The puller apparatus 100 may include an inert gas system for introducing an inert gas (e.g., argon) and extracting the inert gas from the growth chamber 152. The puller apparatus 100 may also include a dopant feeding system (not shown) for introducing dopant into the melt 104.

[0024] According to the Chuklaski single-crystal growth process, a certain amount of polycrystalline silicon (polysilicon) is loaded into crucible 102. Semiconductor or solar-grade material is introduced into the crucible by heat molten from one or more heating elements. The puller apparatus 100 includes a bottom insulation 110 and side insulation 124 for retaining heat within the puller apparatus. In the illustrated embodiment, the puller apparatus 100 includes a bottom heater 126 disposed below the bottom surface 129 of the crucible. The movable crucible 102 is positioned adjacent to the bottom heater 126 to molten the polycrystalline silicon loaded into the crucible 102.

[0025] To form an ingot, seed crystal 122 is brought into contact with the surface 111 of molten material 104. The pulling mechanism 114 is operated to pull seed crystal 122 from molten material 104. (See now for further details.) Figure 2 Ingot 113 includes a crown 142 in which the ingot transforms outward from seed 122 and tapers to reach a target diameter. Ingot 113 includes a constant diameter portion 145 or a cylindrical "body" of a crystal grown by increasing the pulling rate. The body 145 of ingot 113 has a relatively constant diameter. Ingot 113 includes a tail or end cone (not shown) in which the ingot tapers in diameter after the body 145. When the diameter becomes sufficiently small, ingot 113 then separates from melt 104.

[0026] The puller apparatus 100 includes a side heater 135 and a support 106 surrounding a crucible 102 to maintain the temperature of the melt 104 during crystal growth. As the crucible 102 travels upward and downward along the pull axis A, the side heater 135 is positioned radially outward from the crucible sidewall 131. The side heater 135 and the bottom heater 126 can be any type of heater that allows the side heater 135 and the bottom heater 126 to operate as described herein. In some embodiments, heaters 135, 126 are resistance heaters. The side heater 135 and the bottom heater 126 can be controlled by a control system (not shown) such that the temperature of the melt 104 is controlled throughout the pulling process.

[0027] The ingot pulling apparatus 100 may include a heat shield 151. During crystal growth, the heat shield 151 can shield the ingot 113 and can be placed inside the crucible 102. Figure 2 ).

[0028] Now, reference is made to another embodiment of the spindle pulling device 100 shown therein. Figure 3 The crucible 102 and the surface 111 of the molten material form an interface 125 (also referred to herein as the "crucible-molten surface interface," and in the art as a "three-phase interface" or "three-junction point," or more simply, "three points"). As the level of the molten material 104 changes, this interface 125 moves along the crucible sidewall 131 and / or the bottom surface 129 (i.e., along the curved portion of the bottom surface 129 when the molten material is relatively depleted). Without being bound by any particular theory, the crucible (i.e., the liner) corrodes more at the crucible-molten surface interface 125 than other parts of the crucible because oxygen, as SiO evaporates from the molten surface, is more readily replenished in the molten material from the portion of the crucible with the shortest diffusion distance (i.e., both dissolution from the crucible to the molten material and evaporation of oxygen from the molten surface occur at the crucible-molten surface interface).

[0029] According to embodiments of this disclosure, quartz (SiO2)144 ( Figure 4 Quartz 144 is added to the melt 104. Quartz 144 is positioned in the crucible 102 such that it adjoins the crucible sidewall 131 at the surface 111 of the melt (e.g., by migrating to the sidewall or by being held in place by a fixing device discussed further below). The quartz 144 dissolves at the surface 111 of the melt 104 at the crucible sidewall 131, which inhibits corrosion of the crucible at the crucible-melt surface interface 125. Oxygen entering the melt from the quartz 144 evaporates as SiO, which inhibits oxygen dissolving from the crucible 102 at the crucible-melt surface interface 125.

[0030] In some embodiments, the solid quartz 144 added to the melt 104 is synthetic quartz, i.e., quartz synthetically formed, for example, by a hydrothermal process rather than quartz processed from sand. Suitable synthetic quartz includes SUPRASIL from Haraeus (Hanau, Germany) or Type 098WGY quartz from Momentive Performance Materials (Waterford, New York). Synthetic quartz is generally chemically purer than quartz components formed from sand. In some embodiments, the synthetic quartz contains 5.0 ppmw or less of impurities, 1.0 ppmw or less, or even 0.7 ppmw or less of impurities (e.g., 0.5 ppmw or less of aluminum and 0.2 ppmw or less of the sum of lithium, sodium, potassium, and calcium).

[0031] In some embodiments, the quartz (e.g., synthetic quartz) 144 added to the crucible is shaped into rods, hollow tubes, spheres, or irregular shapes (e.g., crushed quartz). Quartz rods can be formed by melting synthetic sand and extruding the melt into rods. The rods can be cut into smaller segments (also referred to as “cullets”) added to the melt 104. The rods can be acid-washed before being added to the melt 104 to reduce impurities added to the melt 104. The solid quartz added to the melt 104 can generally have any size (e.g., diameter and length when using rods) that effectively reduces corrosion of the crucible (e.g., diameter of 1 to 20 mm or 2 to 10 mm and length of 1 to 20 mm or 2 to 10 mm when using rods).

[0032] Quartz 144 may be added to the melt 104 such that the quartz is positioned around the entire circumference of the crucible 102 at the crucible-melt surface interface 125. In other embodiments, quartz 144 is added around a portion of the circumference of the crucible 102. Quartz 144 may be added in any amount sufficient to inhibit corrosion of the crucible at the crucible-melt surface interface 125. In some embodiments, the amount of quartz 144 added to the melt during the drawing cycle is at least 1 g per meter of crucible circumference or at least 10 g, at least 20 g, or at least 50 g of quartz per meter of crucible circumference (e.g., from about 1 g to about 500 g, from about 1 g to about 500 g, or from about 10 g to about 500 g of quartz per meter of crucible circumference).

[0033] Quartz can typically be added to the melt 104 at any point during ingot growth (including, but not limited to, before the growth of the silicon ingot (e.g., after melting and before lowering the seed crystal 122 to contact the melt)). Alternatively or additionally, quartz can be added during ingot growth. Quartz may be added to the melt no more than once or may be added in two or more cycles.

[0034] In some embodiments, the silicon ingot 113 may be characterized by high or ultra-high resistivity (e.g., at least about 3000 ohm-cm, 5000 ohm-cm, or at least 7000 ohm-cm or more). Such ingots 113 may involve longer cycle times and may be more susceptible to crucible corrosion at the crucible-melt surface interface 125, making the addition of quartz 144 particularly advantageous.

[0035] Quartz 144 can be held at the crucible-melt surface interface 125 in any suitable manner. For example, rotation of crucible 102 can apply centrifugal force to a quartz fragment (e.g., a rod), thereby causing the synthetic quartz to adjoin the crucible sidewall 131. In other embodiments, a securing device is used to hold the quartz against the crucible sidewall (e.g., a barrier restricting radial movement of the quartz). Due to the buoyancy of the quartz relative to silicon, the quartz is held at the surface 111 of the melt 104.

[0036] The quartz 144 added to the melt 104 may contain hydroxides (OH) incorporated therein. When using synthetic quartz, the amount of hydroxide can be controlled during quartz production. For example, the atmosphere in which the synthetic quartz is produced can be controlled (e.g., during the extrusion of the quartz rod) to change the amount of hydroxide incorporated into the synthetic quartz rod. Higher dissolution also reduces crystallization on the surface of the quartz 144, as such crystals can detach from the quartz material, which can cause the ingot 113 to lose zero displacement. Less hydroxide allows the quartz to remain in the melt for a longer portion of the ingot cycle time.

[0037] Although the methods disclosed herein have been described as part of a batch Chukraski process (i.e., polysilicon is not added to the melt during ingot growth), in other embodiments, quartz may be added in a continuous Chukraski process in which polysilicon is added to the melt during ingot growth.

[0038] Compared to conventional methods for forming single-crystal silicon ingots, the method disclosed herein offers several advantages. By adding quartz to the melt and positioning it at or near the crucible-melt surface interface, the quartz dissolves, adding oxygen to the melt. This oxygen evaporates from the melt (as SiO), thereby reducing the amount of oxygen dissolved from the crucible at the crucible-melt surface interface (i.e., reducing corrosion at the interface). This is particularly advantageous for high or ultra-high resistivity applications involving long-term cycles where the crucible-melt surface interface is held in the same position on the crucible for extended periods. High or ultra-high resistivity applications are also highly sensitive to impurities (e.g., aluminum, boron, and phosphorus, which affect the resistivity of the resulting ingot) introduced from the crucible into the melt. The method disclosed herein is also advantageous in embodiments where the crucible comprises a synthetic quartz liner backed by a sand-quartz substrate. Such a substrate may contain 50 to 100 times the amount of impurities found in the synthetic quartz liner. By adding quartz to the melt, corrosion or perforation of the synthetic quartz liner can be reduced or prevented. Quartz placed near the sidewall of the crucible can also suppress vibrations of the molten material.

[0039] As used herein, when used in conjunction with ranges of size, concentration, temperature or other physical or chemical properties or characteristics, the terms “about,” “substantially,” “basically” and “approximately” mean to cover variations that may exist in the upper and / or lower limits of the range of properties or characteristics, including, for example, variations arising from rounding, methods of measurement or other statistical variations.

[0040] When describing elements of this disclosure or embodiments thereof, the articles “a” and “the” are intended to mean that one or more elements are present. The terms “comprising,” “including,” “containing,” and “having” are intended to be inclusive and mean that additional elements may be present in addition to the listed elements. Terms indicating a particular orientation (e.g., “top,” “bottom,” “side,” etc.) are used for convenience of description and do not require any particular orientation of the described items.

[0041] Since various changes can be made to the above construction and methods without departing from the scope of this disclosure, it is intended that all matters contained in the above description and shown in the accompanying drawings be interpreted in an illustrative and non-limiting sense.

Claims

1. A method for forming a single-crystal silicon ingot, comprising: Solid polycrystalline silicon is added to a crucible with sidewalls and a bottom; The polycrystalline silicon is heated to form a silicon melt with a surface; The molten silicon is brought into contact with the seed crystals; The seed crystals are extracted from the silicon melt to form a silicon ingot; Synthetic quartz is added to the melt, and due to the buoyancy of quartz relative to silicon, the quartz is held at the surface of the melt. The synthetic quartz is prepared by melting synthetic sand and extruding the melt of the synthetic sand. and The synthetic quartz is at least partially dissolved, and the synthetic quartz adheres to the crucible sidewall at the surface of the melt when it is dissolved.

2. The method according to claim 1, wherein the synthetic quartz is shaped into rods, tubes, spheres, or has an irregular shape.

3. The method according to claim 1, wherein the synthetic quartz contains 5.0 ppmw or less of impurities.

4. The method according to claim 1, wherein the crucible comprises a quartz base layer and a synthetic quartz liner disposed on the quartz base layer.

5. The method of claim 1, wherein the synthetic quartz is disposed around the entire circumference of the crucible at the surface of the melt.

6. The method of claim 1, further comprising rotating the crucible, wherein the centrifugal force caused by the rotation of the crucible causes the synthetic quartz to adjoin the sidewall of the crucible.

7. The method of claim 1, wherein the synthetic quartz is held against the sidewall of the crucible by a fixing device.

8. The method of claim 1, wherein the synthetic quartz is added prior to the growth of the silicon ingot.

9. The method of claim 1, wherein synthetic quartz is added during ingot growth.

10. The method of claim 1, wherein the synthetic quartz is added to the melt no more than once.

11. The method of claim 1, wherein synthetic quartz is added to the melt in two or more cycles.

12. The method of claim 1, wherein the ingot is grown in a batch Chuklaski process and no polysilicon is added to the melt during ingot growth.

13. The method of claim 1, wherein the ingot is grown in a continuous Chuklaski process, in which polysilicon is added to the melt during ingot growth.

14. The method of claim 1, wherein the surface of the melt is unobstructed between the ingot and the synthetic quartz, and a gap exists between the synthetic quartz and the ingot after the synthetic quartz is added to the melt.

15. A method for forming a single-crystal silicon ingot, comprising: Solid polycrystalline silicon is added to a crucible with sidewalls and a bottom; The polycrystalline silicon is heated to form a silicon melt; The molten silicon is brought into contact with the seed crystals; The seed crystal is extracted from the silicon melt to form a silicon ingot, and the ingot is grown in a batch Chuklaski process without adding polycrystalline silicon to the melt during ingot growth; Solid quartz is added to the melt such that the solid quartz extends only partially across the surface of the silicon melt, and is held at the surface of the melt due to the buoyancy of the quartz relative to the silicon; and The quartz is at least partially dissolved to reduce the amount of dissolution in the crucible, wherein the quartz, when dissolved, is adjacent to the entire circumference of the crucible at the surface of the melt.

16. The method of claim 15, wherein the quartz is shaped into a rod, tube, sphere, or has an irregular shape.

17. The method of claim 15, wherein the quartz contains 5.0 ppmw or less of impurities.

18. The method of claim 15, wherein the crucible comprises a quartz substrate and a synthetic quartz liner disposed on the quartz substrate.

19. The method of claim 15, further comprising rotating the crucible, wherein the centrifugal force caused by the rotation of the crucible causes the solid quartz to adjoin the crucible sidewall.

20. The method of claim 15, wherein the solid quartz is held against the sidewall of the crucible by a fixing device.

21. The method of claim 15, wherein the solid quartz is added prior to the growth of the silicon ingot.

22. The method of claim 15, wherein the solid quartz is added during ingot growth.

23. The method of claim 15, wherein solid quartz is added to the melt no more than once.

24. The method of claim 15, wherein solid quartz is added to the melt in two or more cycles.

25. The method of claim 15, wherein the surface of the melt is unobstructed between the ingot and the solid quartz, and a gap exists between the solid quartz and the ingot after the solid quartz is added to the melt.

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