Method for forming a single crystal silicon ingot with reduced carbon contamination and susceptor used in such a method

Abstract

A graphite susceptor for supporting a quartz crucible during a crystal growth process includes a body having an inner surface and a coating deposited on the inner surface. The inner surface of the body defines a cavity, and the cavity has a size and shape complementary to the outer size and shape of the crucible. The coating includes boron nitride and a sintering aid. The sintering aid is configured to promote densification of the boron nitride.

Description

Technical Field

[0001] [Cross - Reference to Related Applications] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 347,897, filed on June 1, 2022, the disclosure of which is hereby incorporated by reference in its entirety.

[0002] The field of the present disclosure relates to a method for forming a single - crystal silicon ingot with reduced carbon contamination in the Czochralski process. In the embodiments disclosed herein, a protective coating is used to reduce or prevent the reaction between the graphite susceptor and the quartz crucible used during the process. In some embodiments, the coating includes boron nitride and a sintering aid that promotes the densification of boron nitride, facilitating the prevention or minimization of cross - contamination of the silicon ingot by boron and / or nitrogen.

Background Art

[0003] Single - crystal silicon is the starting material for most processes for manufacturing many electronic components such as semiconductor devices and solar cells, and is generally prepared by the batch Czochralski (CZ) method or the continuous Czochralski (CCZ) method. In these methods, a polycrystalline raw material such as polycrystalline silicon ( "polysilicon") is charged into a quartz crucible in the form of a solid feedstock and melted, a single seed crystal is brought into contact with the molten silicon or melt, and a single - crystal silicon ingot is grown by slow extraction.

[0004] Polysilicon can also be directly cast into ingots using a directional solidification process. Wafers sliced from polysilicon ingots are commonly used in solar cells for the electronics industry and solar energy production. Polysilicon is generally preferred as a silicon source for solar cells over single crystal silicon because it has a high throughput rate, labor - non - intensive operations, and lower costs due to reduced consumable costs compared to the production of common single crystal silicon. In the directional solidification process, the raw silicon is melted in a quartz crucible and directionally solidified in another crucible or the same crucible. The solidification of the ingot is controlled so that the molten silicon solidifies unidirectionally at the solidification front of the casting. Polysilicon produced in this way is an aggregate of crystal grains, and since there are a high density of heterogeneous nucleation sites on the crucible wall surface, the orientations of the crystal grains are random with respect to each other. When a polysilicon ingot is formed, the ingot is cut into blocks and may be further cut into wafers.

[0005] In these methods, the quartz crucible is dimensionally unstable at the high temperatures required to melt the solid feedstock. To prevent warping or deformation of the crucible during the crystal growth process, the crucible is supported by a susceptor, which is preferably formed from a carbon - containing material such as graphite. Graphite is dimensionally stable at the temperatures required to melt the silicon feedstock.

[0006] During the crystal growth process, carbon from the graphite susceptor and silica from the quartz crucible may react with each other to produce gaseous carbon oxides and silicon products, for example, by the following reactions. SiO 2 + C → SiO(g) + CO(g) SiO 2 + 3C → SiC + 2CO(g) SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0007] These gaseous products and other products (e.g., CO2 ) is carried away by the inert gas flowing around the susceptor and the crucible and may be blown into the silicon melt, thereby contaminating the melt with carbon. When the gaseous reaction products are carried away by the inert gas stream, new carbon and silica surfaces are exposed, promoting the continuous reaction between the crucible and the susceptor, resulting in further contamination of the melt. Silicon crystal ingots grown from the melt and wafers sliced from such ingots may have a relatively high carbon concentration.

[0008] Carbon contamination is known to contribute to current leakage in semiconductor devices and thus to a degradation in the performance of semiconductor devices. Therefore, it is desirable to minimize carbon contamination of the melt and remove potential carbon sources during the CZ and CCZ crystal growth processes.

[0009] The continuous reaction between the crucible and the reactor also promotes erosion of the crucible and the susceptor. Usually, the crucible is used for one growth process, while the susceptor is used for multiple growth processes. However, due to the continuous erosion cycle of the susceptor, the susceptor rapidly deteriorates, shortening the service life of the susceptor. Therefore, in both the CZ and CCZ crystal growth processes, as well as in the directional solidification process, it is desirable to minimize the reaction between the crucible and the susceptor in order to provide a more durable susceptor.

[0010] Known systems and methods for addressing and / or reducing carbon contamination of the melt during the crystal growth process are not satisfactory for meeting the current and future carbon requirements of advanced semiconductor devices and / or for preventing cross-contamination of silicon crystals by the materials used to prevent the reaction between the crucible and the susceptor.

[0011] There is a need for a method of preparing silicon crystal ingots that facilitates reducing the carbon content in the silicon crystals and that facilitates preventing cross-contamination of the silicon crystals by an inert coating material.

[0012] This section is intended to introduce the reader to various aspects of technologies that may be related to various aspects of the present disclosure, which are described and / or claimed hereinafter. This discussion is considered useful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, these descriptions should be read from this perspective and should not be construed as an admission of prior art.

Means for Solving the Problems

[0013] In one aspect, a method for manufacturing a single crystal silicon ingot from a silicon melt includes providing a graphite susceptor having an inner surface that defines a cavity, and depositing a coating on the inner surface of the susceptor. The coating includes boron nitride and a sintering aid. The sintering aid promotes densification of the boron nitride. The method also includes placing a quartz crucible within the cavity of the susceptor. The crucible has an outer surface that contacts the coating. The method further includes adding polycrystalline silicon to the crucible, heating the polycrystalline silicon to form a silicon melt within the crucible, and pulling a single crystal silicon ingot from the silicon melt.

[0014] In another aspect, a graphite susceptor for supporting a quartz crucible during a crystal growth process includes a body having an inner surface that defines a cavity. The cavity has dimensions and a shape that are complementary to the outer dimensions and outer shape of the crucible. The inner surface defines a cavity. The coating includes boron nitride and a sintering aid. The sintering aid is configured to promote densification of the boron nitride.

[0015] In other aspects, a method of forming a coating between the inner surface of a graphite susceptor and the outer surface of a quartz crucible includes providing a particulate mixture of boron nitride and a sintering aid, depositing the particulate mixture of boron nitride and the sintering aid on the inner surface of the susceptor, and sintering the deposited particulate mixture to form a coating. Sintering the deposited particulate mixture includes promoting densification of the boron nitride using the sintering aid.

[0016] There are various improvements to the features mentioned in connection with the above aspects of the present disclosure. Further features can also be incorporated into the above aspects of the present disclosure. These improvements and additional features may exist individually or in any combination. For example, the various features described hereinafter in connection with any of the illustrated embodiments of the present disclosure can be incorporated into any of the above aspects of the present disclosure, either alone or in any combination.

Brief Description of the Drawings

[0017]

Figure 1

[0018]

Figure 2

[0019]

Figure 3A

Figure 3B

[0020] Corresponding reference numerals indicate corresponding parts throughout the drawings.

Modes for Carrying Out the Invention

[0021] Referring to FIG. 1, an ingot pulling apparatus or ingot puller is schematically shown and is generally designated by 100. The ingot puller 100 is used to produce single crystal (i.e., monocrystalline) ingots of semiconductor or solar grade materials such as, for example, single crystal silicon ingots. In some embodiments, the ingot is grown by a so-called Czochralski (CZ) process in which the ingot is pulled from a silicon melt 102 held within a crucible 104 of the crystal puller 100. In some embodiments, the ingot is grown by a batch CZ process in which an amount of polycrystalline silicon sufficient to grow one ingot is charged to the crucible 104 and, after the growth of one ingot, the crucible 104 is substantially depleted of the silicon melt 102. In other embodiments, the ingot is grown by a continuous CZ (CCZ) process in which polycrystalline silicon is continuously or periodically added to the crucible 104 to replenish the silicon melt 102 during the growth process. The CCZ process facilitates the growth of multiple ingots pulled from a single melt 102. However, embodiments of the subject matter described herein are not limited to a particular crystal growth process. For example, in other embodiments, the polycrystalline silicon ingot may be grown using a directional solidification process for solar applications.

[0022] The ingot puller 100 includes a housing 106 that defines a crystal growth chamber 108 and a pull chamber 110 that is laterally dimensioned smaller than the growth chamber 108. The growth chamber 108 has a generally domed upper wall 112 that transitions from the growth chamber 108 to the narrower pull chamber 110. The ingot puller 100 includes an inlet port 114 and an outlet port 116 that can be used for the introduction and removal of process gases to and from the ingot puller 100 during crystal growth.

[0023] In the crucible 104 within the ingot lifting machine 100, a silicon melt 102 from which a silicon ingot is lifted is accommodated. The crucible 104 may be made of quartz or fused silica, and quartz or fused silica has a high melting point and high thermal stability and is generally non-reactive with the molten silicon in the melt 102. It should be understood that the crucible 104 may be made of other materials in addition to quartz without departing from the scope of the present disclosure. For example, the quartz crucible 104 may be made of a composite material including silica and an additional material such as silicon nitride or silicon carbide.

[0024] The silicon melt 102 is obtained by melting the polycrystalline silicon charged into the crucible 104. In a continuous system, a supply system (not shown) is used to supply the solid raw material feed to the crucible assembly 104 and / or the solution 102. The crucible 104 is disposed within the susceptor 118 and supported by the susceptor 118, and the susceptor 118 is further supported by a rotatable shaft 120. The susceptor 118 and the rotatable shaft 120 facilitate the rotation of the crucible 104 about the central longitudinal axis X of the ingot lifting machine 100.

[0025] The heating system 122 (e.g., one or more electrical resistance heaters) surrounds the susceptor 118 and the crucible 104 and supplies heat conductively through the susceptor 118 and the crucible 104 to melt the silicon charge to form the solution 102 and / or to maintain the solution 102 in a molten state. The heater 122 may extend below the susceptor 118 and the crucible 104. The heating system 122 is controlled by a control system (not shown) such that the temperature of the solution 102 is precisely controlled throughout the pull process. For example, the controller may control the current supplied to the heating system 122 to control the amount of thermal energy supplied by the heating system 122. The controller may control the heating device 122 such that the temperature of the melt 102 is maintained at approximately the melting temperature of silicon (e.g., about 1412 °C) or higher. For example, the solution 102 may be heated to a temperature of at least about 1425 °C, at least about 1450 °C, or even at least about 1500 °C. Insulation (not shown) surrounding the heating system 122 may reduce the amount of heat lost through the housing 106. The ingot puller 100 may also include a thermal shield door assembly (not shown) above the surface of the solution 102 to shield the ingot from the heat of the crucible 104 to increase the axial temperature gradient at the solid-melt interface.

[0026] The lifting mechanism (not shown) has a lifting wire 124 extending downward from the mechanism. This mechanism can raise and lower the lifting wire 124 and rotate the lifting wire 124. Depending on the type of ingot lifter, the ingot lifter 100 may have a lifting shaft instead of a wire. The lifting wire 124 terminates at a lifting assembly 126 that includes a seed crystal chuck 128 that holds a seed crystal 130 used in the growth of a silicon ingot. When growing an ingot, the lifting mechanism lowers the seed crystal 130 until it contacts the surface of the silicon melt 102. When the seed crystal 130 begins to melt, the lifting mechanism slowly lifts the seed crystal through the growth chamber 108 and the lifting chamber 110 to grow a single crystal ingot. The speed at which the lifting mechanism rotates the seed crystal 130 and the speed at which the lifting mechanism lifts the seed crystal (i.e., the lifting speed v) are controlled by a control system. As the seed crystal 130 is slowly lifted from the melt 102, silicon atoms from the melt 102 align and attach to the seed crystal 130 to form an ingot.

[0027] A process gas (e.g., argon) is introduced into the growth chamber 108 and the lifting chamber 110 through an inlet port 114 and withdrawn through an outlet port 116. The process gas creates an atmosphere within the housing, and the solution and the atmosphere form a melt-gas interface. The outlet port 116 is in fluid communication with an exhaust system (not shown) of the ingot lifter.

[0028] During operation, the temperature required to melt the silicon input and / or maintain the solution 102 may cause the quartz crucible 104 to soften. The susceptor 118 may be made of graphite or other highly thermally stable graphene or carbon-containing material and provides a rigid external structure to support the softened crucible 104. Contact between the crucible 104 and the susceptor 118 at the draw process temperature allows silica from the quartz crucible 104 to react with carbon from the susceptor 118. This reaction causes the silica to decompose into silicon monoxide (SiO) gas and the carbon to oxidize to form gaseous carbon-containing reaction products such as, for example, carbon monoxide (CO) and carbon dioxide (CO 2 ). As a result, the graphite susceptor 118 may be eroded, creating voids or cavities in the susceptor 118. The structural degradation caused by this erosion shortens the service life of the susceptor 118 and may require replacement of the susceptor 118. In addition, the process gas flowing through the ingot puller 100 can carry the gaseous carbon-containing reaction products away from the crucible 104 and the susceptor 118 and into the solution 102. This exposes fresh carbon and silica surfaces, leading to the continuous formation of gaseous carbon-containing reaction products and erosion of the susceptor 118. The carbon species carried into the melt 102 may move by convection to the solid-melt interface where the ingot grows. As a result, the ingot pulled from the solution 102 may be contaminated with carbon. Carbon contamination of the ingot can lead to performance degradation of the wafers sliced from the ingot in the end-use structure or device.

[0029] Next, referring to FIG. 2, a cross-sectional view of the susceptor 118 and the crucible 104 is shown. The susceptor 118 includes a body 132 that includes an inner surface 134 defining a cavity 136 sized to receive the quartz crucible 104. The crucible 104 has an outer surface 138 that faces the inner surface 134 of the susceptor 118 when the crucible 104 is disposed within the cavity 136. The outer surface 138 of the crucible 104 has a shape corresponding to the shape of the cavity 136 defined by the inner surface 134 of the susceptor 118, whereby the crucible 104 fits against and is supported by the inner surface 134 of the susceptor 118 during the lift process.

[0030] The inner surface 134 of the susceptor 118 is covered with a protective coating 140. The coating 140 may completely or substantially cover the inner surface 134, or may cover at least a portion of the inner surface 134. The crucible 104 contacts the coating 140 during ingot growth. The coating 140 preferably covers a sufficient portion of the inner surface 134 to inhibit or prevent contact between the outer surface 138 of the quartz crucible 104 and the inner surface of the graphite susceptor 118. In some embodiments, the coating 140 may additionally or alternatively cover areas other than the inner surface 134 to inhibit or prevent contact between the graphite susceptor 118 and the quartz crucible 104 and enable it to function as described herein. For example, the coating 140 may cover at least a portion of the outer surface 138 of the crucible 104 in addition to or instead of coating the inner surface 134 of the susceptor 118.

[0031] The coating 140 is preferably formed from a material that is chemically inert to both the carbon of the susceptor 118 and the silica of the crucible 104 under the lift process conditions (e.g., lift temperature). In this regard, the coating 140 provides a chemically inert barrier that limits, inhibits, or prevents direct contact between the graphite susceptor 118 and the quartz crucible 104 and limits, inhibits, or prevents reaction between the carbon of the susceptor 118 and the silica of the crucible 104 under the processing conditions generated inside the ingot lift apparatus 100.

[0032] The inert materials used to form the coating 140 may be boron nitride (BN), silicon nitride (Si 3 N 4 ), or combinations thereof. Boron nitride may be particularly suitable as an inert material because of its high thermal and chemical stability and its resistance to oxidation even at high temperatures. Silicon nitride may tend to form Si-O-N compounds with several stoichiometric compositions at the high temperatures during the pulling process. As the temperature rises, for example, above 1550 °C, the Si-O-N bonds tend to sublime into other gas phases, which degrades the coating 140. Boron nitride (BN), due to the strength of the covalent bond between the boron and nitrogen atoms, may not have a similar tendency to form oxidation products and may function in a higher upper temperature range than silicon nitride.

[0033] Coating 140 may be applied as a coating composition that includes particles of an inert material suspended in a liquid solution. Preferably, the coating composition includes particles of an inert material (e.g., boron nitride) suspended in an amount of liquid solution that facilitates sufficient overlap between the inert material particles when the coating composition is applied to the inner surface 134 of susceptor 118. For example, the coating composition may include at least about 5 wt% of inert material particles. In some embodiments, the coating composition includes at least about 15 wt%, and even at least about 30 wt% of inert material particles. In various embodiments, the coating composition may include from about 5 wt% to about 50 wt% of inert material particles, from about 15 wt% to about 50 wt% of inert material particles, from about 10 wt% to about 40 wt% of inert material particles, from about 20 wt% to about 40 wt% of inert material particles, or from about 30 wt% to about 40 wt% of inert material particles. In some embodiments, the coating composition includes about 37.5 wt% of inert material particles. In some embodiments, the coating composition includes about 25 wt% of inert substance particles. For the purposes of the present disclosure, and unless otherwise specified, the content of the components of the "coating composition" or simply the "composition" refers to the material initially applied to the inner surface 134 of susceptor 118 (e.g., particles of an inert material suspended in a liquid solution), and does not refer to the coating 140 itself formed after additional processing steps (e.g., vaporization of the medium, heating, or sintering, etc.). Unless otherwise stated, the weight percent (%) of a component means the amount of the component in the composition based on the total weight of the composition.

[0034] Without being bound by a particular theory, it is believed that the size of the inert material particles can affect the rheology of the coating composition and can affect the ease of application. In some embodiments, the average nominal diameter of the inert material particles may be less than about 100 μm. In other embodiments, the average nominal diameter of the inert material particles may be less than about 50 μm, less than about 25 μm, or even less than about 10 μm. Generally, decreasing the particle size improves the fluidity of the coating composition. When the inert material particles are boron nitride, the boron nitride particles have a particle size distribution of about 0.5 um d 50 (median), about 0.8 um d 90 (i.e., 90% of the particles have a particle size below about 0.8 um). The inert material particles may be described as powders or nanometer-sized particles.

[0035] When applied in the form of a liquid solution having suspended inert material particles, the coating composition includes additional components such as a medium, a binder, a dispersant, a stabilizer, etc. The components are selected such that the formulation does not crystallize quartz or SiO. Next, the components contemplated for use in the coating composition will be described.

[0036] The composition may include a medium in which the inert material (e.g., boron nitride) remains substantially particulate. Generally, one or more of the inert material and additives such as binders, dispersants, stabilizers, etc. may not dissolve, partially dissolve, or completely dissolve in the medium, and the terms "medium", "diluent", and "solvent" may be used interchangeably, and it should be understood that the embodiments of the present disclosure are not limited to compositions in which one or more components dissolve or do not dissolve in the medium.

[0037] The medium may contain organic compounds and may be aqueous. However, it should be noted that when water is present in an aqueous solution, a large amount of oxygen will be contained in the cured coating, which may react with the graphite susceptor 118 at a high processing temperature. Therefore, while an aqueous solution can be utilized without departing from the present disclosure, in some embodiments, it may be preferable to use a non-aqueous medium. Preferably, the medium easily vaporizes during any drying process. In an example where an aqueous medium is used in a liquid solution, the composition may be adjusted in terms of the amount of any of the additional components present (such as binders, dispersants, and / or stabilizers described later) to achieve the desired fluid viscosity of the liquid coating composition.

[0038] The medium may contain C1-C10 alcohols and may be isopropyl alcohol or ethanol. The amount of the medium is selected to obtain an appropriate flow viscosity of the liquid coating composition. In some embodiments, the coating composition contains at least about 10 wt% of the medium. In other embodiments, the composition may contain at least about 30 wt%, at least about 50 wt%, or even at least about 70 wt% of the medium. In various other embodiments, the composition contains from about 10 wt% to about 80 wt% of the medium, from about 30 wt% to about 70 wt% of the medium, from about 40 wt% to about 60 wt% of the medium, or even from about 45 wt% to about 55 wt% of the medium. In some embodiments, the composition may contain about 49.5 wt% of the medium. The composition may contain a plurality of media where the total weight fraction of the media in the composition is as described above. It should be understood that the composition may contain other amounts of the medium to obtain an appropriate fluid viscosity of the liquid coating composition. The amount of the medium may vary depending on, for example, the type of medium used (such as organic or aqueous). Further, the appropriate fluid viscosity of the liquid coating composition can vary depending on the technique used to apply the liquid coating composition. For example, as described later, aerosol spraying techniques may require a lower fluid viscosity of the liquid coating composition compared to brush coating techniques.

[0039] The coating composition may also include components of the composition, and more particularly, particles of an inert material (e.g., boron nitride), on the inner surface 134 of the susceptor 118, and one or more binders that act to bind the particles to each other after application. Generally, the binder changes the rheology of the coating composition and maintains the distribution of the particles in the medium through application and drying. In some embodiments, the binder is dissolved in the medium. In some embodiments, the binder is a polyvinyl butyral such as BUTVAR® B-76 (available from Eastman). In some embodiments, the coating composition includes at least about 0.5 wt%, at least about 2 wt%, at least about 5 wt%, or at least about 10 wt%, or even at least about 15 wt% of the binder. In various embodiments, the composition includes from about 0.5 wt% to about 20 wt% of the binder, from about 0.5 wt% to about 10 wt% of the binder, from about 0.5 wt% to about 8 wt% of the binder, or from about 2 wt% to about 8 wt% of the binder. In some embodiments, the coating composition includes about 5.6 wt% of the binder. In some embodiments, the composition does not include a binder. The composition may include a plurality of binders where the total weight fraction of the binders in the composition is as described above.

[0040] The coating composition may also include one or more dispersants. Generally, the dispersant acts to prevent the settling of the inert material particles prior to application of the coating composition to the inner surface 134 of the susceptor 118. Suitable dispersants generally do not contribute metal impurities to the coating composition, have clean combustibility, and decompose during the thermal cycle. The combination of a stabilizer (described later) and a dispersant may be selected such that the drying of the coating composition to form the coating 140 is achieved without cracks or with minimal cracking. In some embodiments, the dispersant is a non-aqueous polymeric dispersant such as SOLSPERSE® 20000 (available from Lubrizol Corp.). The coating composition may include at least about 0.05 wt% of the dispersant, in other embodiments at least about 0.1 wt%, at least about 0.5 wt%, at least about 1 wt%, or even at least about 5 wt% of the dispersant. In various other embodiments, the composition includes from about 0.05 wt% to about 10 wt% of the dispersant, from about 0.05 wt% to about 5 wt% of the dispersant, or from about 0.5 wt% to about 2.5 wt% of the dispersant. In some embodiments, the coating composition includes about 1.8 wt% of the dispersant. In some embodiments, the composition does not include a dispersant. The composition may include a plurality of dispersants such that the total amount of the dispersants in the composition is as described above.

[0041] The coating composition may also include a stabilizer. Suitable stabilizers generally do not contribute metal impurities to the coating composition, have a clean flammability, and decompose during heat cycling. The combination of a stabilizer and a dispersant (described above) may be selected such that drying of the coating composition to form the coating 140 is achieved without cracking or with minimal cracking. The stabilizer may be, for example, polyethylene glycol, such as CARBOWAX® Polyethylene Glycol 400 (available from Dow Chemical Co.). The coating composition may include at least about 0.5 wt%, at least about 2 wt%, at least about 5 wt%, at least about 10 wt%, or even at least about 15 wt% of the stabilizer. In various embodiments, the composition includes from about 0.5 wt% to about 20 wt% of the stabilizer, from about 0.5 wt% to about 10 wt% of the stabilizer, from about 0.5 wt% to about 8 wt% of the stabilizer, or from about 2 wt% to about 8 wt% of the stabilizer. In some embodiments, the coating composition includes about 5.6 wt% of the stabilizer. In some embodiments, the composition does not include a stabilizer. The composition may include a plurality of stabilizers where the total amount of stabilizers in the composition is as described above.

[0042] Non-limiting examples of techniques for applying a coating composition having inert material particles suspended in a liquid solution include brushing or aerosol spraying. In a brushing application, the coating composition may be brushed (e.g., foamed brush) onto the inner surface 134 of the susceptor 118. In an aerosol spraying application, the coating composition may be applied to the inner surface 134 of the susceptor 118 using a pressurized spray gun. Generally, a coating composition when applied by aerosol spraying has a lower fluid viscosity than when applied by a brushing technique to enable effective spraying of the coating composition. In this regard, the coating composition may contain a smaller amount of inert material particles when applied by aerosol spraying than when applied by brushing. For example, a brushable coating composition may contain more than 30 wt% of inert material particles, such as from about 30 wt% to about 50 wt% of inert material particles, while a sprayable coating composition may contain less than 30 wt% of inert material particles, such as from about 5 wt% to about 30 wt% of inert material particles. Similarly, the coating composition may have a greater amount of medium when applied by aerosol spraying than when applied by brushing. For example, a brushable coating composition may contain more than 70 wt% of medium, such as from about 70 wt% to about 80 wt% of medium, while a sprayable coating composition may contain less than 60 wt% of medium, such as from about 40 wt% to about 60 wt% of medium.

[0043] Brushing and / or spraying may be carried out under a ventilation hood, preferably at atmospheric pressure and at a temperature below the flash point of the coating composition. The coating composition on the inner surface 134 of the susceptor 118 is then dried either by air drying or heat treatment to remove the medium. The dispersant, binder, and / or stabilizer in the coating composition may be included in an amount sufficient to facilitate delaying the surface evaporation of the medium so that sufficient medium is removed by evaporation from the underlying layer. Otherwise, before sufficient medium evaporates from the thin film bulk, the surface of the outermost layer may peel off too quickly, and pinholes may be formed in the coating 140. In some embodiments, applying the coating composition and then drying to remove the medium may be repeated several times to reach the desired thickness.

[0044] Once the desired thickness is achieved, susceptor 118 may be heated to a temperature sufficient to decompose, vaporize, and / or off-gas any remaining liquid components (such as binders, dispersants, stabilizers, any remaining media, etc.). Generally, heating may be accomplished by heating susceptor 118 having the coating composition applied to its inner surface 134 until the desired temperature is reached. The binder, dispersant, stabilizer, residual media, etc. are removed, and coating 140 is formed. These components may be removed until the residual carbon in coating 140 is less than 5 wt%, or less than about 3 wt%, or less than about 1 wt%. In some embodiments, susceptor 118 is heated to at least about 150 °C, at least about 200 °C, at least about 300 °C, at least about 400 °C, or even at least about 750 °C to remove compounds of the binder, stabilizer, dispersant, and remaining media. In various other embodiments, susceptor 118 may be heated from about 100 °C to 750 °C, or from 400 °C to 750 °C. Susceptor 118 may be heated for at least about 1 hour, in other embodiments, at least about 2 hours, at least about 3 hours, or from about 1 hour to about 5 hours. In some embodiments, susceptor 118 is heated to at least about 300 °C for at least about 2 hours. The atmosphere may be at a pressure (vacuum) from about 60 torr (0.08 atm) to about 1 atm, or from about 150 torr (0.20 atm) to about 1 atm. In other embodiments, pressures above atmospheric pressure are used, such as at least 1 atm, at least 2 atm, or even at least 5 atm.

[0045] Susceptor 118 may be heated in the presence of an inert gas such as nitrogen, helium, argon, etc. Ambient air may be used as the atmosphere during heating, but this is less preferred as it may introduce oxygen into coating 140. Further, as will be understood by those skilled in the art, the furnace structural materials and gas flow can be controlled to avoid oxidation of coating 140.

[0046] As a result, the coating 140 contains particulate materials remaining from the coating composition (e.g., inert material particles and sintering aids further described below). A sintering process may be performed to densify and strengthen the coating 140. This may be performed in addition to and subsequent to the heating described above to remove the liquid component from the coating composition, or both the removal of the liquid component to form the coating 140 and the removal of the liquid component to sinter the coating 140 may be performed as a single process. In certain embodiments, sintering is performed during the dissolution of the silicon feedstock to minimize processing time. To achieve sintering, the susceptor 118 may be heated to a temperature from about 900 °C to about 1600 °C. In some embodiments, the susceptor 118 is heated to a temperature of at least about 1000 °C, at least about 1100 °C, at least about 1400 °C, or even at least about 1500 °C. The coating 140 may be sintered at the temperature for a period of at least about 1 hour to several days. In some embodiments, the coating 140 is sintered for at least about 6 hours. The coating 140 may be sintered in the presence of an inert gas such as nitrogen, helium, or argon. The atmosphere may be at a pressure (vacuum) from about 60 torr (0.08 atm) to about 1 atm, or from about 150 torr (0.20 atm) to about 1 atm. In other embodiments, pressures above atmospheric pressure are used, such as at least 1 atm, at least 2 atm, or even at least 5 atm.

[0047] Coating 140 may be formed by other coating techniques such as, for example, chemical vapor deposition or plasma spraying. In plasma spraying applications, inert material particles and additional powdered or particulate additives (such as, for example, sintering aids detailed below) are fed through one or more powder feeders into a high-temperature plasma jet (such as, for example, a DC plasma torch), where the particles are mixed with a plasma gas (such as, for example, argon, helium, nitrogen, hydrogen, or combinations thereof). The particles have an appropriate size distribution (such as, for example, a size of about 5 - 100 μm) to facilitate sufficient mixing and movement through the plasma jet. The particles are accelerated at high speeds (such as, for example, 50 - 3000 m / s) and rapidly heated to high temperatures (such as, for example, about 2000 °C to about 3000 °C) and directed towards the inner surface 134 of susceptor 118. The softened or melted particles impinge on the inner surface 134, thereby depositing a coating 140 having a desired thickness between the susceptor 118 and the crucible 104. Plasma spraying technology may have several advantages over other film-forming techniques, such as, for example, brush coating or spray coating. For example, plasma spraying may promote high efficiency in terms of throughput and ease of operation. Plasma spraying may also promote a reduction in processing time since the need for a separate heating and / or sintering step to form the coating 140 can be eliminated. Furthermore, the processing conditions of plasma spraying can be controlled to easily adjust the coating properties. Additionally, the coating 140 deposited by plasma spraying may preferably be a substantially permanent layered (interleaved type) coating 140, reducing the need for recoating of the coating 140 after several lift-up steps using the susceptor 118, which may occur when the coating 140 is formed from a brushable or sprayable coating composition.

[0048] Once the coating 140 is formed, the particles contained in the coating 140 (e.g., inert material particles and additional particles such as sintering aids to be described in more detail below) form a thin film interposed between the graphite susceptor 118 and the quartz crucible 104. The layered coating 140 behaves as a sintered monolithic component and exhibits excellent chemical inertness between the graphite susceptor 118 and the quartz crucible 104. However, it has been observed that the layered coating 140 formed essentially of inert material particles (e.g., boron nitride) has minimal strength. Thereby, the inert material particles of the coating 140 may become separated and scattered from each other. The loose inert material particles may be carried into the silicon melt 102 during the pulling process by the process gas flowing through the ingot puller 100, for example. Thereafter, the inert material particles may move to the solid-melt interface where the ingot grows. As a result, the ingot pulled from the melt 102 may be contaminated with inert material particles, which adversely affects the resistivity of the ingot and the wafers sliced from the ingot. For example, boron or nitrogen atoms from loose boron nitride particles may be introduced into the melt 102 as unintended dopants, which may cause fluctuations in the resistivity of the ingot. Further, since the loose powder continues to separate from the coating 140 over time, this problem worsens in subsequent pulling processes. The loose powder accumulates in the region between the susceptor 118 and the crucible 104 that cannot be adequately cleaned even when the pulling process is stopped, causing processing delays and increasing manufacturing time and costs. To make the coating 140 more functional and utilize the reduced reactivity between the carbon surface and the silica surface, it is desirable to provide a more robust layered coating 140.

[0049] For this purpose, it has been found that mixing a sintering aid with the inert material particles facilitates improving the durability of the coating 140 and can easily minimize the possibility that the inert material particles separate from the coating 140 and cause harmful properties in the ingot grown using the puller 100 through cross-contamination.

[0050] Thus, in embodiments of the present disclosure, the coating 140 is formed from a mixture of inert material particles (e.g., boron nitride) and a sintering aid that promotes densification of the inert material particles in the coating 140 and generally improves the adhesion of the inert material particles to the inner surface 134 of the susceptor 118 and to each other. In particular, the sintering aid facilitates the diffusion and bonding of the inert material by forming a grain boundary liquid phase at the sintering temperature (e.g., from about 900 °C to about 1600 °C), thereby promoting densification of the inert material in the sintered coating 140. The aids include, for example, silica (silicon dioxide, SiO 2 ), silicon carbide (SiC), boric acid (hydrogen borate, H 3 BO 3 ), alumina (aluminum dioxide, Al 2 O 3 ), yttria (yttrium oxide, Y 2 O 3 ), zirconia (ZrO 2 ), aluminum nitride (AlN), and one or more of lanthanum oxide (lanthanum oxide, La 2 O 3 ).

[0051] The sintering aid is included in an amount suitable to achieve the target densification characteristics of the coating 140. In this regard, the amount of sintering aid included in the coating 140 can vary quite a bit in order to enable a wide range of achievable strength characteristics of the coating 140. The target densification characteristics may be determined by the acceptable amount of mutual contamination of the inert material (e.g., boron nitride) in the silicon melt during the pull process, and this amount of mutual contamination may depend on the target resistivity of the ingots and the wafers sliced from these ingots.

[0052] In some examples, the ingot may have a high target resistivity. The high resistivity ingot may have a target resistivity of at least about 5000 Ω-cm, at least about 7500 Ω-cm, at least about 10,000 Ω-cm, or even at least about 100,000 Ω-cm. In these examples, since unwanted dopants introduced into the melt can have a relatively large impact on resistivity variations, cross-contamination is preferably minimized. Thus, in these examples, the sintering aid may be included in an amount sufficient to form a thin film that is at least moderately bonded to an inert material (e.g., boron nitride) when the coating 140 is sintered.

[0053] In some examples, the ingot may have a relatively low target resistivity. For example, the ingot may have a target resistivity of less than about 5000 Ω-cm, such as less than about 1000 Ω-cm, or even less than about 500 Ω-cm. In these examples, cross-contamination of the inert material (e.g., boron nitride) in the silicon melt may be tolerated even if it is relatively high. Thus, in these embodiments, the sintering aid may be included in an amount sufficient to provide a loose lubricating system for densifying boron nitride when the coating 140 is sintered.

[0054] Furthermore, the amount of the sintering aid included in the coating 140 must be such that the coating 140 retains the inert properties provided by the inert material (e.g., boron nitride).

[0055] Thus, the mass ratio of the sintering aid to the inert material (e.g., boron nitride) in the coating 140 may be from at least about 1:100 to about 1:1. Usually, the sintering aid is not added in an amount such that the mass ratio of the sintering aid to the inert material exceeds about 1:1. In some embodiments, the mass ratio of the sintering aid to the inert material is about 1:50, about 1:40, about 1:30, about 1:20, about 1:15, about 1:10, about 1:9, about 1:8, about 1:7, about 1:6, about 1:5, about 1:4, about 1:3, about 1:2, or about 1:1. As described above, the mass ratio may be selected as any amount suitable to achieve the targeted densification characteristics of the coating 140. In the brushing or spray coating of the coating 140, the liquid coating composition may contain at least about 0.01 wt% to a maximum of about 30 wt% of the sintering aid. In various embodiments, the liquid coating composition applied by brushing or spray coating may contain about 1 wt% to about 25 wt% of the sintering aid, about 5 wt% to about 20 wt% of the sintering aid, or about 10 wt% to about 15 wt% of the sintering aid. In the plasma spraying application of the coating 140, the sintering aid may be included in a mass ratio with the inert material particles as described above. Preferably, when the coating 140 is applied by plasma spraying, the mass ratio is such that the sintering aid to the inert material is from about 1:20 to about 1:1, from about 1:15 to about 1:1, or from about 1:10 to about 1:1. In an example where the coating 140 is applied by plasma spraying, the mass ratio of the sintering aid to the inert material is about 1:4. When the coating 140 is applied by plasma spraying, the sintering aid may be included in an amount of at least about 0.01 wt% to a maximum of about 50 wt% based on the weight of the inert material particles. In various embodiments, when the coating 140 is applied by plasma spraying, the sintering aid is included in an amount of about 1 wt% to about 50 wt%, about 10 wt% to about 40 wt%, or about 20 wt% to about 30 wt% based on the weight of the inert material particles. In an example, when the coating 140 is applied by plasma spraying, the sintering aid is included in an amount of about 25 wt% based on the weight of the inert material particles.

[0056] As described above, the sintering aid can be applied using a thermal coating process (e.g., plasma spraying) or a wet-like coating process (e.g., brushing or aerosol spraying of a liquid coating composition). The sintering aid is preferably included in an amount sufficient for the inert material particles (e.g., boron nitride) to adhere to each other and form a dense cured coating 140. In this regard, the sintering aid is included to obtain a coating 140 that is much less loose than a coating 140 formed entirely or substantially entirely from an inert material (e.g., boron nitride). Thus, the sintering aid facilitates reducing the possibility that the coating 140 will shed boron nitride as particles and thereby contaminate the solution 102. Further, the sintering aid is included in an amount suitable to prevent reducing the thermal durability and inert functionality of the coating 140 described herein. That is, if the amount of the sintering aid is excessive, it may significantly lower the thermal durability of the coating 140 below that of a coating 140 formed entirely or substantially entirely from an inert material (e.g., boron nitride). In one embodiment, the coating 140 is applied by plasma spraying, and the coating 140 is formed from a particulate mixture of boron nitride as an inert material and a sintering aid selected from silicon carbide (SiC), yttria (yttrium oxide, Y 2 O 3 ), aluminum nitride (AlN), lanthanum (lanthanum oxide, La 2 O 3 ), and mixtures of two or more of these materials, where the mass ratio of the sintering aid to boron nitride is about 1:4 (i.e., the sintering aid is included in an amount of about 25 wt% relative to the weight of boron nitride).

[0057] Sintering the coating 140 (or the coating composition used to form the coating 140) containing inert material particles (e.g., boron nitride) and a sintering aid to densify and strengthen the coating 140 may be carried out as described above. However, the sintering process is not limited to a specific sintering process, and the sintering process may be utilized even if the coating 140 is formed from a coating composition applied as a liquid solution or formed by plasma spraying. The sintering aid can function suitably as described herein regardless of whether sintering is carried out in a vacuum, an inert gas atmosphere, or air. When sintering in air, a sintering aid containing oxides (e.g., silica, boric acid, alumina, yttria, zirconia, and lanthana) is preferably used because it is least reactive in air during sintering. Further, it should be understood that several mechanisms are effective for sintering. For example, mechanisms such as the formation of a glassy phase between grain boundaries, although depending on the viscosity of the glassy system, may start at a low temperature of 900 °C but more generally range from 1200 °C to 1800 °C, which depends on the desired processing time at high temperature. Another mechanism is the evaporation and condensation of the sintering aid, where evaporation of the sintering aid causes transport and subsequent densification, and after the sintering aid is vapor-transported, the components condense in the vapor condensation mode, followed by densification and strength development. Another mechanism is the diffusion of solids within the sintering aid, which also leads to an improvement in strength. The degree of manifestation of strength or physical properties at a given temperature or pressure is similarly influenced by time as a factor.

[0058] Referring to FIGS. 3A and 3B, distribution examples 300A and 300B of a mixture of a sintering aid and inert material particles (e.g., boron nitride) are shown. In particular, these show two example approaches for obtaining a mixture of inert material particles and a sintering aid to facilitate improving the strength of the sintered coating 140.

[0059] In FIG. 3A, the separated particulate mixture 300A includes sintering aid particles 302A directly mixed with inert material particles 304A. The separated particle mixture 300A may be supplied to a high-temperature plasma jet for forming the coating 140 (FIG. 2) by plasma spraying the particles as the inert material particles 304A and the sintering aid particles 302A. Further, the separated particulate mixture 300A may be added to a coating composition as the separated particles 304A of the inert material and the sintering aid particles 302A, whereby the particles 302A and 304A are suspended in a liquid solution containing, for example, a medium, a dispersant, a binder, a stabilizer, and the like.

[0060] In FIG. 3B, the clothing particulate mixture 300B includes inert material particles 304B coated with a sintering aid 302B. To form the coated particulate mixture 300B, the sintering aid 302B is added to an aqueous solution, and the inert material particles 304B are washed with the solution containing the sintering aid 302B. After washing to coat the inert material particles 304B, the slurry is dried to remove the aqueous medium (e.g., water), thereby producing the coated particulate mixture 300B. The coated particulate mixture 300B may then be supplied to a high-temperature plasma jet for forming the coating 140 (FIG. 2) by plasma spraying the coated particulate mixture 300B. Further, the coated particulate mixture 300B may be added to a coating composition, whereby the coated particulate mixture 300B is suspended in a liquid solution containing, for example, a medium, a dispersant, a binder, a stabilizer, and the like.

[0061] These two approaches can promote a wider range of functionality for the mixture of inert material particles and sintering aids according to the present disclosure. The approach selected depends on the amount of sintering aid intended for use, from which sufficient sintering strength can be obtained. Thus, the approach selected depends on the targeted densification characteristics of the coating 140, the targeted inertness of the coating 140, and / or the acceptable level of cross-contamination. One advantage of the coated particulate mixture 300B (shown in FIG. 3B) is that the mixture 300B provides a higher point contact of the sintering aid 302B to other particles that would come into contact with the sintering aid 302B. Thus, it may be necessary to potentially reduce the sintering aid 302B in order to obtain the mechanical strength of the coating 140. However, the pretreatment of the particulate mixture 300B itself not only controls the particle size of the inert material particles 304B coated with the sintering aid 302B that results in the desired mechanical properties of the coating 140, but also requires some additional steps to obtain such a distribution. Conversely, the separated particulate mixture 300A (shown in FIG. 3A) is more of a mechanical mixture that utilizes the influent particle size distribution of the sintering aid particles 302A and the inert material particles 304A. The interparticle contact where the sintering aid particles 302A contact each other sandwiching the particles has a low occurrence rate, so it may be necessary to increase the weight load of the sintering aid particles 302A. The actual form and distribution of the particles may be selected based on factors desired for the final coating 140, such as the desired coating density, scratch resistance, durability, etc.

[0062] Advantages of using the disclosed coating include the reaction of carbon (C) from the graphite susceptor and silica (SiO 2 ) from the quartz crucible (C(s) + SiO 2(s) → CO(g) + SiO(g)) includes reducing or preventing the formation of gaseous products such as carbon monoxide (CO) and silicon monoxide (SiO). Other advantages include promoting the densification of the inert materials used in the formation of the coating, thereby enhancing the strength of the coating and reducing the mutual contamination between the silicon melt and the inert coating material. Further, this coating utilizes materials that promote a wider functionality that can be tailored to the target specifications of the ingot grown by the pulling process. Additionally, the coating can preferably be applied as a semi-permanent thin film, facilitating a reduction in the frequency of coating application required during the pulling process.

[0063] As used herein, the terms "about," "substantially," "essentially," and "approximately," when used in connection with a range of dimensions, concentrations, temperatures, or other physical or chemical properties or characteristics, are intended to encompass variations that may exist at the upper and / or lower limits of the range of the property or characteristic, including, for example, variations resulting from rounding, measurement methods, or other statistical fluctuations.

[0064] When introducing elements of the present disclosure or embodiments thereof, the articles "a," "an," "the," and "said" are intended to mean that there are one or more of the elements. The terms "comprising," "including," "containing," and "having" are intended to be inclusive and mean that additional elements other than the listed elements may exist. The use of terms indicating a particular orientation (e.g., "upper," "bottom," "lateral," etc.) is for convenience of explanation and does not require a particular orientation of the item being described.

[0065] Since various changes can be made in the above configurations and methods without departing from the scope of the present disclosure, all matters included in the above description and shown in the accompanying drawings are intended to be construed as illustrative and not in a limiting sense.

Claims

1. A method for producing a single crystal silicon ingot from a silicon melt, comprising: providing a graphite susceptor having an inner surface defining a cavity; depositing a coating on the inner surface of the susceptor, wherein the coating comprises boron nitride and a sintering aid, and the sintering aid promotes densification of the boron nitride; placing a quartz crucible in the cavity of the susceptor, wherein the crucible has an outer surface that contacts the coating; adding polycrystalline silicon to the crucible; heating the polycrystalline silicon to form a silicon melt in the crucible; and pulling a single crystal silicon ingot from the silicon melt. A method comprising the above steps.

2. Depositing the coating comprises plasma spraying the boron nitride and the sintering aid onto the inner surface of the susceptor, the method according to claim 1. The method according to claim 1.

3. The mass ratio of the sintering aid to the boron nitride in the coating is from 1:20 to 1:1, the method according to claim 1 or claim 2. The method according to claim 1 or claim 2.

4. Depositing the coating comprises applying a coating composition to the inner surface of the susceptor, the coating composition comprising the boron nitride, the sintering aid, an organic medium, and optionally one or more of a stabilizer, a binder, and a dispersant, the method according to claim 1. The method according to claim 1.

5. The method according to claim 4, further comprising removing the organic medium and, if present, one or more of the stabilizer, the binder, and the dispersant from the coating composition applied to the inner surface of the susceptor, thereby forming the coating. The method according to claim 4.

6. The method according to claim 5, further comprising sintering the coating during and / or after removing the organic medium and, if present, one or more of the stabilizer, the binder, and the dispersant. The method according to claim 5.

7. The coating composition comprises 0.1 wt% to 50 wt% of the boron nitride, the method according to any one of claims 4 to 6. The method according to any one of claims 4 to 6.

8. The mass ratio of the sintering aid to the boron nitride in the coating is from 1:100 to 1:1, the method according to any one of claims 4 to 7. The method according to any one of claims 4 to 7.

9. The sintering aid includes silica, silicon carbide, boric acid, alumina, yttria, zirconia, aluminum nitride, lanthana, or a combination thereof. The method according to any one of claims 1 to 8.

10. A graphite susceptor for supporting a quartz crucible during a crystal growth process, A main body having an inner surface defining a cavity, the cavity having a size and shape complementary to the size and shape outside the crucible, the main body, A coating deposited on the inner surface, including boron nitride and a sintering aid, the sintering aid being configured to promote densification of the boron nitride, the coating A susceptor having

11. The mass ratio of the sintering aid to the boron nitride in the coating is from 1:100 to 1:

1. The susceptor according to claim 10.

12. The mass ratio of the sintering aid to the boron nitride in the coating is from 1:20 to 1:

1. The susceptor according to claim 10.

13. The coating is deposited by plasma spraying the boron nitride and the sintering aid onto the inner surface of the susceptor. The susceptor according to any one of claims 10 to 12.

14. The mass ratio of the sintering aid to the boron nitride in the coating is from 1:5 to 1:

1. The susceptor according to any one of claims 10 to 13.

15. The coating is deposited by applying a coating composition including the boron nitride, the sintering aid, and an organic medium onto the inner surface of the susceptor and then removing the medium. The susceptor according to any one of claims 10 to 12.

16. The sintering aid includes silica, silicon carbide, boric acid, alumina, yttria, zirconia, aluminum nitride, lanthana, or a combination thereof. The susceptor according to any one of claims 10 to 15.

17. A method of forming a coating between an inner surface of a graphite susceptor and an outer surface of a quartz crucible, Providing a particulate mixture of the boron nitride and the sintering aid, Depositing the particulate mixture of the boron nitride and the sintering aid on the inner surface of the susceptor, and Sintering the deposited particulate mixture to thereby form the coating, wherein sintering the deposited particulate mixture promotes densification of the boron nitride using the sintering aid. A method comprising. **Claim 18** Sintering the deposited particulate mixture occurs during and / or after deposition of the particulate mixture. The method according to claim 17. **Claim 19** The sintering aid comprises silica, silicon carbide, boric acid, alumina, yttria, zirconia, aluminum nitride, lanthana, or a combination thereof. The method according to claim 17 or claim 18. **Claim 20** Providing the particulate mixture comprises mixing separate particles of the boron nitride and the sintering aid to form a separated particulate mixture. The method according to any one of claims 17 to 19. **Claim 21** Depositing the particulate mixture comprises plasma spraying the separated particulate mixture onto the inner surface of the susceptor. The method according to claim 20. **Claim 22** Providing a coating composition comprising the separated particulate mixture suspended in an organic medium and optionally one or more of a stabilizer, a binder, and a dispersant. Depositing the separated particulate mixture by applying the coating composition to the inner surface of the susceptor, and Removing the medium and optionally one or more of the stabilizer, the binder, and the dispersant during and / or after sintering of the deposited and separated particulate mixture. The method according to claim 20, further comprising. **Claim 23** Providing the particulate mixture comprises providing a coated particulate mixture comprising particles of the boron nitride coated with the sintering aid. The method according to any one of claims 17 to 19. **Claim 24** Depositing the particulate mixture comprises plasma spraying the coated particulate mixture onto the inner surface of the susceptor. The method according to claim 23. **Claim 25** Providing a coating composition comprising the coated particulate mixture suspended in an organic medium and optionally one or more of a stabilizer, a binder, and a dispersant. Depositing the coated particulate mixture by applying the coating composition to the inner surface of the susceptor, and Removing, during and / or after sintering of the deposited and coated particle mixture, the medium and optionally one or more of the stabilizer, the binder, and the dispersant The method according to claim 23, further comprising this.

Images