Ingot pulling apparatus having a composite exhaust tube and method for selecting lengths of lower and upper exhaust tube sections - Patents.com

Abstract

An ingot pulling apparatus having a composite exhaust tube and a method for selecting the length of the lower and upper sections of the composite exhaust tube are disclosed. In some embodiments, the upper section of the composite exhaust tube is made of graphite and the lower section is made of stainless steel.

Description

[Technical field]

[0001] (CROSS REFERENCE TO RELATED APPLICATIONS) This application claims priority to U.S. Provisional Patent Application No. 63 / 153012, filed February 24, 2021, the entirety of which is incorporated herein by reference.

[0002] The field of the disclosure relates to an ingot pulling apparatus having a composite exhaust tube and a method for selecting the length of the lower and upper sections of the composite exhaust tube. [Background technology]

[0003] Single crystal silicon ingots may be prepared by the so-called Czochralski process, in which a single crystal silicon seed is contacted with a silicon melt held in a crucible. The single crystal silicon seed is withdrawn from the melt and a single crystal silicon ingot is pulled from the melt. The ingots may be prepared in a batch system, in which polycrystalline silicon is first melted in a crucible and silicon ingots are withdrawn from the melt until the molten silicon in the crucible is depleted. Alternatively, the ingots may be withdrawn in a continuous Czochralski process, in which polysilicon is intermittently or continuously added to the melt to replenish the silicon melt during ingot growth.

[0004] Pulling of the crystal may be performed in the presence of atmosphere within the housing of the ingot puller. In both the batch Czochralski process and the continuous Czochralski process, a process gas inert to silicon, such as argon, is continuously introduced into the housing and withdrawn through the exhaust system of the puller. As the process gas is withdrawn, compounds (e.g., compounds of silicon carbide and silicon oxide) may deposit in the exhaust system, which is at a lower temperature compared to the puller chamber. Such deposits are periodically removed from the exhaust system, which may require the ingot puller to be shut down, increasing processing costs. Pressure must be maintained within the housing, which is difficult with exhaust tubes of large cross-sectional area, making it difficult to increase the size of the exhaust system.

[0005] A need exists for a crystal pulling system that features reduced formation of deposits on the exhaust system of the ingot puller and that can withstand the environment (eg, temperature) of the ingot puller.

[0006] This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure that are described and / or claimed below. This discussion is believed to be helpful in providing the reader with background information to better understand the various aspects of the present disclosure. As such, it should be understood that these statements are to be read in this light, and not as admissions of prior art. Summary of the Invention

[0007] One aspect of the disclosure relates to an ingot pulling apparatus for growing a single crystal silicon ingot from a silicon melt. The apparatus includes a housing defining a growth chamber. The housing has a bottom and a sidewall extending from the bottom. A crucible is disposed within the growth chamber for holding the silicon melt. An exhaust tube extends from the growth chamber to the bottom of the housing. The exhaust tube defines an exhaust flow path for exhausting exhaust gases from the growth chamber. The exhaust tube includes an upper portion made from a first material and a lower portion coupled to the upper portion. The lower portion is made from a second material different from the first material.

[0008] Another aspect of the disclosure relates to a method for selecting a length of a lower metal portion of a composite exhaust tube of an ingot pulling apparatus. The composite exhaust tube includes an upper portion made from a first material and a lower portion coupled to the upper portion. The lower portion is made from a second material that is a metal. The second material is different from the first material. The composite exhaust tube extends upwardly from a bottom of a housing and through an insulating layer. The method includes determining a threshold temperature for the lower metal portion of the exhaust tube. A radial position of the exhaust tube is determined. An axial temperature profile of the ingot pulling apparatus is determined at the radial position of the exhaust tube during a melting phase of ingot growth. The axial temperature profile decreases from the upper portion of the insulating layer toward the bottom of the housing. The length of the lower metal portion of the exhaust tube is selected such that an upper end of the lower metal portion is at an axial position where a temperature of the lower metal portion is lower than the threshold temperature during the melting phase of ingot growth.

[0009] Various refinements exist in the features described in relation to the above-mentioned aspects of the disclosure. Likewise, further features may be incorporated into the above-mentioned aspects of the disclosure. These refinements and additional features may exist individually or in any combination. For example, various features described below in relation to any of the illustrated embodiments of the disclosure may be incorporated into any of the above-mentioned aspects of the disclosure, alone or in any combination. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional view of an ingot pulling apparatus having a charge of molten polycrystalline silicon. [Diagram 2] FIG. 2 is a cross-sectional view of the ingot pulling apparatus after melting and before the silicon ingot is grown. [Diagram 3] FIG. 3 is a cross-sectional view of an ingot pulling apparatus during growth of a silicon ingot. [Figure 4] FIG. 4 is a perspective view of an exhaust tube of an ingot pulling apparatus. [Diagram 5] FIG. 5 is an exploded view of the exhaust tube showing the upper and lower portions of the exhaust tube.

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

[0012] The present disclosure provides for an ingot puller having a composite exhaust tube and a method for selecting a length of a lower portion (e.g., a metal lower portion) of the exhaust tube. The ingot puller may generally be implemented with any ingot puller configured to pull a monocrystalline silicon ingot. An example of an ingot puller (or more simply, an ingot puller) is generally designated "100" in FIG. 1. The ingot puller 100 includes a crucible 102 for holding a melt 104 (FIG. 2) of a semiconductor or solar grade material, such as silicon, supported by a susceptor 106. The ingot puller 100 includes an ingot puller housing 108 that defines a growth chamber 152 for pulling a silicon ingot 113 (FIG. 3) from the melt 104 along a pull axis A. The crucible 102 is disposed within the growth chamber 152. The ingot puller 100 is generally suitable for batch Czochralski growth, in which an ingot is grown entirely from an initial charge 101 of polysilicon. In another embodiment, polysilicon is added during ingot growth, such as in a continuous Czochralski process.

[0013] Crucible 102 includes a floor 129 and a sidewall 131 extending upwardly from floor 129. Sidewall 131 is generally vertical. Floor 129 includes a curvature of crucible 102 that extends below sidewall 131. Within crucible 102 is a silicon melt 104 (FIG. 3) having a melt surface 111 (i.e., a melt-ingot interface).

[0014] In some embodiments, the crucible 102 is layered. For example, the crucible 102 may be made from a quartz base layer and a synthetic quartz layer disposed on the quartz base layer.

[0015] The susceptor 106 is supported by a shaft 105. The susceptor 106, crucible 102, shaft 105, and ingot 113 have a common longitudinal axis A or "pull axis" A.

[0016] The ingot pulling housing 108 includes a housing bottom 116 and a housing sidewall 121 extending upwardly from the bottom 116. The ingot pulling apparatus 100 includes an insulating cap 125 to prevent contaminants from entering the growth chamber 152 through the bottom insulation layer 110. Side insulation 124 extends radially inward from the housing sidewall 121.

[0017] Within the ingot pulling apparatus 100, a pulling mechanism 114 is provided for growing and pulling an ingot 113 (FIG. 3) from the melt 104. The pulling mechanism 114 includes a pulling cable 118, a seed holder or chuck 120 coupled to one end of the pulling cable 118, and a silicon seed crystal 122 coupled to the seed holder or chuck 120 for initiating crystal growth. One end of the pulling cable 118 is connected to a pulley (not shown) or drum (not shown) or any other suitable type of lifting mechanism, e.g., a shaft, and the other end is connected to the chuck 120 that holds the seed crystal 122. During operation, the seed crystal 122 is lowered and contacts the melt 104. The pulling mechanism 114 operates to raise the seed crystal 122. This causes the single crystal ingot 113 (FIG. 3) to be pulled from the melt 104.

[0018] A crucible drive unit 107 (e.g., a motor) rotates the crucible 102 and the susceptor 106 during heating and crystal pulling. A lifting mechanism 112 raises and lowers the crucible 102 along the pulling axis A during the growth process. For example, as shown in FIG. 1, the crucible 102 may be at a lowest position (near the bottom heater 126) where an initial charge 101 of solid phase polycrystalline silicon previously added to the crucible 102 is melted. Crystal growth begins by contacting the melt 104 with a seed crystal 122 and raising the seed crystal 122 by the pulling mechanism 114. As the ingot 113 grows, the silicon melt 104 is consumed and the height of the silicon melt 104 in the crucible 102 decreases. The crucible 102 and the susceptor 106 may be raised to maintain the melt surface 111 at or near the same position relative to the ingot pulling apparatus 100 (FIG. 3).

[0019] A crystal drive unit (not shown) may rotate the pulling cable 118 and ingot 113 (FIG. 3) in a direction opposite to (e.g., counter-rotation) the direction that the crucible drive unit 107 rotates the crucible 102. In embodiments using unidirectional rotation, the crystal drive unit may rotate the pulling cable 118 in the same direction that the crucible drive unit 107 rotates the crucible 102. The crystal drive unit also raises or lowers the ingot 113 relative to the melt surface 111 as desired during the growth process.

[0020] The ingot pulling apparatus 100 may include an inert gas system for introducing or withdrawing an inert gas, such as argon, into or from the growth chamber 152. The ingot pulling apparatus 100 may also include a dopant delivery system (not shown) for introducing dopants into the melt 104.

[0021] According to the Czochralski single crystal growth process, a quantity of solid polycrystalline silicon 101, i.e., polysilicon, is loaded into a crucible 102. The initial charge 101 of semiconductor or solar grade material introduced into the crucible 102 melts with heat provided from one or more heating elements to form a silicon melt 104 (FIG. 2) within the crucible 102. The ingot pulling apparatus 100 includes a bottom insulation layer 110 and side insulation 124 to maintain heat in the growth chamber 152. Portions of the ingot pulling apparatus 100 may be cooled (e.g., fluid cooled at the bottom of the apparatus 100). In the illustrated embodiment, the ingot pulling apparatus 100 includes a bottom heater 126 disposed below the crucible floor 129. As shown in FIG. 1, the crucible 102 may be moved relatively close to the bottom heater 126 such that the bottom heater 126 melts the polycrystalline charge 101 within the crucible 102.

[0022] To form an ingot 113 (FIG. 3), a seed crystal 122 is brought into contact with a surface 111 of the melt 104. A pulling mechanism 114 is operated to pull the seed crystal 122 from the melt 104. Referring to FIG. 3, the ingot 113 includes a crown portion 142 where the ingot 113 tapers as it transitions outward from the seed crystal 122 to reach a target diameter. The ingot 113 includes a constant diameter portion 145 or cylindrical "body" of the crystal 113 that is grown by increasing the pulling rate. The body 145 of the ingot 113 has a relatively constant diameter. The ingot 113 includes a tail or end cone (not shown) where the ingot tapers radially after the body 145. When the diameter is small enough, the ingot 113 is separated from the melt 104.

[0023] The ingot pulling apparatus 100 includes a side heater 135 and a susceptor 106 that surrounds the crucible 102 to maintain the temperature of the melt 104 during crystal growth. The side heater 135 is disposed radially outward of the crucible sidewall 131 as the crucible 102 moves up and down the pulling axis A. The side heater 135 and bottom heater 126 may be any type of heater that is operable as described herein. In some embodiments, the heaters 135, 126 are resistive heaters. The side heater 135 and bottom heater 126 may be controlled by a control system (not shown) such that the temperature of the melt 104 is controlled throughout the pulling process. The bottom heater 126 and the side heater 135 are exemplary and the ingot pulling apparatus 100 may include different arrangements of heaters (eg, heaters in different locations and / or additional heaters).

[0024] The ingot pulling apparatus 100 may include a heat shield 151. The heat shield 151 may surround the ingot 113 and may be partially disposed within the crucible 102 during crystal growth (FIG. 3).

[0025] The ingot pulling apparatus 100 includes an exhaust tube 140. The exhaust tube 140 extends through the bottom 116 of the housing 108, through the bottom insulation layer 110, through the insulation cap 125, and into the growth chamber 152. The exhaust tube 140 includes an upper end 143 that may be aligned with the insulation cap 125, and a lower end 149 that is aligned with the bottom 116 of the housing 108. The exhaust tube 140 has an overall length L140 (FIG. 4) that extends from the upper end 143 to the lower end 149. The exhaust tube 140 defines an exhaust passage 166 (FIG. 1) for exhausting exhaust gases from the growth chamber 152. The exhaust tube 140 may be integrated with an exhaust port (not shown). The exhaust port may be connected (e.g., by a flange connection) to an external conduit that directs the exhaust gases to a filtration system and a vacuum pump.

[0026] Although the ingot pulling apparatus 100 is shown with a single exhaust tube 140, the ingot pulling apparatus 100 may include multiple exhaust tubes 140 (and associated exhaust ports), such as at least 2, 3, 4, 5, 6, or more exhaust tubes 140. In such embodiments, each exhaust tube 140 may be a compound exhaust tube as described herein.

[0027] 4, exhaust tube 140 includes an upper portion 147 and a lower portion 155 coupled to upper portion 147. Upper portion 147 is made from a first material and lower portion 155 is made from a second material different from the first material. According to some embodiments of the present disclosure, the first material from which upper portion 147 is made has a melting temperature higher than the melting temperature of the second material from which lower portion 155 is made. In some embodiments, the first material from which upper portion 147 of exhaust tube 140 is made is graphite. The second material from which lower portion 155 of exhaust tube 140 is made may be a metal such as stainless steel (e.g., upper portion 147 is graphite and lower portion 155 is stainless steel).

[0028] It should be noted that the exhaust tube 140 shown in Figures 4-5 is exemplary. For example, the upper portion 147 and / or the lower portion 155 may include two or more layers (e.g., the upper portion 147 may include a layer of graphite on the inside and a layer of metal on the outside). In some embodiments, the composite exhaust tube 140 includes additional portions made from different materials.

[0029] The upper portion 147 and the lower portion 155 of the exhaust tube 140 may be joined together by any known method for connecting two dissimilar materials. For example, as shown in FIG. 5, the lower portion 155 includes a collar 158. When the two portions 147, 155 are joined together, the collar 158 is received in the upper portion 147 (e.g., a counterbore formed in the upper portion 147). The collar 158 may be a friction fit within the upper portion 147 (e.g., the weight of the upper portion 147 and / or a bore in the bottom insulation layer 110 helps maintain the connection).

[0030] The upper portion 147 of the exhaust tube has a length L147 extending from the upper end 154 to the lower end 156. 162 The collar 158 extends from the distal end 162 to the distal end 164 of the proximal end 162 (i.e., the length L155 includes the collar 158).

[0031] According to some embodiments of the present disclosure (e.g., when the lower portion 155 of the exhaust tube 140 is a metal such as stainless steel), the length L155 of the lower portion 155 of the exhaust tube 140 may be selected to prevent deformation and / or breakage of the lower portion 155 of the exhaust tube 140. To select the length L155 of the lower portion 155 of the exhaust tube 140, a threshold temperature for the lower (e.g., metal) portion of the exhaust tube 140 is determined. The threshold temperature may be related to the melting temperature of the second material of the lower portion 155 of the exhaust tube 140 (e.g., 10° C. below the melting temperature of the second material from which the lower portion 155 of the exhaust tube 140 is made, or 25° C. below the melting temperature, or 50° C. below the melting temperature, or 100° C. below the melting temperature of the second material from which the lower portion 155 of the exhaust tube 140 is made). In some embodiments, the threshold temperature is the melting temperature of the second material of the lower portion 155 of the exhaust tube 140.

[0032] In embodiments in which the lower portion 155 is made from stainless steel (e.g., 304L stainless steel, which generally has a melting temperature in the range of 1400°C to 1420°C), the threshold temperature may be 1400°C or less, 1375°C or less, or 1330°C or less.

[0033] An embodiment of a method for selecting the length L155 of the lower portion 155 of the exhaust tube 140 includes determining the axial temperature profile of the ingot pulling apparatus 100 (i.e., the temperature within the ingot pulling apparatus along the length L140 of the exhaust tube 140) at the radial position R140 (FIG. 1) of the exhaust tube 140 during the melting phase of ingot growth. In an existing ingot puller 100 where a composite exhaust tube 140 is retrofitted to a conventional exhaust tube, the radial position 140 (FIG. 1) of the exhaust tube 140 is simply the position of the tube in the ingot puller 100. In a new ingot pulling apparatus 100, the radial position R140 is set by determining where the exhaust tube 140 will be located when designing the new ingot pulling apparatus 100.

[0034] The axial temperature profile at the radial position of the exhaust tube 140 is determined by measuring the temperature within the ingot pulling apparatus 100 during the melting phase of the ingot growth (e.g., measuring the actual temperature within the ingot pulling apparatus 100 with one or more temperature sensors). In some embodiments, the axial temperature profile at the radial position of the exhaust tube 140 is determined by modeling the temperature within the ingot pulling apparatus 100 during the melting phase. The temperature profile of the ingot pulling apparatus 100 during melting may be modeled using commercially available tools such as CrysVun (CrysVun software from the Crystal Growth Laboratory at Fraunhofer Institute IISB, Erlangen, Germany), COMSOL (COSMOL multiphysics software from COSMOL, Inc., Burlington, Massachusetts, USA), Fluent (commercial software managed by Ansys, Canonsburg, Pennsylvania, USA), and CGSim (CGSim software commercialized by STR Group, Inc., St. Petersburg, Russia). These tools are exemplary and other software codes that model heat transfer in a puller during melting (eg, any steady-state 2D simulation) may be used.

[0035] According to an embodiment of the present disclosure, the temperature profile of the ingot pulling apparatus 100 is determined during a melting phase when an initial charge 101 (FIG. 1) of solid phase polycrystalline silicon added to the crucible 102 melts. During melting, the crucible 102 is typically lowered to a lowest position closest to the bottom heater (or heaters) 126. During melting, the exhaust tube 140 may experience the highest temperature, with the lower portion 155 closest to the threshold temperature. The axial temperature profile at radial position R140 of the exhaust tube 140 generally decreases in temperature from the upper end 143 to the lower end 149.

[0036] Once the temperature profile of the ingot pulling apparatus 100 at the radial position of the exhaust tube 140 is determined during melting, the length L155 of the lower metal portion 155 of the exhaust tube 140 is selected. 162 The length L155 of the lower metal portion 155 of the exhaust tube 140 (FIG. 5) is selected such that the length L155 of the lower metal portion 155 of the exhaust tube 140 is at an axial position where the temperature of the entire lower metal portion 155 is below the threshold temperature during the melting phase of ingot growth (i.e., when the silicon charge 101 (FIG. 1) is molten). In this regard, it is noted that the length L155 of the lower portion 155 of the exhaust tube may include a collar 158 to ensure that the entire lower portion 155 of the exhaust tube 140 is below the threshold temperature.

[0037] Note that because the length L140 (FIG. 4) of the exhaust tube 140 is relatively fixed by the thickness of the bottom insulation layer 110, selecting the length of the lower portion 155 of the exhaust tube sets the length L147 of the upper portion 147 of the exhaust tube 140 (i.e., in embodiments where the composite exhaust tube 140 includes only two portions and the height of the collar 158 is fixed).

[0038] In some embodiments, a second threshold temperature is established for the upper portion 147 of the exhaust tube 140 (i.e., the threshold temperature described above is the first threshold temperature). In such embodiments, the length L155 of the lower portion 155 of the exhaust tube is 162The length L155 may be selected such that the bottom 155 is long enough to allow the entire length L147 of the upper portion 147 of the exhaust tube 140 to be above the second threshold temperature, while the bottom 155 may be shortened to ensure that the entire length L147 of the upper portion 147 of the exhaust tube 140 is below the first threshold temperature. For example, the second threshold temperature may be the maximum temperature at which at least some deposits form on the upper portion 147 of the exhaust tube 140 (i.e., the second threshold temperature may be a temperature at which such deposits do not form at temperatures above the second threshold temperature). Examples of deposits include silicon oxide (SiOx), silicon (e.g., by silicon condensation), and silicon carbide (SiC) (including composites of SiC and SiOx). In embodiments in which upper portion 147 is made from graphite, upper portion 147 is maintained at a temperature higher than the second threshold temperature (i.e., by making lower portion 155 of sufficient length L155 to maintain the entire upper portion 147 in the hotter portion of bottom insulation layer 110), and the second threshold temperature may be 1200°C.

[0039] Compared to conventional ingot pullers, the ingot puller of the present disclosure has several advantages. By using a composite exhaust tube, the exhaust tube can be adapted to reduce deposits that form on the exhaust tube. In embodiments where the upper portion is made from graphite, the upper portion is relatively simple and inexpensive to manufacture. Graphite is a relatively inexpensive material. In such embodiments, the graphite is limited to the upper portion of the exhaust tube, where temperatures are relatively high and deposits are less likely to form. In embodiments where the lower portion of the exhaust tube is made from stainless steel, the stainless steel is resistant to deposit formation, which is beneficial in the lower portion of the exhaust tube, which is cooler and more prone to deposits. Reducing deposits allows for longer run times for the ingot puller, thereby improving productivity. Longer run times allow for more stable processing (which improves yields) and less frequent breakdown of the hot zone, which reduces damage to the hot zone. Additionally, the exhaust tube can be replaced less frequently, which reduces operating costs. By modeling the temperature of the ingot pulling equipment, the length of the lower section of the tube may be selected such that the upper section of the exhaust tube is at a temperature below the threshold temperature at which the material of the lower section (e.g., stainless steel) begins to deform or melt. Alternatively or additionally, the length of the lower section of the tube may be selected to be short enough to ensure that the length of the upper section of the tube is at a temperature above the temperature at which deposits form on the material from which the upper section of the tube is made (e.g., above 1200° C. for graphite).

[0040] (Example) The processes of the present disclosure are further illustrated by the following examples, which should not be construed in a limiting sense. Example 1: Use of a composite exhaust tube

[0041] The composite exhaust tube was used in an ingot puller similar to the ingot puller 100 shown in Figures 1 through 3. The exhaust tube included an upper section made of graphite and a lower section made of stainless steel. During ingot growth, the portion of the exhaust tube above the stainless steel lower section melted. The temperature profile of the ingot puller was modeled using conventional tools. The boundary between the melted portion of the stainless steel portion of the tube and the unmelted portion below it generally corresponded to the location of the 1330°C contour in the ingot puller temperature profile (i.e., the contour where actual melting occurred may be used as the maximum threshold temperature for the lower metal portion of the exhaust tube).

[0042] A second exhaust tube was used in the ingot puller that included an upper section made from graphite and a lower section made from stainless steel. The length of the lower stainless steel section of the second exhaust tube was shortened (and the length of the upper graphite section was correspondingly increased) so that the entire stainless steel section was located below the 1330°C contour of the modeled temperature profile of the ingot puller. The stainless steel section did not melt during ingot growth. The exhaust tube did not form excessive deposits on the upper graphite section and had minimal deposits on the lower stainless steel section.

[0043] As used herein, the terms "about," "substantially," "essentially," and "approximately," when used in connection with a range of a dimension, density, temperature, or other physical or chemical property or characteristic, are meant 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 that result from rounding, measurement methods, or other statistical variations.

[0044] When introducing elements of the present disclosure or embodiments of the present disclosure, 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 there may be additional elements other than the listed elements. The use of specific orientation terms (e.g., "top," "bottom," "side") is for convenience of description and does not require a specific orientation of the articles being described.

[0045] Because various changes may be made in the structure and methods described above 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 in an illustrative and not a limiting sense.

Claims

1. 1. An ingot pulling apparatus for growing single crystal silicon from a silicon melt, comprising: a housing defining a growth chamber, the housing having a bottom and a sidewall extending from the bottom; a crucible disposed within the growth chamber for holding the silicon melt; an exhaust tube extending from the growth chamber to the bottom of the housing, the exhaust tube defining an exhaust flow path for exhausting exhaust gases from the growth chamber; Equipped with The exhaust tube is an upper portion made from a first material; a lower portion joined to said upper portion and made from a second material different from said first material; An ingot pulling apparatus comprising:

2. 2. The ingot pulling apparatus of claim 1, wherein said first material is graphite.

3. 2. The ingot pulling apparatus of claim 1, wherein the second material is a metal.

4. 4. The ingot pulling apparatus of claim 3, wherein said second material is stainless steel.

5. 2. The ingot pulling apparatus of claim 1, wherein said first material is graphite and said second material is stainless steel.

6. 2. The ingot pulling apparatus of claim 1, wherein the first material has a melting temperature greater than a melting temperature of the second material.

7. the lower portion includes a collar that is received in the upper portion; 2. The ingot pulling apparatus of claim 1, wherein said collar is a friction fit within said upper portion.

8. the ingot pulling apparatus includes a plurality of exhaust tubes; Each exhaust tube is an upper portion made from the first material; a lower portion joined to the upper portion and made from the second material; 2. The ingot pulling apparatus of claim 1 , comprising:

9. 1. A method for selecting a length of a lower metallic portion of a composite exhaust tube of an ingot pulling apparatus, the composite exhaust tube comprising an upper portion made of a first material and a lower portion joined to the upper portion and made of a second material that is metallic, the second material being different from the first material, the composite exhaust tube extending upwardly from a bottom of a housing and through an insulating layer; The method comprises: determining a threshold temperature for the lower metal portion of the exhaust tube; determining a radial position of the exhaust tube; determining an axial temperature profile of the ingot pulling apparatus at the radial position of the exhaust tube during a melting phase of ingot growth, the axial temperature profile decreasing from an upper portion of the insulation layer toward a bottom of the housing; selecting a length of the lower metal portion of the exhaust tube such that an upper end of the lower metal portion is at an axial position where the temperature of the lower metal portion is lower than the threshold temperature during the melting phase of ingot growth; The method includes:

10. 10. The method of claim 9, wherein the axial temperature profile at the radial position of the exhaust tube is determined by measuring temperatures within the ingot pulling apparatus during the melting phase.

11. 10. The method of claim 9, wherein the axial temperature profile at the radial position of the exhaust tube is determined by modeling temperatures within the ingot pulling apparatus during the melting phase.

12. The method of claim 9 , wherein the threshold temperature is related to a melting temperature of the second material of the lower metallic portion of the exhaust tube.

13. The method of claim 9 , wherein the threshold temperature is a melting temperature of the second material of the lower metallic portion of the exhaust tube.

14. the threshold temperature is a first threshold temperature, The method of claim 9 , wherein a length of the lower metallic portion of the exhaust tube is selected such that a full length of the upper portion of the exhaust tube is above a second threshold temperature.

15. The method of claim 14 , wherein the second threshold temperature is a maximum temperature at which at least some deposits form on the upper portion of the exhaust tube.

16. The method of claim 15, wherein the deposit is at least silicon oxide, silicon, and silicon carbide.

17. 15. The method of claim 14, wherein the first material from which the upper portion is made is graphite and the second threshold temperature is 1200 degrees Celsius.

18. The method of claim 9 , wherein the second material from which the lower portion is made is stainless steel.

19. The method of claim 9 , wherein the first material from which the upper portion is made is graphite.

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