WO2017062949A1

WO2017062949A1 - System and method for degassing granular polysilicon

Info

Publication number: WO2017062949A1
Application number: PCT/US2016/056268
Authority: WO
Inventors: Steven Lawrence KIMBEL; Salvador Zepeda; Harold Wilfred Korb; Richard George SCHRENKER
Original assignee: Sunedison, Inc.
Priority date: 2015-10-10
Filing date: 2016-10-10
Publication date: 2017-04-13

Abstract

A system for degassing granular polysilicon includes a baking container having an interior space for containing granular polysilicon. The system also includes a heat source for heating the granular polysilicon within the interior space of the baking container. The system further includes a drive mechanism operatively connected to the baking container for mechanically agitating the granular polysilicon within the interior space during heating.

Description

SYSTEM AND METHOD FOR DEGASSING GRANULAR POLYS ILICON

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application Serial No. 62/239,865, filed 10 October 2015, the disclosure of which is hereby incorporated by reference in its entirety.

FIELD

[0002] The field of the disclosure relates generally to systems and methods for producing silicon and, more particularly, to systems and methods for degassing granular polysilicon.

BACKGROUND

[0003] Single-crystal silicon may be manufactured using a Czochralski (Cz) process. With the Cz process, a single-crystal silicon ingot is produced by melting polycrystalline silicon (polysilicon) in a

crucible, dipping a seed crystal into the molten silicon, withdrawing the seed crystal in a manner sufficient to achieve the diameter desired for the ingot, and growing the single crystal at that diameter.

[0004] Often granular polysilicon is melted to form the molten silicon used in the Cz process. Granular polysilicon can be produced using a variety of methods, one of which is a fluidized bed reaction method. With such a method, the granular polysilicon is typically subjected to trichlorosilane or silane growth mechanisms having a high gas component (e.g., hydrogen in the case of silane growth, and chlorine in the case of trichlorosilane growth) , and the gas typically becomes entrained (e.g., absorbed or trapped) in the polysilicon granules.

[0005] The gas entrained in the granular polysilicon can pose problems in the Cz process. Under the low pressure conditions typically employed during the Cz process, the entrained gas tends to release from the polysilicon granules, causing the polysilicon melt of the Cz process to splatter molten silicon onto components of the Cz apparatus. This can result in costly damage to the Cz apparatus by, for example, increasing the risk of exhaust line blockage, more rapid graphite deterioration by conversion to SiC, or breakage during cooldown caused by coefficient of thermal expansion mismatch.

[0006] Heating the granular polysilicon at high temperatures prior to the Cz process can effectively remove the gas entrained within the polysilicon granules, thereby reducing the amount of gas released from the granules during the Cz process. One common heating method is a stagnant bake method, and another common heating method is heating in a fluidized bed (FB) . Both of these methods have significant drawbacks, however.

[0007] The stagnant bake method is labor intensive, in that the granular polysilicon must be manually loaded onto and unloaded from baking containers that are used to bake the granular polysilicon in a furnace. Because the cost of labor is high for this method, the baking containers tend to be sized larger for baking larger quantities of granules at a time. Along with the increased quantity of granular polysilicon in each baking container comes an increase in the number of gas pockets in each baking container. Together, the increased quantity of granules and gas pockets in each baking container can somewhat insulate the granules located near the center of the container, which can result in the centrally located granules taking longer to reach their desired temperature. As a result, it is often the case that the baking containers need to be heated at higher temperatures or for longer periods of time in order to ensure proper degassing of the centrally located granules. However, higher temperatures or longer heating cycles can cause increased sintering of the granules located near the sides of the container. As a result, it is common for either the granules near the sides of the container to be become sintered, or for the granules located near the center of the container to be under-degassed, both of which are undesirable.

[0008] The FB method, on the other hand, involves less direct labor and tends to result in less sintering, due to continuous gas -borne particle movement. However, it is difficult to obtain uniform degassing with known FB methods. Typical FB methods utilize a carrier gas to fluidize the reactor bed to bake granular polysilicon at temperatures from 1000-1200C. In order to maintain a semi- continuous FB operation, however, a portion of the heated granules (e.g., about 10% of the granules) within the reactor needs to be periodically removed from the fluidized bed and replenished with an equal portion of unheated granules. However, this process of fractional addition and subtraction of granules from the reactor often results in the premature removal of under-degassed granules, given the difficulty of distinguishing the under-degassed granules from the completely degassed granules in a fully mixed FB. Thus it is hard to ensure that all of the granules in the reactor have been uniformly degassed using known FB methods. Additionally, purchasing and maintaining the FB reactor, in addition to supplying the necessary raw materials (e.g., the precursor gas) needed to perform the FB process, can be quite expensive.

[0009] It would be useful, therefore, to provide systems and methods for degassing granular

polysilicon in a manner that facilitates more uniform heating and less sintering of the granules, while also promoting a continuous, automated process with less labor, machinery, and raw material costs.

[0010] This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure.

Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art .

BRIEF SUMMARY

[0011] In one aspect, a system for degassing granular polysilicon includes a baking container having an interior space for containing granular polysilicon. The system also includes a heat source for heating the granular polysilicon within the interior space of the baking container. The system further includes a drive mechanism connected to the baking container for mechanically

agitating the granular polysilicon within the interior space during heating.

[0012] In another aspect, a method for degassing granular polysilicon includes heating granular polysilicon within a baking container, and mechanically agitating the granular polysilicon as the granular

polysilicon is heated.

[0013] In yet another aspect, a method for producing single-crystal silicon includes making gas- entrained granular polysilicon. The method also includes degassing the gas-entrained granular polysilicon within a baking container, and mechanically agitating the gas- entrained granular polysilicon in the baking container during degassing. The method further includes growing a single -crystal ingot of silicon from the degassed granular polysilicon using a Czochralski process.

[0014] Various refinements exist of the features noted in relation to the above-mentioned aspects. Further features may also be incorporated in the above- mentioned aspects as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to any of the illustrated embodiments may be incorporated into any of the above-described aspects, alone or in any combination . BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 is a schematic of a combined granular polysilicon and single-crystal silicon production system;

[0016] Figure 2 is a schematic of a subsystem of the system shown in Figure 1;

[0017] Figure 3 is a schematic perspective of a baking container of the subsystem shown in Figure 2;

[0018] Figure 4 is a schematic cross - section of the baking container shown in Figure 3 during operation of the subsystem shown in Figure 2;

[0019] Figure 5 is a schematic cross - section of the baking container shown in Figure 4 taken along plane 5-5 of Figure 4; and

[0020] Figure 6 is a schematic perspective of another embodiment of a baking container for use in the subsystem shown in Figure 2.

[0021] Corresponding reference characters indicate corresponding parts throughout the several views of the drawings .

DETAILED DESCRIPTION

[0022] Referring to Figure 1, a combined granular polysilicon and single-crystal silicon production system is shown schematically and is indicated generally at 100. The system 100 is configured to produce single- crystal ingots of silicon, and the system 100 includes: a first subsystem 102 configured for making a gas -entrained granular polysilicon product 104; a second subsystem 106 configured for converting the gas -entrained granular polysilicon product 104 of the first subsystem 102 into a degassed granular polysilicon product 108; and a third subsystem 110 configured for melting the degassed granular polysilicon product 108 and growing single-crystal ingots of silicon via suitable Czochralski (Cz) or continuous Czochralski (CCz) processing of the degassed granular polysilicon product 108 of the second subsystem 106. As used herein, the term "degas" and any variation thereof refers to the removal of a substantial amount of gas entrained in non-gaseous matter such as, for example, granular polysilicon. In other embodiments, the system 100 may have any suitable number of subsystems arranged in any suitable manner that facilitates enabling the second subsystem 106 to function as described herein.

[0023] In the illustrated embodiment, at least one of the subsystems 102, 106, 110 of the system 100 may operate in an automated, continuous manner (e.g., any one or more of the subsystems 102, 106, 110 may be configured to perform its respective functions over an extended duration without stoppage and substantially without manual labor) . Similarly, the illustrated subsystems 102, 106, 110 of system 100 may interact with one another in an automated, continuous manner (e.g., the first subsystem 102 may be configured to supply the gas -entrained granular polysilicon product 104 to the second subsystem 106 over an extended duration without stoppage and substantially without manual labor; and/or the second subsystem 106 may be configured to supply the degassed granular polysilicon product 108 to the third subsystem 110 over an extended duration without stoppage and substantially without manual labor) . In other embodiments, however, any one or more of the subsystems 102, 106, 110, and/or their respective interactions with one another, may not be automated or may not be continuous.

[0024] The first subsystem 102 of the illustrated embodiment makes the gas-entrained granular polysilicon product 104 using a suitable trichlorosilane or a silane growth process. The granules of the product 104 are substantially spherical, bead-like particles that are free-flowing in their aggregate, and the entrained gas is either hydrogen or chlorine (e.g., the granules of the product 104 may contain anywhere from about 100 to about 1000 ppma (about 3.6 to about 35.9 ppm by weight) of hydrogen) .

[0025] If the gas -entrained granular polysilicon product 104 was to be delivered directly to the third subsystem 110, without intermediate processing in the second subsystem 106, the product 104 would ultimately be degassed (e.g., dehydrogenated) upon its melting in the third subsystem 110. Such degassing in the melt of the third subsystem 110 would result in the splattering of melted polysilicon onto components of the third subsystem 110. Therefore, it is desirable to degas the product 104 in advance of the third subsystem 110 in an effort to minimize such splattering and, hence, much of the component damage that could occur overtime as a result. In this manner, the primary purpose of the second subsystem 106 is to degas the gas-entrained granular polysilicon product 104 in advance of the third subsystem 110. [0026] Figure 2 is a schematic illustration of the second subsystem 106. In the illustrated embodiment, the second subsystem 106 includes a first pressure or vacuum load-lock vessel (e.g., a first hopper 112), a displaceable baking container (e.g., a rotary baking container 114) in granular flow communication with the first hopper 112, a second pressure or vacuum load- lock vessel (e.g., a second hopper 116) in granular flow communication with the baking container 114, and a drive mechanism (e.g., an electric motor 118) operably connected to the baking container 114 to facilitate agitating polysilicon granules disposed within the baking container 114, as set forth in more detail below.

[0027] Notably, the baking container 114 of the illustrated embodiment is situated at least in part within or adjacent a suitable heat source 120 (e.g., a 200mm tube furnace from Centrotherm Photovoltaics USA Inc.) for heating the baking container 114 in the manner set forth below. Alternatively, rather than having the heat source 120 to heat the baking container 114, the baking container 114 may be surrounded by, or carry, any suitable heating element (s) that facilitate enabling the baking container 114 to be heated as set forth herein. Suitable cooling systems may be provided for the components of the heat source 120 and/or the baking container 114 (e.g., a liquid cooling system) . Moreover, the second subsystem 106 may have any suitable drive mechanism that facilitates displacing the baking container 114 in any suitable manner (e.g., the baking container 114 may not be a rotary baking container as described herein, but instead may be a baking container having at least one component that is

displaceable in a vibrating manner or translating manner) .

[0028] The illustrated first hopper 112 is configured to receive gas -entrained granular polysilicon product 104 made by the first subsystem 102, and to automatically and continuously feed the gas-entrained granular polysilicon product 104 to the baking container 114. Similarly, the second hopper 116 is configured to receive the degassed granular polysilicon product 108 from the baking container 114 for subsequent supplying of the degassed granular polysilicon product 108 to the third subsystem 110.

[0029] In this manner, the first and second hoppers 112, 116, with their pressure or vacuum load-lock functionality, facilitate maintaining a low pressure environment within the baking container 114 and/or the heat source 120 while the gas -entrained granular polysilicon product 104 is fed into the baking container 114, and while the degassed granular polysilicon product 108 is discharged from the baking container 114. Other contemplated

embodiments of the second subsystem 106 may have any suitable type(s) of pressure or vacuum load-lock vessels in any suitable quantity, and arranged in any suitable manner in granular flow communication with any suitable number of baking containers 114, that facilitate enabling the second subsystem 106 to function as described herein.

[0030] In the illustrated embodiment, a controlled flow of inert purge gas is channeled through the heat source 120 and/or the baking container 114 to

facilitate removing tramp gases (e.g., granule-released gases such as hydrogen) and/or process gasses (e.g., gases released from surfaces of the heat source 120 and/or the baking container 114) and maintaining a desired pressure within the heat source 120 and/or the baking container 114. For example, in some embodiments, a vacuum pump may be communicatively coupled to the heat source 120 and/or the baking container 114 via a control valve, and an inert purge gas source (e.g., a source argon) may be

communicatively coupled to the heat source 120 and/or the baking container 114 via a flow control device (e.g., a mass flow controller) such that inert purge gas flows through the heat source 120 and/or the baking container 114 in a direction opposite the direction in which granular polysilicon flows through the heat source 120 and/or the baking container 114, as set forth in more detail below. In other embodiments, if the degassing operation is to be conducted at a pressure above atmospheric pressure within the heat source 120 and/or the baking container 114, no vacuum pump would be utilized.

[0031] Figures 3-5 illustrate one embodiment of the baking container 114 of the second subsystem 106. In the illustrated embodiment, the baking container 114 includes a hollow tube or shell 122 that defines a

generally cylindrical interior space 124 with a granule receiving end 126 and a granule discharge end 128. The receiving end 126 has at least one opening (indicated generally by 130) for receiving the gas -entrained granular polysilicon product 104 into the interior space 124.

Similarly, the discharge end 128 has at least one opening 132 for discharging the degassed granular polysilicon product 108 from the interior space 124. In some embodiments, to facilitate removing tramp gases and/or process gases from the baking container 114 in the manner set forth above, the shell 122 may further include at least one port 134 in which a suitable pneumatic valve is positioned to facilitate maintaining a pressurized (e.g., low pressure) environment within the interior space 124 while providing a continuous flow of inert purge gas 136 (e.g., argon) through the interior space 124. This minimizes gaseous contaminants within the interior space 124.

[0032] The example baking container 114 is a rotary baking container in that the baking container 114 includes a rotatable agitator 138 disposed within the interior space 124 of the shell 122. The agitator 138 is operatively connected to the electric motor 118 for rotation within the shell 122 about a rotation axis 140. The agitator 138 of the illustrated embodiment is oriented obliquely (i.e., at an incline) relative to the direction 142 of the gravitational force acting on the baking container 114. In other suitable embodiments, however, the agitator 138 may be oriented level or substantially level (or perpendicular to the direction 142 of the gravitational force acting on the baking container 114) . Alternatively, the various components of the baking container 114 may be oriented in any suitable manner that facilitates enabling the baking container 114 to function as described herein.

[0033] The illustrated agitator 138 has a shaft 144 and a plurality of baffles 146 emanating from the shaft 144. The shaft 144 is oriented substantially coaxial with the rotation axis 140 such that the shaft 144 and, therefore, the baffles 146 are rotatable about the rotation axis 140. Each of the baffles 146 is generally disc-shaped and is sized to extend from the shaft 144 so as to nearly make contact with the shell 122, thereby spanning nearly the entire interior space 124 and segmenting the interior space 124 into a plurality of axially sequential zones (indicated generally by 148) . Alternatively, the agitator 138 may be in the form of an auger, having a single, substantially helical baffle that wraps around at least a segment of the length of the shaft 144.

[0034] In the illustrated embodiment, each of the baffles 146 is oriented obliquely relative to the rotation axis 140, and each of the baffles 146 has an aperture 150 formed therein, with the apertures 150 of the various baffles 146 being angularly offset relative to one another in terms of their positioning about the rotation axis 140. The apertures 150 of the baffles 146 enable flow communication between adjacent zones 148 across the respective baffles 146 as the apertures 150 rotate (e.g., as the apertures 150 of the baffles 146 periodically submerge beneath a level of granular polysilicon if the baking container 114 is only partially filled with granular polysilicon) . It is contemplated that the baffles 146 may have any suitable shape and orientation, and may have any suitable number of apertures 150 that are arranged in any suitable manner relative to one another, that facilitate enabling the agitator 138 to function as described herein.

[0035] Suitably, the shell 122 and/or the agitator 138, or the granule contacting surfaces thereof, may be fabricated from quartz, a machined silicon material, a silicon-carbide material, a silicon-coated graphite material, or other suitable material, and may optionally be coated in silicon, to minimize contamination of the interior space 124 during operation of the second subsystem 106. In some embodiments, because the shell 122 may be more susceptible to heat-related deformation if fabricated from quartz, a separate support shell (e.g., a support shell fabricated from a composite material such as a graphite material or a silicon graphite material) may be provided to mechanically support the shell 122 by at least partially enveloping the shell 122 and rotating together with the shell 122 to facilitate preventing the shell 122 from deforming during a degassing operation.

[0036] To degas the gas-entrained granular polysilicon product 104 using the second subsystem 106, the baking container 114 is heated by the heat source 120 such that a desired thermal field is present within the interior space 124 (e.g., by heating the baking container 114 to about 2100°F or about 1150°C) . The gas -entrained granular polysilicon product 104 is then delivered to the interior space 124 of the shell 122 from the first subsystem 102 via the first hopper 112 at a predetermined rate. The electric motor 118 is operated to rotate the agitator 138 about the rotation axis 140 such that the baffles 146 agitate the granules within the interior space 124, slowly pushing the granules from the receiving end 126 to the discharge end 128 sequentially across the zones 148 via the apertures 150 of the baffles 146. The granules thereby move across the interior space 124 in a cascade- like manner for subsequent discharge into the second hopper 116 via the opening (s) 132 for subsequent delivery to the third substation 110.

[0037] As shown in Fig. 5, when the granules 152 are agitated by the baffles 146, the granules 152 are kept in motion within the interior space 124. More specifically, while the granules 152 continuously move in the direction 154 of rotation within, and across, the respective zones 148, each of the granules 152 has a slightly different displacement vector 156. In this manner, the granules 152 flow freely within the interior space 124. This free flow prevents the granules 152 from sintering or otherwise bonding together. Moreover, this free flowing motion of the granules 152 within the interior space 124 promotes uniform heating and, therefore, uniform degassing of all of the granules 152.

[0038] Furthermore, the parameters of the process by which the granules 152 are heated and agitated within the interior space 124 can be selected to optimize degassing. More specifically, the temperature to which the interior space 124 is heated by the heat source 120, the time/rate at which the product 104 is input into the interior space 124, and the time/speed at which the agitator 138 rotates, can be fixed or modulated to

facilitate heating the granules 152 to a desired

temperature while promoting uniform degassing of the granules 152 and inhibiting the granules 152 from

sintering. In one particular embodiment, for example, the granules 152 may be agitated throughout the entire duration of their heating within the interior space 124.

[0039] In the illustrated embodiment, the agitation of the granules 152 within the interior space 124 is directly caused by movement of the agitator 138 and therefore the granules 152 are mechanically agitated by the agitator 138. To agitate the granules 152 in other embodiments, the shell 122 may be displaced by the drive mechanism in lieu of, or in conjunction with, the agitator 138 being displaced by the drive mechanism (e.g., the drive mechanism may have any suitable number of electric motors 118 arranged in any suitable manner for displacing any suitable component (s) of the baking container 114) .

Alternatively, the granules 152 may be mechanically agitated within the interior space 124 using any suitable agitation mechanism such as, for example, a mechanically vibrated bed (or surface) underneath the granules 152.

[0040] Figure 6 illustrates another embodiment of a rotary baking container (indicated generally by 200) for use in the second subsystem 106. The baking container 200 is in the form of a hollow, generally helically extending tube or shell 206 having a receiving end 202 and a discharge end 204. The baking container 200 may be operatively connected to the electric motor 118 and suitably disposed within or adjacent the heat source 120. The baking container 200 may also be connected in granular flow communication with the first hopper 112 and the second hopper 116 via the receiving end 202 and the discharge end 204, respectively.

[0041] Unlike the baking container 114 above, the baking container 200 does not have an internal

agitation mechanism such as the agitator 138. Rather, the shell 206 serves as an agitation mechanism in this

embodiment, in that rotation of the shell 206 causes mechanical agitation of the granules inside of the shell 206 and, hence, free-flowing movement of the granules from the receiving end 202 to the discharge end 204 in a manner that inhibits sintering and facilitates uniform degassing. Other than the construction of the baking container 200 being different than the construction of the baking container 114, the baking container 200 is otherwise configured in the system 100 and subsystem 106 in the same manner as the baking container 114.

[0042] When introducing elements of the present invention or the embodiment (s ) 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" and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements.

[0043] As various changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

WHAT IS CLAIMED IS:

1. A system for degassing granular polysilicon, the system comprising:

a baking container having an interior space for containing granular polysilicon;

a heat source for heating the granular polysilicon within the interior space of the baking container; and

a drive mechanism operatively connected to the baking container for mechanically agitating the granular polysilicon within the interior space during heating;

wherein the baking container includes a receiving aperture and a discharge aperture, the drive mechanism operatively connected to the baking container to

mechanically agitate the granular polysilicon such that the granular polysilicon flows from the receiving aperture to the discharge aperture within the interior space.

2. The system of claim 1, further comprising a first hopper connected in granular flow communication with the receiving aperture.

3. The system of claim 2, further comprising a second hopper connected in granular flow communication with the discharge aperture.

4. The system of claim 3, wherein the first hopper and the second hopper are configured as load- lock vessels that facilitate maintaining a controlled atmosphere within the interior space during heating.

5. The system of claim 1, wherein the baking container comprises a shell operatively connected to the drive mechanism for displacing the shell via the drive mechanism to mechanically agitate the granular polysilicon.

6. The system of claim 5, wherein the shell is generally helical and operatively connected to the drive mechanism for displacement by rotating the shell.

7. The system of claim 1, wherein the baking container comprises a shell and an agitator disposed within the shell, the agitator operatively connected to the drive mechanism for displacing the agitator via the drive mechanism to mechanically agitate the granular polysilicon.

8. The system of claim 7, wherein the agitator is operatively connected to the drive mechanism for displacement by rotating the agitator.

9. The system of claim 8, wherein the agitator has a shaft and a substantially helical baffle around the shaft .

10. The system of claim 9, wherein the agitator has a shaft and a plurality of baffles emanating from the shaft .

11. The system of claim 10, wherein the agitator is rotatable about a rotation axis along which the shaft extends .

12. The system of claim 11, wherein the baffles are oriented obliquely relative to the rotation axis.

13. The system of claim 10, wherein the interior space is generally cylindrical and wherein the baffles extend from the shaft such that the interior space is segmented by the baffles into a plurality of zones.

14. The system of claim 13 , wherein each of the baffles comprises an aperture providing granular flow communication between adjacent zones.

15. The system of claim 14, wherein the apertures are angularly offset relative to one another about the rotation axis.

16. A method for degassing granular polysilicon, the method comprising:

heating granular polysilicon within a baking container; and

mechanically agitating the granular polysilicon as the granular polysilicon is heated to degas the granular polysilicon .

17. The method of claim 16, wherein the granular polysilicon is mechanically agitated by displacing a shell of the baking container.

18. The method of claim 16, wherein the granular polysilicon is mechanically agitated by moving an agitator disposed within an interior space of the baking container.

19. The method of claim 16, wherein the granular polysilicon is mechanically agitated throughout the entire duration of its heating.

20. A method for producing single-crystal silicon, said method comprising:

making gas -entrained granular polysilicon;

degassing the gas-entrained granular polysilicon within a baking container;

mechanically agitating the gas-entrained granular polysilicon in the baking container during degassing; and growing a single-crystal ingot of silicon from the degassed granular polysilicon using a Czochralski process .