METHOD FOR PREVENTING CONTAMINATION OF A BASE PLATE

Abstract

Method for preventing contamination of a base plate with: after production of polycrystalline silicon in a reactor with the base plate and a lid covering the base plate, removing the lid from the base plate; and Isolating a space comprising the base plate by an insulating device, after removing the cover, wherein the insulating device comprises a filter unit configured to blow filtered air through a filter onto the base plate, and Blowing filtered air through the filter unit onto the base plate.

Description

BACKGROUNDTechnical field

[0001] The present invention relates to a method for preventing contamination of a base plate for the production of polycrystalline silicon.

[0002] This application claims priority from Japanese patent application JP 2021 - 143 083 A, filed on March 10, 2020, the contents of which are hereby incorporated by reference in their entirety. State of the art

[0003] Polycrystalline silicon is a raw material for single-crystal silicon used in semiconductor manufacturing and silicon used in solar cell manufacturing. The Siemens process is known as a process for producing polycrystalline silicon. In this process, a silane-based raw material gas is brought into contact with a heated silicon core wire to deposit polycrystalline silicon on the surface of the silicon core wire by chemical vapor deposition (CVD).

[0004] The reactor used in the Siemens process generally consists of a bell-shaped cover called a “bell cover” and a bottom part called a base plate, which is equipped with an electrode, an inlet port for raw material gas, an air outlet port, etc., which are connected to each other by a flange.

[0005] The electrode passes through the base plate with an insulator between them and is connected to another electrode by wiring or to a power supply outside the reactor. The electrode, base plate, and bell jar lid are cooled using a coolant such as water to prevent polycrystalline silicon from settling during vapor-phase growth or to prevent metal temperatures from rising and leading to heavy metal contamination in the polycrystalline silicon.

[0006] The silicon core wire, which is fixed to the electrode or core wire holder, is heated by Joule heat. A raw material gas, such as a mixture of trichlorosilane and hydrogen, is sprayed onto the silicon wire from a gas nozzle to vapor deposition of high-purity silicon on the silicon core wire, forming a silicon rod.

[0007] After the silicon rod growth is complete, the silicon rod is sufficiently cooled. After the reactor is filled with a harmless gas, the flange bolts on the bell-cap and base plate are removed. The bell-cap is lifted by a crane and transported to a bell-cap washing facility for washing with high-pressure wash water or similar. The base plate is cleaned of any silicon rod fragments that have fallen onto the base plate after the silicon rod has been harvested. Harvesting the silicon rod and cleaning the base plates are performed manually by the operator.

[0008] After cleaning the base plate, a new core wire and core wire holder are placed on the electrode on the base plate. The cleaned bell cover is transported again by crane and connected to the base plate with its flange.

[0009] In recent years, the demand for reducing impurities in polysilicon rods has increased significantly. In particular, contamination of the inner surface (bulk) of polysilicon rods, which is not washed away and removed in subsequent processes, significantly impacts product quality, unlike surface contamination.

[0010] Bulk impurities can be attributed to factors such as impurities in the raw material gas itself, reactor material, and surface contamination on the reactor's inner surface. It is known that surface stains on the reactor's inner surface are influenced by the ambient atmosphere while the reactor is open and by the cleanliness of the reactor's inner surface immediately before the reactor is closed.

[0011] Therefore, the base plate is cleaned, for example, as described in JP 6 395 924 B2. Furthermore, efforts are made to keep the entire space surrounding the reactor clean, as described in JP 2016 - 536 249 A and JP 2016 - 521 239 A.

[0012] JP 2018 - 100 805 A addresses the problem of how to provide a simple cleaning booth that is not a large cleaning booth, is easy to assemble on site, provides a quiet and consistently clean space, and is economically excellent. A frame with support columns and beams is proposed as a solution. The frame includes a booth body with a ceiling 5c, side walls 5a, 5b, and a gate; an air blower 4 installed above an upper part of a side wall 5b of the booth body; and a sock filter 6 connected to the air blower 4 and suspended near the ceiling in the booth body.Air sucked in by the air blower 4 is purified by the sock filter 6, then the purified air flows downward from the sock filter 6 toward a bottom surface of the cabin body, and a space is provided between the side walls 5a, 5b and the bottom surface for discharging the purified air to the outside of the cabin body.

[0013] JP 2016 - 033 437 A addresses the problem of how to achieve a cleanroom booth that circulates air supplied by an air purification device with a simple structure. As a solution, a cleanroom booth is proposed that comprises: an air purification device; a housing body forming a space above a floor surface (G) into which air is supplied from the air purification device; and an air passage from the space to the air purification device. The housing body comprises a top plate (R) and a side wall connected to the top plate. Part or all of the side wall has a double structure consisting of an outer wall and an inner wall. An internal space between the outer wall and the inner wall forms part of the passage. SUMMARY

[0014] However, cleaning the base plate according to JP 6395924 B2 is not sufficient. The base plate has a complex design with electrodes, a raw material gas supply port, an exit gas port, and the like, and can suffer countless small scratches due to the silicon rod collapsing or partially falling off. It is very difficult to remove all contamination from the contaminated base plate and return the base plate to a clean state.

[0015] Additionally, excessive cleaning can cause new scratches on the base plate, trapping dirt. Furthermore, the cleaning tool itself can be lost and remain on the base plate, causing contamination.

[0016] According to JP 2016 - 536 249 A, contamination can be reduced by keeping the entire room containing the reactor clean in a cleanroom environment. However, this must be done from the very beginning of the room design. Furthermore, it is economically difficult to create and maintain a cleanroom in which a large number of Siemens reactors can be installed.

[0017] Even if the entire room is a cleanroom environment, contamination sources within the room include particles generated by the work of operators, metal powder, and oil generated by the use of cranes and counterweight devices used to move the bell jar lid, and the like. One problem is that such newly generated contamination cannot be removed immediately.

[0018] According to JP 2016 - 521 239 A, the room is cleaned at regular intervals, but there is the problem that the cleaning effect cannot be achieved immediately against newly emerging contamination, as described above.

[0019] The invention therefore provides a device for preventing contamination of the base plate used for the production of polycrystalline silicon in a cost-effective manner.

[0020] The invention is defined in the claims.

[0021] In the invention, by separating the space comprising the base plate by the insulating device, it is possible to prevent the contamination of the base plate used to produce polycrystalline silicon. BRIEF DESCRIPTION OF THE DRAWINGS Fig.Fig. 1 is a diagram showing a mode in which a base plate is protected in an insulated manner by using an insulating device according to an embodiment of the invention, diagrammatically showing a side cross-section of a bell shell and a side cross-section of the insulating device: Fig. Fig. 2 is a side sectional view showing a state in which the insulating device according to the embodiment of the invention has a filter unit; Fig. Fig. 3 is a top plan view of a state in which the insulating device according to the embodiment of the invention has a filter unit; and Fig. 4 shows a flowchart for explaining a mode in which an isolation device is used. DESCRIPTION IN DETAIL

[0022] The reaction has a base plate 8 and a bell bowl 4, which represents a lid that is connected to the base plate 8. Fig. 1 shows a mode in which the bell shell 4 is separated from the base plate 8, but in operation the bell shell 4 is connected to the base plate via a flange or the like.

[0023] The base plate 8 is provided with a raw material gas supply nozzle 9 for introducing a raw material gas, an electrode 10, a core wire holder 11 on the electrode 10, and a silicon core wire 12 arranged on the core wire holder 11. To produce polycrystalline silicon, polycrystalline silicon is deposited on the surface of the silicon core wire 12, for example, by chemical vapor deposition (CVD).

[0024] An insulation device 30 may include a partition 31 extending vertically. Furthermore, a cover member 32 is disposed on the upper surface of the partition 31. The partition 31 may be supported by a support column 33 extending vertically and a support column 33 extending horizontally. The partition 31 may be made of vinyl or the like.

[0025] The bell jar 4, which has been removed from the base plate 8, can be moved by a crane 1 via a counterweight 2 and hook 3. The base plate 8 can be opened and closed by moving it vertically or horizontally. The bell jar 4 can be moved from the base plate 8 to a bell jar washing device, e.g., a bell jar washstand. When the bell jar is moved, contaminated air A1, which contains lubricating oil used in the components and fine-particle metal powder generated by the interaction of the components, flows onto the base surface. In the present embodiment, in order to prevent the contaminated air A1 from reaching the base plate 8, an insulating device 30 capable of insulating the space is arranged to cover the entire base plate 8. This makes it possible for the base plate to be insulated and protected.As a result, contamination of the surface of the base plate 8 can be prevented.

[0026] In this embodiment, the bell shell 4 can be moved to the extent that a space is created for installing the insulation device 30, and at this time, the movement of the bell shell 4 is stopped. Then, after the base plate 8 has been covered with the insulation device 30, the movement of the bell shell 4 can be restarted. In this mode, it is advantageous that the combination of the surface of the base plate 8 due to the movement of the bell shell 4 can be prevented as much as possible.

[0027] As in Fig. 2, the insulation direction 30 may have fan filter units (FFUs) 36 on its cover part 32. When this mode is applied, purified air A2 can be blown directly onto the base plate 8. Thus, when an operator 90 (see Fig.1) By performing work such as placing the silicon core wire 12 in the insulated and protected space, it is possible to prevent the air 15 including the powder dust generated by the work from reaching the base plate 8, so that the base plate 8 is kept clean.

[0028] HEPA filters are preferred for FFUs 36. It is even more advantageous to use UPLA filters. For FFUs 36, it is even more advantageous to use UPLA filters with low organic matter / low boron.

[0029] The isolation device 30 does not necessarily have to be used with the base plate 8 for the entire time the reactor is open (where the bell-shaped shell 4 is removed from the base plate 8). For example, the isolation device 30 can be used only for the time when the generation of dust, such as metal powder, is expected during movement of the bell-shaped shell or during cleaning of the room.

[0030] The insulation device 30 can be assembled on-site or carried by a crane or the like. From the point of view of avoiding the generation of powder dust, it is advantageous for the insulation device 30 to be provided with movable parts 7, such as rollers.

[0031] When polycrystalline silicon is grown in a reactor using the Siemens process, hydrochloric acid is produced as a byproduct. Therefore, the insulation device 30 is used in an acidic atmosphere in this case. For this reason, it is desirable to protect the metal parts, such as the exterior of the FFUs 36 and the clean room frame, with tape wrapping to prevent them from rusting / corroding due to the acidic atmosphere.

[0032] It is also advisable to use a plastic cover on the metal parts, such as the exterior of the FFUs 36 and the frame of the cleanroom enclosure. It is advantageous to reduce outgassing of the coating plastic, as this has the effect of further reducing contamination of the base plate 8.

[0033] The cause of metal contamination, particularly zinc contamination, was previously unknown. This invention is based on careful investigations into the sources of contamination, which led to the discovery that the contamination is primarily due to zinc plating on the interior surface of the air conditioner filter chamber, as well as to the flaking of the air duct due to corrosion and the penetration of the zinc plating into the chamber.

[0034] In this way, it was found that working on the base plate 8, and in particular cleaning the base plate 8 and installing a silicon core wire for growing polycrystalline silicon according to the Siemens method, as well as isolating the working space by the insulating device 30 for a short time, can be effectively used to avoid metal contamination.

[0035] In addition, the operator 90 working in the area isolated by the isolation device 30 wears a safety helmet and mask, eyeglasses, nitrile gloves, and dustproof clothing to clean the base plate 8 and install a new core wire.

[0036] During and after removal of the polycrystalline silicon, fragments of polycrystalline silicon are removed using Kepler gloves, an apron, a protective shield, and the like to prevent the fragments from causing cuts.

[0037] At this time, it is desirable to direct a downward flow of air without turbulence to the outside of the system through the FFUs 36 in the isolation device 30, so that contaminated Na and Ca caused by the operator 90 do not adhere to the base plate 8 (see Fig.2). To create the downward airflow over the FFUs 36, it is advantageous to provide openings in the bottom of the partition wall 31 of the isolation device 30 so that the downward airflow is directed to the outside of the system without disrupting its airflow within the isolation device 30.

[0038] The openings are preferably evenly distributed across the entire partition wall 31. Considering the amount of air blown into the insulation device 30 by the FFUs 36, the higher the ventilation frequency of the room in a given period of time, the greater the amount of air blown into the insulation device 30 by the FFUs 36. In order not to disrupt the air flow, it is desirable to adjust the width of the openings as required. The width of the openings can be changed by altering the height of the insulation device 30. It can also be changed by adjusting the vertical length of the partition wall 31.

[0039] Furthermore, as in Fig.3, it should be noted that if the shape of the insulation device 30 is square, as seen from above, then there is a difference in the air volume at the lower part of the partition wall 31. Therefore, it is also preferable to make the openings in the center of the partition wall 31 larger than the openings at the ends (corners) of the partition wall 31 in the plan view from above, so that the air flow is not disturbed. Fig. 2, the air including powder dust is designated by the reference symbol A3.

[0040] In the isolation device 30, the downward flowing air from the FFUs 36 may be disturbed by the operator 90 (see Fig.1). If the airflow is disturbed in this way, it may become unlikely that the airflow from the insulating device 30 will reach the outside of the system. In this case, Na and Ca may remain in the insulating device 30, which is not recommended because the amount of Na and Ca adhering to the base plate 8 and the installed silicon core wire increases. Therefore, it is preferable to direct the downward airflow from the FFUs 36 to the outside of the insulating device 30.

[0041] More specifically, it is conceivable to choose a method in which the air flow is attenuated from the center of the insulating device 30 to the outside. As in Fig.2, the air volume from the central FFUs 36 can be increased and the air flow from the peripheral FFUs 36 can be reduced, allowing the air flow within the insulation device 30 to the outside of the system without disruption of the air flow. Looking at the operation according to Fig. 3, one can ensure that the air flow from the FFU 36 located in the center is greater than the air volume from the eight FFUs 36 surrounding the central FFU 36.

[0042] Instead of or in addition to this procedure, an oblique air flow can be generated, which leads from the center of the insulating device 30 to the outside (see arrow A2 in Fig. 2) so that the air flow that hit the operator 90 can be forced out of the system.

[0043] This is preferable because the higher the speed of the total airflow, the faster the contamination is discharged to the outside of the system. The ventilation frequency in the room is preferably 30 times per hour or more because the effect of reducing contamination is remarkably large. The ventilation frequency in the room is even more preferably 90 times per hour. In the present embodiment, the ventilation frequency is denoted as nx / h, which means that air equal to nx the volume of the space in the isolation device 30 is blown out in one hour. If the ventilation frequency in the room is 30 times per hour or more, this means that an air volume 30 times greater than the capacity of the space in the isolation device 30 is blown out per hour.If the frequency of ventilation in the room in question is 90 times per hour or more, this means that a volume of air 90 times greater than the capacity of the room is blown out in the isolation device 30 per hour.

[0044] When the insulation device 30 is viewed from above, the air volume within ½ the area of ​​a circle in the center (see reference numeral 80 in Fig. 3) divided by the partition 31 preferably 1.5 times the air volume outside the circle, more preferably 1.8 times, and even more preferably 2.0 times.

[0045] Next, an example of an operation in which the insulating device 30 is incorporated will be shown with reference to Fig. 4 is described.

[0046] Certain polycrystalline silicon (the current polycrystalline silicon) is produced according to the Siemens method in a reactor having a base plate 8 and a bell-shaped shell 4, which consists of a lid constituting the base plate 8 (polycrystalline silicon production step S1).

[0047] After completion of the polycrystalline silicon in question, the bell shell 4 is removed from the base plate 8 (separation step S2).

[0048] Next, the produced polycrystalline silicon in question is extracted (extraction step S3).

[0049] After the polycrystalline silicon has been recovered, the space enclosing the base plate 8, from which the bell-shaped shell 4 has been removed, is separated by the insulation device 30 (insulation step S49). The insulation device 30 can be moved by moving the movable parts 7, including rollers or the like. Once the space enclosing the base plate 8 has been isolated by the insulation device 30, the produced polycrystalline silicon is recovered and the base plate 8 is cleaned.

[0050] The insulation by the insulation device 30 is performed after the bell-shaped shell 4 is removed from the base plate 8 after the completion of the polycrystalline silicon production and the polycrystalline silicon extraction is completed. Then, the time from the insulation by the insulation device 30 until the removal of the insulation device 30 is set as the insulation time for the next manufacturing step.

[0051] If the time from removing the bell-shaped shell 4 from the base plate 8 after the production of the polycrystalline silicon to reinstalling the bell-shaped shell 4 on the base plate 8 to start the production of the next polycrystalline silicon is set as the dissolution time, the insulation time may be 70% or more of the dissolution time. To shorten the exposure time to the outside air, the insulation time may be 80% or more of the dissolution time.

[0052] After the bell-shaped shell 4 has been removed from the base plate 8, the polycrystalline silicon is extracted immediately. The time required for extraction is, for example, between 0.5 and 1.5 hours, inclusive of the aforementioned values. The time required after extracting the polycrystalline silicon to insulate the base plate 8 with the insulation device 30 is, for example, 5 to 10 minutes. The time from removing the bell-shaped shell 4 from the base plate 8 to reinstalling the bell-shaped shell 4 on the base plate 8 to start production of the next polycrystalline silicon is, for example, 3 to 12 hours, inclusive.

[0053] While the space including the base plate 8 is insulated by the insulation device 30, the filter units attached to the insulation device 30 supply a gas with a volume 30 times greater than the capacity of the insulation device 30 per hour (possibly 30 times per hour or more). Preferably, while the space including the base plate 8 is insulated by the insulation device 30, the filter units attached to the insulation device 30 can supply a gas with a volume 90 times or more of the capacity of the insulation device 30 per hour (possibly 90 times per hour or more). The capacity of the insulation device 30 means the capacity of the space surrounded by the insulation device 30 and also means the capacity of the space enclosed by the partition wall 31 and the lid 32 (see an upper portion as the opening).

[0054] The insulation device 30 is removed, and the bell-shaped shell 4 is reinstalled on the base plate 8 (installation step S5). Production of the next polycrystalline silicon (subsequent polycrystalline silicon) then begins using the Siemens method. The steps described above are then repeated. [Examples]

[0055] Examples are described next.

[0056] Using the Siemens method, the air near the upper portion of the base plate 8 was collected through a baffle plate for 7.0 hours from the time the polycrystalline silicon rod was collected until the bell-shaped cup was closed, including the complete erection of the core wire. The influence of the air on the air was examined. 230 g of pure water was used as the collection liquid. The suction rate was set to 2.0 L / min. After collection, the metal components in the collection liquid were directly analyzed by ICP-MS. In both examples and the comparative example described later, the time from the time the bell-shaped cup 4 was removed from the base plate 8 until the polycrystalline silicon was collected was 1.0 hour. [Comparison example 1]

[0057] Work was carried out for 8.0 hours after opening the reactor, specifically without protection of base plate 8. [Example 1]

[0058] After the reactor was opened and the polycrystalline silicon rod was obtained in 1.0 hour, the base plate 8 was protected and insulated from the surrounding space by the insulation device 30 with the support column 33 and the vinyl partition 31. In this case, the insulation time was 6.4 hours (0.1 hour was required to protect the base plate 8 with the insulation device 30 after obtaining the polycrystalline silicon rod; further, 0.5 hours were required to remove the insulation device 30 and reinstall the bell jar 4 on the base plate 8). As a result, contamination with metals such as Zn, Ni, and Fe was significantly prevented. In addition, contamination with Ca, which might have been mixed in from outside the insulation device 30, was reduced to about 1 / 4. In Example 1, the insulation time was 80% (= 6.4 / 8.0) of the dissolution time. [Example 2]

[0059] After the reactor was opened and the polycrystalline silicon rod was obtained in 1.0 hour, the base plate 8 was protected and isolated from the surrounding space by the insulation device 30 with the support column 33 and the vinyl partition 31. The ventilation frequency in the isolated space through the FFUs 37 (ULPA filters made of NITTA) mounted on the upper part was set to 30 times per hour. Metal parts, such as the exterior of the FFUs 36 and the frame of the housing of the insulation device 30, were protected with tape as an anti-rust measure when working in an acidic atmosphere. This further reduced Ca and Na, which were suspected of being introduced by the operator 90.In Example 2, as in Example 1, the insulation time was 6.4 hours (it took 0.1 hour to protect the base plate 8 by the insulating device 30 after obtaining the polycrystalline silicon rod, and it took 0.5 hour to remove the insulating device 30 and re-install the bell shell 4 on the base plate 8). [Example 3]

[0060] After the reactor was opened and the polycrystalline silicon rod was obtained in one hour, the base plate 8 was protected and isolated from the surrounding space by the insulation device, which included the support column 33 and the vinyl partition 31. The ventilation frequency in the space isolated by the FFUs 36 mounted on the upper part was set to 90 times per hour. This further reduced Na and Ca, presumably generated by the operator 90. In Example 3, as in Examples 2 and 3, the insulation time was 6.4 hours (it took 0.1 hour to protect the base plate 8 with the insulation device 30 after obtaining the polycrystalline silicon rod, and it took 0.5 hour to remove the insulation device 30 and reinstall the bell jar 4 on the base plate 8).

[0061] The results of Comparative Example 1 and Examples 1 to 3 are shown in the table below: [Table 1] [Unit: pptw] element N / a Cr Fe Ni Cu Zn Ca Comparison example 1 397 1 22 84 5 239 2335 Example 1 303 2 3 27 1 30 641 Example 2 120 1 8 9 0 2 203 Example 3 13 0 9 1 0 3 37

[0062] It was confirmed that Fe, Ni, Zn, and Ca could be reduced, particularly by isolating the base plate 8 from the surrounding space with an insulating device. It was further confirmed that Na and Ca, which were presumably generated by the operator 90, were reduced by directing clean air onto the base plate 8.

[0063] It was further confirmed that Na and Ca were further reduced by increasing the frequency of ventilation to 30 times per hour and 90 times per hour. List of reference symbols 1 crane 2 Counterweight 3. Hook 4 bell bowl (lid) 7 moving part 8 Base plate 9 Raw material gas supply nozzle 10 Electrode 11 core wire holders 12 silicon core wire 30 Isolation device 31 Partition wall 36 Filter unit (FFU) 80 Circle showing the area ½ from the center of the isolation device 90 operators A1 contaminated air A2 Clean air A3 Air with powder dust

Claims

[1] Method for preventing contamination of a base plate with: after production of polycrystalline silicon in a reactor with the base plate and a lid covering the base plate, removing the lid from the base plate; and Isolating a space comprising the base plate by an insulating device, after removing the cover, wherein the insulating device comprises a filter unit configured to blow filtered air through a filter onto the base plate, and Blowing filtered air through the filter unit onto the base plate. [2] A method for preventing contamination of the base plate according to claim 1, characterized by that the filter unit delivers filtered air per hour with a volume 30 times or more greater than the capacity of the isolation device. [3] A method for preventing contamination of the base plate according to claim 1, characterized bythat the filter unit delivers air volumes 90 times or more greater than the capacity of the isolation device per hour. [4] A method for preventing contamination of the base plate according to any one of claims 1 to 3, characterized by that the insulating device has a movable part, and that before insulating the space enclosing the base plate, the insulating device is moved by the movable part. [5] A method for preventing contamination of the base plate according to any one of claims 1 to 4, characterized by starting the next production of a polycrystalline silicon after removing the insulation device and installing the cover on the base plate. [6] A method for preventing contamination of the base plate according to any one of claims 1 to 5, characterized bythat the insulation of the space enclosing the base plate by the insulation device is carried out in 70% or more of the time from the removal of the cover from the base plate after the end of one production of the polycrystalline silicon to the installation of the cover on the base plate for starting the next production of polycrystalline silicon.

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