WO2011045320A1

WO2011045320A1 - Heating device for polysilicon reactors

Info

Publication number: WO2011045320A1
Application number: PCT/EP2010/065285
Authority: WO
Inventors: Gerhard Schwankhart
Original assignee: Inocon Technologie Ges.M.B.H
Priority date: 2009-10-14
Filing date: 2010-10-12
Publication date: 2011-04-21

Abstract

Heating device for silicon rods in polysilicon reactors, in which a plasma generator with an anode/cathode arrangement (7, 10, 12) through which a plasma gas (PG) flows so as to form an arc is provided, wherein the plasma gas (PG) is a temporal sequence of a gas that is ionized more easily than hydrogen and hydrogen and the anode/cathode arrangement (7, 10, 12) is formed by a first anode nozzle (10), through which the plasma gas (PG) flows, and a cathode (12) which is located at a short distance from the nozzle in the axial direction in the inlet region of the first anode nozzle (10), and a second anode nozzle (7), which is arranged axially in the outlet region of the first anode nozzle (10) and is separated from the first anode nozzle (10) by an insulating body. The anode/cathode arrangement (7, 10, 12) and the insulating body can be cooled with hydrogen. The heating device according to the invention can therefore be operated in a hydrogen atmosphere, with the result that the polysilicon reactor does not have to be opened. The heating device according to the invention can therefore remain in the polysilicon reactor during the silicon deposition process which lasts approximately 100 hours.

Description

Hei zVorrichtung for polysilicon reactors

The invention relates to a heater for silicon rods in polysilicon reactors, according to the preamble of claim 1.

Starting material for solar modules or semiconductors made of silicon are the abundant quartz occurrences worldwide. Reduction with carbon produces silicon and the gas carbon dioxide. To increase the degree of purity and to produce polycrystalline silicon, the silicon is first converted into gaseous trichlorosilane (HSiCls). In the conventional Siemens process, also referred to as CVD ("Chemical Vapor Deposition") method, trichlorosilane is subsequently passed together with hydrogen (H ₂ ) into a so-called polysilicon or CVD reactor in which thin silicon rods are located which are electrically heated to high temperatures up to 1000 ° C. At high pressure and temperature, trichlorosilane and hydrogen react again to form silicon that is vapor deposited and subsequently attaches to the silicon rods to form large blocks of polysilicon are formed, which are then "harvested" from the reactor.

In addition to the polysilicon reactor, suitable STC-TCS converters are used, which convert the silicon tetrachloride produced in the Siemens process back into trichlorosilane. This is then fed back to the polysilicon reactor.

The silicon rods are about 8x8 mm wide and up to 2m long rods, which are arranged concentrically in several rows in the pressure-resistant polysilicon reactor and electrically contacted. However, in order for these silicon rods to have sufficient electrical conductivity in order to subsequently heat them up electrically, they must first be preheated to a temperature of at least 400 ° C. This preheating is done in conventional Way with long infrared heater rods, which are located in the vicinity of the silicon rods. After preheating, these infrared heaters must be removed from the hot polysilicon reactor, otherwise they would hinder the growing diameter silicon rods. Moreover, the infrared heater bar would contaminate the silicon via its surface outgassing in the subsequent high temperature process.

The expansion of infrared heating bars is time consuming and dangerous. In addition, the polysilicon reactor must be purged with nitrogen each time it is opened to reliably remove the entraining air oxygen and allow subsequent introduction of hydrogen and trichlorosilane. The silicon rods must be kept conductive by current flow. Furthermore, because of the nitrogen purging, silicon nitride may form on the surface of the silicon rods, preventing the subsequent deposition of silicon. The expansion of the infrared heating bars thus significantly delays the entire production cycle for polysilicon, thus increasing the overall cost of the polysilicon.

It is therefore the object of the invention to provide a new heating device which can be operated in a hydrogen atmosphere and in which the polysilicon reactor no longer has to be opened. The heating device according to the invention should thus be such that it can remain in the polysilicon reactor during the approximately one hundred-hour deposition process of silicon. However, such a hot device must not only be resistant to the trichlorosilane-hydrogen atmosphere, but also remain functional at temperatures up to 1100 ° C.

These objects are achieved by a heating device according to the features of claim 1. Claim 1 refers to a heater for silicon rods in polysilicon reactors, in which according to the invention a plasma generator with a flowed through by a plasma gas Anode-cathode arrangement is provided for forming an arc, wherein the plasma gas is a chronological sequence of a gas and hydrogen which is easier to ionize relative to hydrogen, and the anode-cathode arrangement comprises a first anode nozzle through which the plasma gas flows, and one in the inlet region the first anode nozzle in the axial direction of a slightly spaced cathode, and an axially disposed in the outlet region of the first anode nozzle, the second anode nozzle is formed, which is separated from the first anode nozzle by an insulating body.

According to the invention, a plasma generator is thus used for preheating the silicon rods, hydrogen being used as the plasma gas. This hydrogen can preferably be taken from the hydrogen cycle of the polysilicon reactor, and is subsequently fed back to the reactor after passing through the plasma generator. Hydrogen, however, is difficult to ionize so that it can not generate an arc in the context of high-frequency ignition. Therefore, it is first necessary to use a gas that is easier to ionize relative to hydrogen, such as argon. Only after the formation of a stable arc, which is the case after about 0.5-20 seconds, hydrogen can be supplied, and the supply of the gas to be ionized easier to turn off. In order to obtain the high thermal power required for the deposition process (corresponding to at least 80kW), a working voltage of at least 200V DC is needed. Since plasma has a very low electrical resistance, a correspondingly long arc must be produced. Therefore, according to the invention, an electrode-anode arrangement is provided in which two anode nozzles are arranged at a distance from one another and separated from an insulating body. First, an arc between the first anode nozzle and the slightly spaced cathode is ignited using the more easily ionized gas, such as argon. The distance is about 3mm. After forming a stable arc after about 20 seconds, the first anode nozzle becomes switched off and switched on the second anode nozzle, so that the argon arc burns to the second anode nozzle. The distance between the second anode nozzle and the cathode depends on the required power requirement, and is about 20mm. Once the longer arc to the second anode nozzle is sufficiently stable, again after about 20 seconds, switching to hydrogen or other difficult to ionize gas such as helium or nitrogen can be used.

The insulating body is preferably composed of an alternating sequence of insulating disks and cooling disks, wherein the cooling disks are flowed through by an insulating body cooling medium.

So that the plasma generator during the entire

Separation process can remain in the polysilicon reactor, the lying in the interior of the polysilicon reactor portion of the plasma generator is up to the outlet region of the second anode nozzle enclosed by a double-walled protective tube, which is traversed by a thermowell cooling medium. The cooling of the protective tube is hermetically sealed against the polysilicon reactor, and is charged from outside the reactor with the protective tube cooling medium.

The first anode nozzle and the second anode nozzle are each made of silver or molybdenum. The insulating disks are preferably made of a silicon nitride ceramic, and the cooling disks made of silver or molybdenum. These materials are resistant to high temperatures.

A particularly preferred embodiment is that the first anode nozzle, the second anode nozzle and the cooling disks are made of molybdenum, and the insulator cooling medium is hydrogen gas. In the preheating phase, a high thermal power is dissipated. In order to be able to cool with hydrogen gas already in the preheating phase, the critical components must be exposed to higher temperature loads, which can be done by choice of molybdenum as a material succeeds. In addition, molybdenum behaves neutrally with respect to silicon, so that any evaporated molybdenum in the reactor does not adversely affect the deposition process of silicon.

To increase safety, the plasma generator may be flushed with a purge gas, preferably hydrogen gas, prior to the initiation of trichlorosilane to remove any moisture present and to avoid hazardous reactions of the water with the subsequently introduced trichlorosilane.

Preferably, furthermore, the electrode

Cathode arrangement and the insulating executed as a replaceable structural unit, which is held via a nozzle nut in the plasma head of the plasma generator. Maintenance work on the cathode-anode assembly can thus be made easier without having to remove the entire plasma generator. For this purpose, the cathode, the first anode nozzle, and the second anode nozzle are each connected to a connecting bolt made of electrically conductive material, which is sheathed by an insulating sleeve. The connecting bolts can be inserted into bores of the carrier, electrical contacting of the connecting bolts being provided in a region of the carrier which is at a distance from the plasma head.

The nozzle nut is preferably designed so that it ensures a resilient clamping of the cathode-anode assembly and the insulating body, so that their thermal expansion can be compensated.

The invention will be explained in more detail below with reference to an embodiment with reference to the accompanying drawings. This show the

Fig. 1 is a perspective view of an embodiment of a plasma head according to the invention with the three Connecting bolt for the first anode nozzle, the second anode nozzle and the cathode,

2 shows a sectional view of the plasma head according to the invention according to FIG. 1 according to the section line D-D of FIG. 5, FIG.

3 shows a sectional view of the plasma head according to the invention according to FIG. 1 according to the section line C-C of FIG. 5, FIG.

4 shows a sectional view of the plasma head according to the invention according to FIG. 1 according to the section line B-B of FIG. 5, FIG.

5 is a view of the plasma head according to the invention according to FIG. 1 seen from above,

6 shows an embodiment of a plasma generator according to the invention without inserted plasma head in a sectional view along the section line A-A of FIG. 7,

FIG. 7 shows the plasma generator according to FIG. 6 seen from above, FIG.

8 shows a detailed view of the area shown in rectangular fashion in FIG. 7, in which the contacting of the connecting bolt for the cathode can be seen,

9 shows an embodiment of a plasma generator according to the invention with inserted plasma head in a sectional view along the section line A-A of FIG. 10, FIG.

FIG. 10 shows the plasma generator according to FIG. 9 seen from above, FIG.

11 is a sectional view of the plasma generator according to the invention along the section line B-B of Fig. 10,

12 is a sectional view of the plasma generator according to the invention along the section line CC of FIG. 10, 13 is a sectional view of the plasma generator according to the invention along the section line DD of FIG. 10,

FIG. 14 shows an embodiment of the plasma generator according to the invention according to FIG. 9 in a sectional view according to the section line A-A of FIG. 10 with a course of the cooling media for the plasma generator carrier and the cathode.

FIG. 15 shows an embodiment of the plasma generator according to the invention according to FIG. 9 in a sectional view according to the section line C-C of FIG. 10 with the course of the cooling media for the insulating body and the purge gas drawn in, FIG.

FIG. 16 shows an embodiment of the plasma generator according to the invention according to FIG. 9 in a sectional view according to the section line D-D of FIG. 10 with a course of the cooling media for the protective tube and the plasma generator carrier,

17 is an enlarged view of FIG. 4 with marked course of the cooling medium for the insulating body and the cathode,

Fig. 18 is an enlarged view of FIG. 3 with marked course of the cooling medium for the cathode, as well as the plasma gas, and the

Fig. 19 is an enlarged view of FIG. 2 with marked course of the outlet of the cooling medium for the insulating body and the cathode.

In Fig. 1, a perspective view of an embodiment of a plasma head 57 according to the invention is shown first. The plasma head 57 comprises the anode-cathode arrangement consisting of a first anode nozzle 10, a second anode nozzle 7, and a cathode 12 for Forming an arc over which a plasma jet 56 is generated by means of a plasma gas PG, as will be explained in more detail. In FIG. 1, the first connecting bolt 22 for the first anode nozzle 10, the second connecting bolt 16 for the second anode nozzle 7, and the third connecting bolt 24 for the cathode 12 can be seen for this purpose. The first connecting bolt 22 is encased by a first insulating sleeve 23, the second connecting bolt 16 by a second insulating sleeve 17, and the third connecting bolt 24 by a third insulating sleeve 25, which are each preferably made of a ceramic material.

The construction of the plasma head 57 according to the invention according to FIG. 1 will be explained with reference to FIGS. 2-4, which show different sectional views, as can be seen with reference to FIG. 5, of the plasma head 57 according to the invention according to FIG. 1, with a detailed reference in FIG Fig. 4 finds. On a cathode plate 1, a cathode 12 is attached via a cathode nut 13, which is connected via the cathode plate 1 and the third terminal pin 24 with a negative pole of a power supply (see Fig. 3). In the cathode plate 1, a cathode cooling gas passage 20 is further provided, which is sealed with a blind plug 15, and communicates with a cathode cooling tube 14. Via the cathode cooling tube 14, the cathode 12 is cooled with a cathode cooling medium KK, as will be explained in more detail. The cathode plate 1 is separated from the nozzle holding plate 3 by a first insulating plate 2 in which the first anode nozzle 10 is held. The first anode nozzle 10 is traversed by a plasma gas PG, as will also be explained in more detail. In the inlet region of the plasma gas PG into the first anode nozzle 10, an insulating nozzle 11 is arranged between the first anode nozzle 10 and the cathode 12, which electrically shields the cathode 12 from the first anode nozzle 10 to the tip of the cathode 12. The first anode nozzle 10 is connected via the nozzle holding plate 3 and the first connecting bolt 22 with the positive pole of a power supply (see Fig. 3). at Applying a voltage and passing a plasma gas PG can thus form an arc between the cathode 12 and the first anode nozzle 10. The plasma gas PG flows through the free channel between the cathode 12 and the first anode nozzle 10 and the insulating nozzle 11. The first anode nozzle 10 is preferably made of silver or molybdenum, and the cathode 12 made of tungsten.

The nozzle holding plate 3 is separated by a second insulating plate 4 from a front housing 5, in which by means of a nozzle nut 6, a second anode nozzle 7 is held. Also, the second anode nozzle 7 is preferably made of silver or molybdenum, and connected via the front housing 5 and the second terminal bolt 16 to the positive pole of a power supply. The second anode nozzle 7 is further separated via an insulating body of the first anode nozzle 10, wherein the insulating body of an alternating sequence of cooling disks 8 and insulating 9 is composed, each forming a free, central channel, and the cooling disks 8 of the insulating body cooling medium KI flow through cascade, as will be explained in more detail. The cooling disks 8 and insulating disks 9 are held in an insulating sleeve 58, which contacts the second anode nozzle 7 in the contact region 26. When a voltage is applied and a plasma gas PG is passed through, an arc can therefore also be formed between the cathode 12 and the second anode nozzle 7, the arc extending along the entire central channel formed by the cooling disks 8 and insulating disks 9.

The cathode plate 1 is further attached via an insulating support 19 to an insulating flange 21 by means of a support ring 18. The entire structure of the plasma head 57 is held together via fixing pins 30, which are arranged electrically isolated within the structure of the plasma head 57 (see FIG. 2).

With reference to FIG. 6, an embodiment of a plasma generator according to the invention without being used Plasma head 57 in a sectional view according to the section line AA of Fig. 7 explained. FIG. 7 shows the plasma generator according to FIG. 6 seen from above, with reference being made to FIG. 10 with regard to its referencing. The plasma generator comprises a carrier body 31, which is installed in a double-walled protective tube 27. The protective tube 27 is preferably made of electropolished, acid-resistant steel and has channels which are traversed by a protective tube cooling medium KS. As a protective tube cooling medium KS water can be used, as will be explained in more detail. The carrier body 31 further has bores 34 into which the connection bolts 16, 22, 24 can be inserted, leaving a free space through which the cathode cooling medium KK can be introduced, which cools the cathode 12. The cathode cooling medium KK is preferably a hydrogen gas. At the lower, free end of the plasma generator, a protective ring 28, preferably made of a ceramic material, is provided, which reduces the cross section between the protective tube 27 and the front housing 5 of the plasma head 57 and so in connection with the in the space between the protective tube 27 and support body 31st introduced purge gas SG (depending on the operating state of the reactor such as trichlorosilane or nitrogen) prevents the entry of the hot reactor gases and the heat radiation from the silicon rods in the space. The plasma generator is attached to the polysilicon reactor (not visible in Figs. 1-19) by means of a mounting flange 32 and mounting bolts 29. Attachment points 59 for the connection of a stop means such as hook and ring hangers, wreath chains, round slings, loops, etc. can be provided on the attachment flange 32 in order to be able to handle the heating device more easily. Furthermore, the cathode power supply 33, as well as the outlet 43 for a carrier body cooling medium KT can be seen, which cools the carrier body 31 of the plasma generator, as will also be explained in more detail. In FIG. 6, furthermore, a rectangular area is marked, which is shown enlarged in FIG. This area shows the contacting of the connecting bolt 24 for the cathode 12, which is inserted in the assembled state in the bore 34 and at its upper, exposed end, where it is not covered by the insulating sleeve 25 (see also Fig. 1) a first leg of a U-connection tab 46 contacted. The U-connection tab 46 is supported on a Isolierverkleidung 47 and is attached to its second leg to a power bolt 48, which is the lower continuation of a bolt passage 50. The bolt passage 50 is by means of a

Bolzendurchführungs insulating sleeve 49 arranged electrically insulated from the rest of the structure, and has at its upper end an extension 51 which is electrically contacted with a bolt shoulder 53. The bolt shoulder 53 is held in a glass insulator 54 and silicone gaskets 52. Finally, the electrical connection to the cathode power supply 33 is established via a ground cable lug 55.

Finally, FIGS. 9-13 show an embodiment of a plasma generator according to the invention with inserted plasma head 57, FIG. 9 showing a sectional view according to section line AA of FIG. 10, FIG. 11 a sectional view along section line BB of FIG. Fig. 12 is a sectional view taken along the line CC of Fig. 10, and Fig. 13 is a sectional view along the section line DD of Fig. 10. Fig. 10 shows the overall structure of the plasma generator of FIG. 9 seen from above, and is also provided with a detailed referencing of the different components.

FIG. 9 shows, in particular, the third connecting bolt 24 for the cathode 12, which is now inserted in a bore 34. Furthermore, the flow 42 for the support body cooling medium KT is shown, wherein the support body cooling medium KT the support body 31 of the plasma generator via the incoming cooling channel 35 (see FIG. 13) and the flows through outflowing cooling channel 36, and leaves it again via the outlet 43.

The inlets and outlets for the protective tube cooling medium KS of the protective tube 27 are shown in FIG. 11, namely in the form of a protective tube flow 45, and a protective tube return 44. The course of this protective tube cooling medium KS is based on FIG. 14-19 will be explained in more detail. Furthermore, a first connection 39 for the insulating body cooling medium KI, which flows through the cooling disks 8, can also be seen, wherein this insulating body cooling medium KI leaves the heating device according to the invention downwards, as will be explained in more detail with reference to FIGS. 13 and 19.

FIG. 12 shows a second connection 41 for the cathode cooling medium KK, which cools the cathode 12. The cathode cooling medium KK also leaves the heating device according to the invention downwards, as will be explained in more detail with reference to FIGS. 14, 17 and 18. FIG. 12 also shows the inserted second connecting bolt 16 for the second anode nozzle 7, as well as the purge gas inlet 38, can be introduced via the purge gas SG to flush the heater with hydrogen gas, trichlorosilane or nitrogen and remove residual moisture and oxygen.

Finally, FIG. 13 shows the plasma gas connection 40, via which plasma gas PG can be fed to the anode-cathode arrangement. The plasma gas PG leaves the heating device according to the invention as a plasma jet 56. FIG. 13 also shows the inserted first connecting bolt 22 for the first anode nozzle 10.

The path of the cooling media and the plasma gas PG within the plasma generator is explained in more detail with reference to FIGS. 14-16. FIGS. 17-19 show the progressions of the cooling media and the plasma gas PG in the plasma head 57. FIG. 14 initially shows a sectional view according to the section line AA of FIG. 10, and shows the profile of the carrier body cooling medium KT for the carrier body 31 of the plasma generator. The carrier body cooling medium KT is supplied via the feed 42, flows through the carrier body 31 of the plasma generator via the incoming cooling channel 35 (see FIG. 16) and the outflowing cooling channel 36, and leaves the plasma generator via the outlet 43. The cathode cooling medium KK becomes supplied via the second port 41, which is visible approximately in FIG. 15, and flows through the free space within the bore 34, in which the second connecting bolt 24 is used for the cathode 12, to the cathode plate 1. The further course of the cathode Cooling medium KK passes through the upper cathode cooling gas channel 60 and the cathode cooling tube 14 within the cathode 12 (see FIG. 18) into the lower cooling gas channel 20 (see FIG. 17), into the free space of the bore 34, in which the second connecting bolt 16 for the second anode nozzle 7 is inserted, opens. The cathode cooling medium KK mixes here with the Isolierkörper- cooling medium KI.

FIG. 14 also shows the negative cathode contact K _e for the cathode 12, which leads from the power supply 33 via the bolt leadthrough 50 and the third connecting bolt 24 to the cathode plate 1 and the cathode 12 held therein (see FIG ).

Fig. 15 shows in a sectional view along the section line CC of FIG. 10, the course of the insulating body cooling medium KI and the purge gas SG. The insulating body cooling medium KI is fed via the first connection 39 to the free space in the bore for the second connecting bolt 16, which contacts the second anode nozzle 7. It subsequently flows along the second connecting bolt 16 as far as the plasma head 57, where it meets the second insulating plate 4 and is fed via an opening in the overlying nozzle holding plate 3 to the first anode nozzle 10 and surrounds it from the outside (see FIG. 17). , The insulating body cooling medium KI is in another sequence together with the cathode cooling medium KK of the first cooling disk 8 of the insulating supplied and flows through the first cooling disk 8, which thus cools the immediately adjacent insulating 9 flat. The cooling disks 8 have in their outer regions breakthroughs, through which the insulating body cooling medium KI can reach the next cooling disk 8, it flows through subsequently and the next cooling disk 8 is supplied. Thus, a cascade cooling of the insulator is realized. The insulator cooling medium KI and the cathode cooling medium KK, after passing through the insulator, finally strike the second anode nozzle 7, which is flowed around along its outer surface, and leave the plasma head 57 through the gap between the second anode nozzle 7 and the nozzle nut 6 (see FIG Fig. 19).

Furthermore, FIG. 15 also shows the path of the purge gas SG which is introduced via a purge gas connection 38, flows through the heating device between carrier body 31 and protective tube 27 and is conducted into the exterior between guard ring 28 and front housing 5.

Furthermore, the positive contact AD2 _e for the second anode nozzle 7 can be seen in FIG. 15, which leads via a pin bushing 50 analogous to the cathode contacting K _e and the second connecting pin 16 to the front housing 5 and the second anode nozzle 7 (see FIG also Fig. 17).

FIG. 16 shows in a sectional view along the section line DD of FIG. 10 the course of the protective tube cooling medium KS for the protective tube 27, the carrier body cooling medium KT for the carrier body 31, and of the plasma gas PG. The course of the carrier body cooling medium KT has already been explained with reference to FIG. 14. The protective tube cooling medium KS is supplied via the protective tube feed 45 (see also FIGS. 10 and 11), flows through the protective tube 27 in this provided channels, and is discharged via the return 44 again.

The plasma gas PG is supplied via the plasma gas connection 40 to the free space in the bore for the first connection bolt 22, which contacts the first anode nozzle 10. It subsequently flows along the first connecting bolt 22 as far as the plasma head 57, where it meets the first insulating plate 2 and is fed to the cathode 12 via an opening in the overlying cathode plate 2 (see FIG. 18). The plasma gas PG then flows through the opening of the first anode nozzle 10, as well as the axial channel which runs centrally through the insulating body, and finally reaches the second anode nozzle 7, where it leaves the heating device as a plasma jet 56.

Further, in Fig. 16, the positive contact ADl _e for the first anode nozzle 10 can be seen, which leads via an analogous to the cathode contacting K _e bolt bushing 50 and the first terminal bolt 22 to the nozzle holder plate 3 and the first anode nozzle 10 held therein (see also Fig. 18).

The operation of the heating device according to the invention is now as follows. First, a gas that is more easily ionizable, such as argon, is introduced via the plasma gas port 40 and a high voltage is applied between the cathode 12, the first anode nozzle 10, and the second anode nozzle 7. As a result, an arc is formed between the cathode 12 and the first anode nozzle 10. After forming a stable arc after about 1-5 seconds, the power supply for the first anode nozzle 10 is turned off, so that an arc is formed between the cathode 12 and the second anode nozzle 7, which extends through the central channel of the insulating body. After forming a stable arc to the second anode nozzle 7, which after about l-20s is the case, the argon supply is turned off and introduced as plasma gas hydrogen gas.

The hydrogen gas leaves as plasma jet 56 the heating device according to the invention and heats the silicon rods. During this preheating process, it is possible to use water as carrier-body cooling medium KT and as protective-tube cooling medium KS, the carrier-body cooling medium KT being supplied via the feed 42, and the protective-tube cooling medium KS via the protective-tube feed 45. The use of water This is recommended because about 25kW of thermal power has to be dissipated in the preheat phase. However, if there are leaks through which water enters the polysilicon reactor, there are still no problems in the preheating phase because water does not generate dangerous reactions with hydrogen, and trichlorosilane is not yet present in the reactor in the preheating phase. As insulator cooling medium KI, hydrogen gas can be used, which is supplied via the first port KT. Also, as the cathode cooling medium KK, hydrogen gas is preferably used, which is supplied via the second port 38.

As soon as the silicon rods reach a temperature of about 400 ° C, they become conductive and heat up spontaneously. The power supply to the plasma generator is now interrupted so that the arc goes out. Subsequently, the plasma gas supply is reduced to a small amount of purge gas through the plasma gas channel to prevent the entry of silicon dust during the deposition process in the plasma gas channel. By means of a valve circuit any existing moisture can be blown out of the heating device according to the invention by supplying purge gas SG, preferably hydrogen gas, via the purge gas channel inlet 38. Subsequently, trichlorosilane can be introduced into the hydrogen atmosphere in the polysilicon reactor, and the deposition process of pure silicon can begin. During the deposition process, the heating device according to the invention remains within the reactor and therefore must continue to be cooled because of the radiant heat in the reactor. Due to the hermetically sealed against the reactor, water-cooled protective tube 27, however, only about 360 watts of thermal power must be dissipated, for which the flow through the cooling disks 8 and the cathode 12, and the support body 31 is sufficient with hydrogen gas. If, due to leaks, carrier-body cooling medium KT now enters the polysilicon reactor, only hydrogen would be fed to the reactor without further negative consequences.

If the plasma generator is made of high temperature resistant materials such as molybdenum or ceramic, the plasma generator can operate at temperatures up to 500 ° C as there are no seals or cooling water. As a cooling medium can be used in this case only hydrogen gas, which is injected at temperatures up to 400 ° C in the reactor and supports the heating of the silicon rods. To dissipate a thermal power of 25kW, about 300 m ^{3 of} hydrogen gas per hour are needed. Since in this case the loss of about 25kW of cooling power by water cooling is eliminated, the efficiency of the preheating can be increased to approximately 100%. Also, the hydrogen gas used for cooling can be taken from the reactor circuit, and is therefore not lost. Furthermore, a possible calcification of the components can be prevented with the exclusive use of hydrogen gas as the cooling medium.

The heating device according to the invention can thus be operated in a hydrogen atmosphere, so that the

Polysilicon reactor no longer needs to be opened. The heater according to the invention is further adapted to remain in the polysilicon reactor during the approximately one hundred hour deposition process of silicon. The heating device according to the invention is Not only resistant to trichlorosilane-hydrogen atmosphere, but also at temperatures up to 1100 ° C functional.

Claims

Claims:

1. Hei zVorrichtung for silicon rods in polysilicon reactors, characterized in that a plasma generator with a, of a plasma gas (PG) flowed through anode-cathode assembly (7, 10, 12) is provided for forming an arc, wherein the plasma gas (PG) is a time sequence of a gas and hydrogen which is easier to ionize relative to hydrogen, and the anode-cathode arrangement (7, 10, 12) comprises a first anode nozzle (10) through which the plasma gas (PG) flows, and one in the inlet region the first anode nozzle (10) in the axial direction of slightly spaced cathode (12), and one in the outlet region of the first anode nozzle (10) axially arranged second anode nozzle (7) is formed, which is separated from the first anode nozzle (10) by an insulating body ,

2. Heating device according to claim 1, characterized in that the insulating body is composed of an alternating sequence of insulating disks (9) and cooling disks (8), wherein the cooling disks (8) are traversed by an insulating body cooling medium (KI).

3. Heating device according to claim 1 or 2, characterized in that the lying in the interior of the polysilicon reactor portion of the plasma generator to the outlet region of the second anode nozzle (7) by a double-walled protective tube (27) is enclosed by a thermowell cooling medium (KS) is flowed through.

4. Heating device according to one of claims 1 to 3, characterized in that the first anode nozzle (10) and the second anode nozzle (7) are each made of silver or molybdenum.

5. Heating device according to one of claims 2 to 4, characterized in that the insulating discs (9) are made of a silicon nitride ceramic.

6. Heating device according to one of claims 2 to 5, characterized in that the cooling disks (8) are made of silver or molybdenum.

7. Heating device according to one of claims 2 to 6, characterized in that the first anode nozzle (10), the second anode nozzle (7) and the cooling disks (8) are made of molybdenum, and the insulating body cooling medium (KI) is hydrogen gas.

8. Heating device according to one of claims 1 to 7, characterized in that the electrode-cathode arrangement (7, 10, 12) and the insulating body are designed as interchangeable, structural unit, via a nozzle nut (6) in the plasma head (57) of Plasma generator is held.

9. heating device according to claim 8, characterized in that the nozzle nut (6) as a resilient mounting of

Electrode cathode assembly (7, 10, 12) and the insulating body is executed.

10. Heating device according to claim 8 or 9, characterized in that the plasma head (57) is attached to a carrier body (31) of the plasma generator, and the cathode (12), the first anode nozzle (10), and the second anode nozzle (7). each with a connecting bolt (24, 22, 16) of electrically conductive material, which is surrounded by an insulating sleeve (25, 23, 17) is connected, wherein the connecting pins (24, 22, 16) in bores (34) of the carrier body (31) are insertable, and an electrical contacting of the connecting pins (24, 22, 16) in a, from the plasma head (57) spaced portion of the carrier body (31) is provided.

11. Use of a plasma generator for preheating silicon rods in polysilicon reactors.