EP3160903A1

EP3160903A1 - Fluidized bed reactor and method for producing polycrystalline silicon granules

Info

Publication number: EP3160903A1
Application number: EP15734088.6A
Authority: EP
Inventors: Simon PEDRON; Bernhard Baumann; Gerhard FORSTPOINTNER
Original assignee: Wacker Chemie AG
Current assignee: Wacker Chemie AG
Priority date: 2014-06-24
Filing date: 2015-06-19
Publication date: 2017-05-03
Also published as: DE102014212049A1; TWI555888B; KR20160148601A; TW201600655A; CN106458608A; CN106458608B; KR101914535B1; US20170158516A1; WO2015197498A1

Abstract

The invention relates to a fluidized bed reactor for producing polycrystalline silicon granules, comprising a reactor vessel (1), a reactor tube (2), and a reactor bottom (15) within the reactor vessel (1), wherein an intermediate jacket (3) is located between an outer wall of the reactor tube (2) and an inner wall of the reactor vessel (1), and comprising a heating device (5), at least one bottom gas nozzle (9) for feeding fluidizing gas, at least one secondary gas nozzle (10) for feeding reaction gas, a feeding apparatus (11) for feeding silicon nucleus particles, a removal line (14) for polycrystalline silicon granules, and an apparatus for leading away reactor exhaust gas (16), wherein a main body of the reactor tube (2) is composed of 60 wt% silicon carbide and comprises a CVD coating having a layer thickness of at least 5 µm, composed of at least 99.995 wt% SiC, or wherein a main body of the reactor tube (2) is composed of sapphire glass, containing at least 99.99 wt% α-Al₂O₃. The invention further relates to a method for producing polycrystalline silicon granules in such a fluidized bed reactor.

Description

Fluidized bed reactor and process for producing polycrystalline

silicon granules

The invention relates to a fluidized bed reactor and a method for producing polycrystalline silicon granules.

Polycrystalline silicon granules are an alternative to the polysilicon produced in the Siemens process. Whereas the polysilicon in the Siemens process is obtained as a cylindrical silicon rod which, before being further processed, must be reduced in time and cost to so-called chippolyol and, if necessary, must be cleaned again, polycrystalline silicon granules have bulk material properties and can be used directly as raw material, e.g. used for single crystal production for the photovoltaic and electronics industry. Polycrystalline silicon granules are produced in a fluidized bed reactor. This is done by fluidization of silicon particles by means of a gas flow in a fluidized bed, which is heated by a heater to high temperatures. By adding a silicon-containing reaction gas, a precipitation reaction takes place on the hot particle surface. Here, elemental silicon is deposited on the silicon particles and the individual particles grow in diameter. By the regular withdrawal of grown particles and the addition of smaller silicon seed particles, the process can be operated continuously with all the associated advantages. As the silicon-containing educt gas, there are described silicon-halogen compounds (e.g., chlorosilanes or bromosilanes), monosilane (SiH4), and mixtures of these gases with hydrogen.

Such deposition methods and devices for this purpose are known for example from US 4786477 A and US 49004 1 A. No. 4,90041,1 A discloses a process for obtaining high-purity polycrystalline silicon by precipitation of silicon onto high-purity silicon particles of silicon-containing gas, such as silane, dichlorosilane, trichlorosilane or tribromosilane, characterized by a reactor with a fluidized bed, in which together with Silicon seed particles, a reaction gas is introduced through an introduction tube, microwaves are supplied to heat the fluidized particles, so that deposited on it polysilicon. US 4786477 A discloses an apparatus for carrying out the process comprising a reactor having a gas inlet tube for the reaction gas mixture at the lower end, a gas outlet tube at the top and a feed tube for the silicon seed particles, characterized in that the reactor made of quartz vertically on the Center line of a heat generator is located in which a shield against microwave in the middle part is installed and which communicates with microwave generators via microwave guide tubes, below the reactor, a gas distribution plate and within each microwave guide tube a Gasabsperrmembran is arranged and that cooling channels between the wall of the heat generator and the outer wall of the reactor and in the gas distribution plate are provided.

The Siliziumimpfteilchen be heated by means of microwave radiation to a temperature of 600-1200 ° C.

US Pat. No. 6,007,869 A discloses a process for producing silicon granules having a chlorine contamination of less than 50 ppm by weight by precipitation of elemental silicon on silicon particles in a fluidized bed reactor having a heating zone and a reaction zone, wherein the silicon particles in the heating zone are heated by means of an inert, silicon-free carrier gas are fluidized and heated by means of microwave energy, and exposed in the reaction zone to a reaction gas consisting of a silicon-containing feed gas and the carrier gas, characterized in that the average temperature of the reaction gas in the reaction zone, while flowing through the fluidized silicon particles, below 900 ° C is.

The reactor tube made of metal, for example of stainless steel, is lined on the inside with high-purity quartz and on the outside with insulating material of low thermal conductivity, such as silica, coated. US7029632 B2 discloses a fluidized bed reactor consisting of:

a) a pressure-bearing shell; b) an inner reactor tube made of a material having a high transmission for heat radiation; c) an inlet (4) for silicon particles; d) an inlet device (6) for supplying a reaction gas containing a gaseous silicon compound, the inlet device being tubular and dividing the fluidized bed into a heating zone and an overlying reaction zone; e) a gas distribution device for supplying a fluidizing gas into the heating zone; f) an outlet for unreacted reaction gas, fluidizing gas and the gaseous or vaporous products of the reaction; g) an outlet for the product; h) a heating device; i) a power supply for the heating device, characterized in that the heating device is a radiation source for heat radiation, which is arranged outside the inner reactor tube and without direct contact to this ring around the heating zone and is designed such that they by means of thermal radiation, the silicon particles in the Heating zone heats up in such a way that adjusts the reaction temperature in the reaction zone.

All product-contacting components of the reactor are preferably made of an inert material or are coated with such a material.

Particularly suitable materials for this purpose are silicon or quartz.

In any case, the inner reactor tube must also have a high transmission for the heat radiation emitted by the selected heater. For example, with quartz glass of equivalent quality, the transmission for infrared radiation with wavelengths smaller than 2.6 pm is greater than 90%. Thus, quartz in combination with an infrared radiation heater (range from 0.7 to 2.5 μm), for example a radiator with SiC surface whose maximum of the emitted radiation lies at a wavelength of 2.1 μm, is particularly good suitable.

In the deposition of high-purity polysilicon from a silicon-containing gas more flow can be achieved if the deposition temperature is selected as high as possible. Increasing the deposition temperature accelerates the deposition kinetics. The equilibrium yield with respect to silicon increases. If chlorosilanes are used as precursors, a high deposition rate is expected to result in a lower chlorine level in the product. However, an increase in temperature is limited by the nature of the reactors. In a quartz glass reactor such as in US 4786477 A or US7029632 B2, the maximum permissible temperature is about 1 150 ° C. If this temperature is permanently exceeded locally, the glass softens and deforms.

Therefore, it would be desirable to find materials that have higher temperature resistance.

At the same time, the material should have a similarly high degree of transmission as quartz glass or a combination of high emissivity and high thermal conductivity.

The materials should also be inert against chemical attack, in particular by H ₂ , chlorosilanes, HCl, N ₂ at high temperatures.

Metals form silicides with chlorosilanes.

Free silicon reacts with nitrogen to form silicon nitride.

Nitrogen is often used as an inert gas in the pressure-bearing shell or in the heating chamber, which limits the reaction space, cf. e.g. US 490041 1 A. If nitrogen is used in the pressure-bearing casing, the reactor tube should be gas-tight in order to prevent nitrogen from passing from the casing into the interior of the reactor tube.

Free carbon methanized with H ₂ .

Therefore, it is proposed in the prior art to coat or line carbonaceous materials with silicon. Due to the fluidized bed, reactor tubes can cause abrasion on the walls.

The reactor tube can also be exposed to high voltages, namely compressive stress due to the clamping of the tube, thermal stresses due to high temperature gradients in the axial and radial directions. The latter occur preferably when the fluidized bed is locally heated from the outside.

EP1337463B1 discloses a reactor for producing high-purity, granular silicon by decomposition of a silicon-containing gas, characterized in that the reactor consists of a carbon-fiber-reinforced material based on silicon carbide, wherein the heat-insulating regions at the bottom of the reactor, and at the reactor head of a carbon fiber reinforced Silicon carbide material with low thermal conductivity exist, while the remaining areas are made of a carbon fiber-reinforced silicon carbide material with high thermal conductivity.

The disadvantage is that such a reactor tube is not gas-tight with respect to nitrogen in the intermediate jacket. In addition, a contamination of the silicon granules with carbon is to be expected.

US 8075692 B2 describes a fluidized bed reactor having a metal alloy reactor tube and a removable concentric sleeve inside the reactor tube, the sleeve being silicon carbide, silicon nitride, silicon, quartz, a molybdenum alloy, molybdenum, graphite, a cobalt alloy or a nickel alloy or a coating can have with the materials mentioned. The sleeve should withstand a temperature of at least 870 ° C, the temperatures in the vicinity of the sleeve amount to 700-900 ° C.

EP1984297 B1 discloses a fluidized bed reactor for the production of granular polycrystalline silicon comprising a) a reactor tube; b) a reactor shell surrounding the reactor tube; c) an inner zone formed in the reactor tube and an outer zone between the reactor shell and the reactor tube, wherein a silicon particle bed is present in the inner zone and a silicon deposition takes place while in the outer zone does not have a silicon particle bed and no silicon deposition takes place; d) a gas distributor device for introducing a gas into the silicon particle bed; e) an outlet for polycrystalline silicon particles and an outlet for reacted gas from the fluidized bed f) an inert gas inlet for maintaining a substantially inert gas atmosphere in the outer zone; g) a pressure control means for measuring and controlling the inner zone pressure Pi or the outer zone pressure Po; h) pressure difference control means for maintaining the value of Po - Pi within a range of 0 to 1 bar; wherein the inner zone pressure or the outer zone pressure is in the range of 1 to 15 bar.

The reactor tube preferably consists of an inorganic material which has a high temperature resistance, such as e.g. Quartz, silica, silicon nitride, boron nitride, silicon carbide, graphite, amorphous carbon.

US 8431032 B2 discloses a process for producing polysilicon by means of a fluidized bed reactor for the production of granular polysilicon

(i) A silicon particle production step in which the reaction gas is supplied through the reaction gas supply device such that silicon deposit occurs on the surface of the silicon particles in contact with the reaction gas, forming silicon deposits on the inner wall of the reactor tube surrounding the reaction zone

(ii) a step discharging a silicon particle following the silicon particle manufacturing step; and

(iii) a step of removing silicon deposits following the silicon particle removal step, wherein the silicon deposits are removed by supplying an etching gas into the reaction zone which reacts with the silicon deposits to form gaseous silicon mixtures. The deposition temperature is 600 ~ 850 ° C for monosilane as feed gas and 900-1 150 ° C for trichlorosilane. The following materials are mentioned: quartz, silica, silicon nitride, silicon carbide, graphite, amorphous carbon.

Because of possible contamination of the product with carbon using silicon carbide, graphite, amorphous carbon, liners or coatings of silicon, silica, quartz, or silicon nitride are suggested. The disadvantage is that, during cooling or because of irregularities in the process, damage due to the different thermal expansion of the two materials, such as flaking or breakouts, can even lead to material failure.

In addition, such a reactor tube is not inert to nitrogen in the intermediate jacket.

The etching process described in US Pat. No. 8431032 B2 makes it possible to etch wall covering on the reactor tube and on internals by means of a gas mixture. The etching gas comprises e.g. HCl.

Free silicon is etched with HCl. However, if free silicon is present in the tube itself, the reactor tube is also chemically attacked. JP 63225514 A discloses a reactor tube of silicon carbide having a lining or coating of silicon for use in fluidized bed separation of high purity polysilicon from monosilane (SiH) at a deposition temperature of 550-1000 ° C. In an etching process to remove wall covering, the coating would be attacked by silicon.

The requirements for a material for a reactor tube of a fluidized bed reactor for the production of polycrystalline silicon granules are therefore diverse, with all the measures proposed in the prior art being unsatisfactory for various reasons.

From the problem described, the problem of the invention resulted. The object is achieved by a fluidized bed reactor for the production of polycrystalline silicon granules, comprising a reactor vessel (1), a reactor tube (2) and a reactor bottom (15) within the reactor vessel (1), wherein between an outer wall of the reactor tube (2) and a Inner wall of the reactor container (1), further comprising a heating device (5), at least one bottom gas nozzle (9) for supplying fluidizing gas and at least one secondary gas nozzle (10) for supplying reaction gas, a supply device (1 1) to silicon To supply seed particles, a removal line (14) for polycrystalline silicon granules and a device for removing reactor exhaust gas (16), characterized in that a base body of the reactor tube (2) consists of at least 60 wt .-% of silicon carbide and at least on its inner side a CVD coating having a layer thickness of at least 5 μm, which is at least 99.995% by weight of silicon carbide.

The fluidized-bed reactor according to the invention provides for the use of silicon carbide for the main body of the reactor tube and for the coating of the reactor tube. Silicon carbide (SiC) has a high thermal conductivity of 20 to 150 W / m-K at 1000 ° C and an emissivity of 80 to 90%.

The CVD coating with SiC preferably has a layer thickness of 30 to 500 μm, particularly preferably a layer thickness of 50 to 200 μm.

Preferably, both the pipe inside and the pipe outside are coated.

The main body preferably consists of sintered SiC (SSiC).

SSiC is temperature resistant up to 1800-1900 ° C and already gas-tight without further treatment. In manufacturing, compounds containing electron acceptors (e.g., boron) are commonly incorporated as sintering aids. The SiC content of the SSiC basic body in this case is more than 90% by weight.

The main body can also consist of nitride-bonded SiC. This material is temperature resistant up to about 1500 ° C. Main constituents are SiC (65-90% by weight) and less than 6% by weight of metallic impurities or sintering aids. Other ingredients are Si ₃ N ₄ and free silicon.

Without further treatment, nitride-bonded SiC is not gas-tight. The CVD coating, however, the gas tightness is accomplished. The main body can also consist of recrystallized SiC (RSiC). RSiC is temperature resistant up to about 1800-2000 ° C and has a high purity of greater than 99 wt% SiC. However, the material is open-porous without further treatment and thus not gas-tight.

One possible treatment to achieve gas tightness is to infiltrate with liquid silicon to fill the pores. As a result, the maximum operating temperature is lowered to about 1400 ° C. Subsequent CVD coating ensures chemical inertness and required surface cleanliness. The CVD coating would be obsolete if no wall covering is to be etched and high-purity polysilicon is used for infiltration.

Alternatively, the gas tightness can be ensured by a SiC-CVD coating with a layer thickness of 200 to 800 μιτι.

The main body can also consist of reaction-bonded SiC (RBSiC or SiSiC). This consists of 65 to 95 wt .-% of SiC and less than 1 wt .-% of metallic impurities. Other ingredients are free silicon and free carbon. The material can be used up to 1400 ° C, but is not inert to a corrosive atmosphere due to an excess of silicon. If C-fibers are used for mechanical stabilization and control of thermal conductivity of the material, free carbon may be present on the surface. This is prone to methanation and thereby affects the gas tightness. A CVD coating of at least 5 μιη layer thickness with at least 99.995 wt .-% SiC, however, ensures the chemical inertness and the surface purity of the material.

Thus, the preferred materials can be used up to a temperature of at least 1400 ° C, which has an advantage, e.g. compared with the silicon nitride proposed in the prior art, which is stable only up to 1250 ° C. The base body and the coating have essentially the same coefficient of thermal expansion.

On the other hand, coating SiC bodies with Si ₃ N would cause the coating to flake off. The object is also achieved by a fluidized bed reactor for the production of polycrystalline silicon granules comprising a reactor vessel (1), a reactor tube (2) and a reactor bottom (15) within the reactor vessel (1), wherein between an outer wall of the reactor tube (2) and an inner wall of the reactor vessel (1) is an intermediate casing (3), further comprising a heating device (5), at least one bottom gas nozzle (9) for supplying fluidizing gas and at least one secondary gas nozzle (10) for supplying reaction gas, a feed device (1 1) to supply silicon seed particles, a polycrystalline silicon granule discharge line (14) and a reactor exhaust gas discharge device (16), characterized in that a main body of the sapphire glass reactor tube (2) containing at least 99.99% by weight of a -AI ₂ 0 ₃ , exists.

A reactor tube of high-purity sapphire glass (a-Al ₂ 0 ₃ ) with a purity of at least 99.99 wt .-% can be used up to 1900 ° C and has similar transmission properties as glass, a high abrasion resistance and is chemically resistant to all reaction gases ,

Furthermore, the material can be provided with a SiC-CVD coating because of an almost identical coefficient of thermal expansion (4.6- 10 ^"6 K ^-1 at 1000 ° C.), which is preferred.

Preferably, the reactor tube at least on its inside a CVD coating containing at least 99.995 wt .-% SiC and a layer thickness of at least 5 pm. The CVD coating with SiC preferably has a layer thickness of 30-500 μm, more preferably between 50 and 200 μm.

Alternatively, both the inside of the pipe and the outside of the pipe are coated. In both devices according to the invention, the intermediate casing preferably contains an insulating material and is filled with an inert gas or is purged with an inert gas. Nitrogen is preferably used as the inert gas. The pressure in the intermediate jacket is preferably higher than in the reaction space.

The high purity of the SiC coating of at least 99.995% by weight of SiC ensures that dopants (electron donors and acceptors, for example B, Al, As, P), metals, carbon, oxygen or chemical compounds of these substances in the near-surface Regions of the reactor tube are present only in low concentrations, so that the individual elements can reach neither by diffusion nor by abrasion in appreciable amount in the fluidized bed. There is no free silicon and no free carbon on the surface. This gives inertness to H ₂ , chlorosilanes, HCl and N ₂ .

Contamination of the polycrystalline silicon granules with carbon is prevented by using a high-purity CVD coating on the SiC reactor tube. Notable amounts of carbon would be transferred from pure SiC only to contact with liquid silicon.

The invention also relates to a process for the production of polycrystalline silicon granules in one of the above-described fluidized bed reactors with a novel reactor tube, comprising fluidizing silicon seed particles by means of a gas flow in a fluidized bed, which is heated by means of a heater, wherein polycrystalline silicon by addition of a silicon-containing reaction gas the hot Siliziumkeimpartikeloberflächen is deposited, whereby the polycrystalline silicon granules formed.

Preferably, the resulting polycrystalline silicon granules are removed from the fluidized bed reactor. Subsequently, Siiiciumablagerungen are preferably removed on walls of the reactor tube and other reactor components by supplying an etching gas into the reaction zone.

Likewise, it is preferred to continuously supply etchant gas to poly Si silicon deposits on the hot silicon seed particle surfaces to provide silicon deposits on reactor tube and other reactor wall walls avoid. The supply of the etching gas is preferably carried out locally in the so-called. Free board zone, which designates the gas space above the fluidized bed.

The wall covering can therefore be etched cyclically in alternation with the deposition process. Alternatively, locally etching gas can be continuously added in the deposition operation in order to avoid the formation of wall covering.

The process is preferably operated continuously by removing particles grown by deposition from the reactor and metering in fresh silicon seed particles.

Preferably, trichlorosilane is used as the silicon-containing reaction gas.

The temperature of the fluidized bed in the reaction zone in this case is more than 900 ° C and preferably more than 1000 ° C.

Preferably, the temperature of the fluidized bed is at least 1100 ° C, more preferably at least 1 150 ° C and most preferably at least 1200 ° C. The temperature of the fluidized bed in the reaction zone may also be 1300-1400 ° C. The temperature of the fluidized bed in the reaction zone 1 is particularly preferably from 150 ° C. to 1250 ° C. In this temperature range, a maximum of the deposition rate is achieved, which drops again at even higher temperatures.

It is likewise preferred to use monosilane as the silicon-containing reaction gas. The temperature of the fluidized bed in the reaction zone is preferably 550-850 ° C.

Furthermore, it is preferred to use dichlorosilane as the silicon-containing reaction gas. The temperature of the fluidized bed in the reaction region is preferably 600-1000 ° C.

The fluidizing gas is preferably hydrogen. The reaction gas is injected via one or more nozzles in the fluidized bed. The local gas velocities at the outlet of the nozzles are preferably 0.5 to 200 m / s. The concentration of the silicon-containing reaction gas is preferably from 5 mol% to 50 mol%, particularly preferably from 15 mol% to 40 mol%, based on the total amount of gas flowing through the fluidized bed.

The concentration of the silicon-containing reaction gas in the reaction gas nozzles is preferably 20 mol% to 80 mol%, particularly preferably 30 mol% to 60 mol%, based on the total amount of gas flowing through the reaction gas nozzles. The silicon-containing reaction gas is preferably trichlorosilane.

The reactor pressure ranges from 0 to 7 barü, preferably in the range 0.5 to 4.5 barü.

For a reactor with a diameter of z. B. 400 mm, the mass flow of silicon-containing reaction gas is preferably 200 to 600 kg / h. The hydrogen volume flow is preferably 100 to 300 NrrrVh. For larger reactors, higher amounts of silicon-containing reaction gas and H _{2 are} preferred.

It will be understood by those skilled in the art that some process parameters are ideally selected depending on the reactor size. For this reason, standardized operating data are referred to below on the reactor cross-sectional area in which the method according to the invention is preferably used.

The specific mass flow of silicon-containing reaction gas is preferably 1600-6500 kg / (h ^* m ² ). The specific hydrogen volume flow is preferably 800-4000

Nm ³ / (h * m ² ).

The specific bed weight is preferably 700-2000 kg / m ² . The specific silicon seed particle dosage rate is preferably 7-25 kg / (h ^* m ² ). The specific reactor heating power is preferably 800-3000 kW / m ² .

The residence time of the reaction gas in the fluidized bed is preferably 0.1 to 10 s, more preferably 0.2 to 5 s.

The features specified with regard to the above-mentioned embodiments of the method according to the invention can be correspondingly transferred to the device according to the invention. Conversely, the features specified with regard to the embodiments of the device according to the invention described above can be correspondingly transferred to the method according to the invention. These and other features of the embodiments according to the invention are explained in the description of the figures and in the claims. The individual features can be realized either separately or in combination as embodiments of the invention. Furthermore, they can describe advantageous embodiments that are independently protectable. Brief description of the figure

Fig. 1 shows the schematic structure of a fluidized bed reactor.

LIST OF REFERENCE NUMBERS

1 reactor vessel

2 reactor tube

3 intermediate jacket

4 fluidized bed

5 heating device

6 reaction gas

7 fluidizing gas

8 reactor head 9 floor gas nozzle

10 secondary gas nozzle

11 Seedzuführeinrichtung

12 seed

13 Polycrystalline silicon granules

14 sampling line

15 reactor bottom

16 Reactor exhaust gas The fluidized bed reactor consists of a reactor vessel 1 into which a reactor tube 2 is inserted.

Between the inner wall of the reactor vessel 1 and the outer wall of the reactor tube 2 is an intermediate casing. 3

The intermediate casing 3 contains insulating material and is filled with an inert gas or is purged with an inert gas.

The pressure in the intermediate jacket 3 is higher than in the reaction space, which is limited by the walls of the reactor tube 2.

Inside the reactor tube 2 is the fluidized bed 4 of polysilicon granules. The gas space above the fluidized bed (above the dashed line) is commonly referred to as a "free board zone".

The fluidized bed 4 is heated by means of a heater 5.

As an addition to the reactor, the fluidizing gas 7 and the reaction gas mixture 6 are fed.

The gas supply takes place specifically via nozzles. The fluidizing gas 7 is supplied via bottom gas nozzles 9 and the reaction gas mixture via so-called secondary gas nozzles (reaction gas nozzles) 10.

The height of the secondary gas nozzles 10 may differ from the height of the bottom gas nozzles 9.

Due to the arrangement of the nozzles, a bubble-forming fluidized bed 4 with additional vertical secondary gas injection is formed in the reactor. The reactor head 8 may have a larger cross section than the fluidized bed 4.

About a Seedzuführeinrichtung 11 with motor M seed 12 is fed to the reactor head 8. The polycrystalline silicon granules 13 is removed via a removal line 14 at the reactor bottom 15.

At the reactor head 8, the reactor exhaust gas 16 is withdrawn. Examples and Comparative Examples Deposition

In a fluidized bed reactor, high-purity polysilicon granules of trichlorosilane are deposited.

Hydrogen is used as the fluidizing gas.

The deposition takes place at a pressure of 3 bar (abs) in a reactor tube with an inner diameter of 500 mm.

Product is continuously withdrawn and the seed supply is controlled so that the Sauter diameter of the product is 1000 ± 50 μιη. The intermediate coat is purged with nitrogen. The residence time of the reaction gas in the fluidized bed is 0.5 s.

A total of 800 kg / h of gas is supplied, 17.5 mol% of which consist of trichlorosilane and the remainder of hydrogen.

example 1

If the reactor tube consists of SSiC with a SiC content of 98% by weight and with a 150 μηπ thick CVD coating, a fluidized bed temperature of 1200 ° C. can be achieved.

The reaction gas reacts to equilibrium. Thus, 38.9 kg of silicon can be deposited per hour.

The result is a surface yield of 198 kg h ^"1 m ^{" 2} silicon and a chlorine content of 14 ppmw in the product.

Comparative Example 1

Conversely, if the reactor tube is made of quartz glass, only a fluidized bed temperature of 980 ° C can be achieved, since otherwise a temperature of 1 150 ° C is exceeded long term on the heated reactor tube outside.

It can be deposited per hour 29.8 kg of silicon (90% of the equilibrium yield).

This results in a surface-related yield of 152 kg h ^"1 m ^{" 2} silicon and a chlorine content of 26 ppmw in the product. The differences in the average values of dopant, carbon and metal contents in the product between the two processes are less than the statistical variance. etching

In alternation with the deposition process of Example 1 or Comparative Example 1, an etching process is run.

The bed is lowered and instead of trichlorosilane now 30 kg / h of HCl are fed.

The reaction temperature is chosen to be similar to that used for the deposition process in order to avoid thermal stresses between reactor pipe and wall covering.

Example 2

If the reactor tube consists of SSiC with a SiC content of 98% by weight with a high-purity SiC coating with a thickness of 150 μm, the reactor tube is not chemically attacked and can continue to be used without restriction after the etching process.

Comparative Example 2

However, if the reactor tube is made of silicon or SiSiC without surface treatment, the reactor lining is also etched with the wall covering.

This leads to impairment of the mechanical stability of the reactor tube up to the failure of the component. The consequence is mass transfer between the intermediate shell and the reaction space.

During the etching process, hydrogen can react with a carbon-containing heater and the nitrogen used as the inert gas to the toxic product HCN.

In the separation process, the product comes into contact with contaminants from the boiler room. Nitrogen also builds up in the product.

Chlorosilanes react on the hot surface of the heater to form silicon nitride, which forms white growths there.

Contact with hot, conductive granules can in extreme cases also lead to an earth fault of the heater.

The reactor must still be taken out of service during the etching. The reactor tube is no longer usable for further runs.

The above description of exemplary embodiments is to be understood by way of example. The disclosure thus made makes it possible for a person skilled in the art on the one hand to understand the present invention and the associated advantages, and on the other hand, in the understanding of the person skilled in the art, also encompasses obvious modifications and modifications of the described structures and methods. It is therefore intended that all such alterations and modifications as well as equivalents be covered by the scope of the claims.

Claims

claims:

A fluidized bed reactor for the production of polycrystalline silicon granules, comprising a reactor vessel (1), a reactor tube (2) and a reactor bottom (15) within the reactor vessel (1), wherein between an outer wall of the reactor tube (2) and an inner wall of the reactor vessel ( 1) an intermediate casing (3), further comprising a heating device (5), at least one bottom gas nozzle (9) for supplying fluidizing gas and at least one secondary gas nozzle (10) for supplying reaction gas, a feed device (1 1) to silicon seed particles supply a removal line (14) for polycrystalline silicon granules and a device for removing reactor exhaust gas (16), characterized in that a main body of the reactor tube (2) consists of at least 60% by weight of silicon carbide and at least on its inside a CVD Coating with a layer thickness of at least 5 pm, containing at least 99.995% by weight of silicon carbide consists, has.

2. Fluidized bed reactor according to claim 1, wherein in addition an outer side of the reactor tube (2) has a CVD coating with a layer thickness of at least 5 pm, which consists of at least 99.995 wt .-% of silicon carbide.

3. Fluidized bed reactor according to claim 1 or claim 2, wherein the main body of the reactor tube (2) consists of sintered silicon carbide, nitride bonded silicon carbide, recrystallized silicon carbide or reaction-bound silicon carbide.

4. fluidized bed reactor according to one of claims 1 to 3, wherein the CVD coating has a layer thickness of 30 - 500 pm. 5. fluidized bed reactor according to claim 4, wherein the CVD coating has a layer thickness of 50 - 200 pm.

6. fluidized bed reactor for the production of polycrystalline silicon granules comprising a reactor vessel (1), a reactor tube (2) and a reactor bottom (15) within the reactor vessel (1), wherein between an outer wall of the reactor tube (2) and an inner wall of the reactor vessel ( 1) an intermediate casing (3), further comprising a heating device (5), at least one bottom gas nozzle (9) for supplying fluidizing gas and at least one secondary gas nozzle (10) for supplying reaction gas, a feed device (1 1) to silicon seed particles supply, a discharge line (14) for polycrystalline silicon granules and a device for removing reactor off-gas (16), marked thereby characterized that a base body of the reactor tube (2) made of sapphire glass containing at least 99.99 wt .-% a-AI ₂ 0 ₃ exists.

7. fluidized bed reactor according to claim 6, comprising at least on an inner side of the main body of the reactor tube (2) a CVD coating with a layer thickness of at least 5 pm, which consists of at least 99.995 wt .-% of silicon carbide.

8. Fluidized bed reactor according to claim 7, wherein in addition an outer side of the reactor tube (2) comprises a CVD coating with a layer thickness of at least 5 pm, which consists of at least 99.995 wt .-% of silicon carbide.

9. fluidized bed reactor according to claim 7 or claim 8, wherein the CVD coating has a layer thickness of 30 - 500 pm. 10. fluidized bed reactor according to claim 9, wherein the CVD coating has a layer thickness of 50 - 200 pm.

1 1. fluidized bed reactor according to one of claims 1 to 10, wherein the intermediate casing (3) comprises an insulating material and is filled with an inert gas or rinsed.

12. A process for the production of polycrystalline silicon granules, which is carried out in a fluidized bed reactor according to one of claims 1 to 1 1, comprising send fluidization of silicon seed particles by means of a gas flow in a fluidized bed, which is heated by means of a heater, wherein polycrystalline silicon is deposited on the hot silicon seed particle surfaces by adding a silicon-containing reaction gas, whereby the polycrystalline silicon granules formed.

13. The method of claim 12, wherein the resulting polycrystalline silicon granules are removed from the fluidized bed reactor and then silicon deposits on walls of the reactor tube and other reactor components are removed by supplying an etching gas into the reaction zone.

14. The method of claim 12, wherein during the deposition of polycrystalline silicon on the hot silicon seed particle surfaces, an etching gas is continuously supplied to prevent silicon deposits on walls of the reactor tube and other reactor components. 5. The method of claim 14, wherein the supply of the etching gas takes place locally in a gas space above the fluidized bed. 16. The method according to any one of claims 12 to 15, wherein used as the silicon-containing gas trichlorosilane and the fluidized bed is heated to a temperature of more than 900 ° C.

17. The method of claim 16, wherein the fluidized bed is heated to a temperature of at least 1100 ° C.

18. The method according to any one of claims 12 to 15, wherein monosilane used as the silicon-containing gas and the fluidized bed is heated to a temperature of 550-850 ° C.

19. The method according to any one of claims 12 to 15, wherein used as the silicon-containing gas dichlorosilane and the fluidized bed is heated to a temperature of 600-1000 ° C.