Reactor and process for preparing granular polysilicon

Abstract

A reactor for preparing granular polysilicon by deposition of polycrystalline silicon on silicon seed particles has a reaction vessel, an inner reactor tube for a fluidized bed comprising granular polysilicon and a reactor bottom within the reactor vessel, a heating device for heating the fluidized bed in the inner reactor tube, at least one bottom gas nozzle for introduction of fluidizing gas and at least one reaction gas nozzle for introduction of reaction gas, a feed device to introduce silicon seed particles, an offtake line for granular polysilicon, and a device for discharging reactor offgas from the reactor vessel, and has a cylindrical component which has openings on its cylindrical surface, with at least 5% and not more than 95% of the cylindrical surface being open located between the inner reactor tube and the heating device.

Claims

1. A reactor for preparing granular polysilicon by deposition of polycrystalline silicon on silicon seed particles, comprising: a reaction vessel having a reactor bottom; an inner reactor tube for a fluidized bed comprising granular polysilicon, a heater for heating the fluidized bed in the inner reactor tube; at least one bottom gas nozzle for introduction of fluidizing gas and at least one reaction gas nozzle for introduction of reaction gas; a silicon seed particle feed; an offtake line for granular polysilicon; a reactor offgas discharge for discharging reactor offgas from the reactor vessel; and a cylindrical component which has openings on its cylindrical surface such that at least 5% and not more than 95% of the cylindrical surface is open, the cylindrical component located between the inner reactor tube and the heater, and comprising a thermally conductive material such that heat energy is transferred by heat radiation and heat conduction to the cylindrical component such the cylindrical component can be brought to incandescence, or wherein the component comprises a material which is permeable to radiation energy from the heater.

2. The reactor of claim 1, wherein 40-70% of the cylindrical surface of the component is open.

3. The reactor of claim 2, wherein 45-60% of the cylindrical surface of the component is open.

4. The reactor of claim 1, wherein the heater comprises a resistance heater having a meandering shape or comprises a plurality of individual heating elements.

5. The reactor of claim 1, wherein the heater comprises a plurality of heating elements arranged concentrically around the inner reactor tube and the cylindrical component is also arranged concentrically around the inner reactor tube and within the heater.

6. The reactor of claim 1, wherein the heater comprises heating elements which are positioned in openings of the cylindrical component.

7. The reactor of claim 1, wherein the cylindrical component is constructed of at least one material selected from the group consisting of graphite, CFC, silicon, SiC and fused silica, or wherein the cylindrical component is coated with one or more of these materials.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically shows, in two views, how the component is arranged between heaters and reactor tube.

(2) FIG. 2 shows a first embodiment of the component.

(3) FIG. 3 shows a second embodiment of the component.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) For the purposes of the present invention, the feature that at least 5% and not more than 95% of the cylindrical surface of the cylindrical component is open means that the ratio of free area (sum of the open areas) to the total area of the cylindrical surface of the component is 5-95%. This ratio is preferably 40-70%, more preferably 45-60%. The openings can be slits, cut-outs, meshes, drilled holes, etc. The component can, for example, have the form of a cylindrical mesh.

(5) The component is preferably open in the upward or downward direction or in both directions (base and top area of the cylinder). This facilitates the disassembly of the reactor.

(6) The heating device can be a heater having a meandering shape or a plurality of heating elements or heating strips. The heating device preferably consists of a plurality of heating elements arranged concentrically around the inner reactor tube. In this case, the component, which is likewise arranged concentrically around the inner reactor tube, is preferably located between the heating elements and the inner reactor tube.

(7) The component preferably consists of a material which has good thermal conductivity. The heating energy is transferred by heat radiation and heat conduction to the component and brings the latter to glowing. Preference is also given to using a component which consists of a material which is permeable for the radiation energy from the heating device. The heating elements are preferably located in openings of the cylindrical surface of the component. The openings can be cut-outs in which the heating strips are located.

(8) The component preferably comprises a material selected from the group consisting of graphite, CFC, silicon, SiC and fused silica. The component can consist of one or more of the materials mentioned. The component can likewise be coated with one or more of the materials mentioned.

(9) The fluidized-bed reactor consists of a reactor vessel in which an inner reactor tube is installed. The fluidized bed comprising the polysilicon granules is present in the interior of the reactor tube. The fluidized bed is heated by means of the heating device. As feed gases, the fluidizing gas and the reaction gas mixture are fed into the reactor. The gases are fed in a targeted manner through nozzles. The fluidizing gas is fed in via a bottom gas nozzle and the reaction gas mixture is fed in through secondary gas nozzles (reaction gas nozzles). The height of the secondary gas nozzles can differ from the height of the bottom gas nozzles. Due to the arrangement of the nozzles, a bubble-forming fluidized bed with additional vertical secondary gas introduction is formed in the reactor. Silicon seed particles are introduced into the reactor via a feed device. The polysilicon granule product is taken off through an offtake line at the bottom of the reactor. The reactor offgas is taken off via a facility for discharging reactor offgas.

(10) It has surprisingly been found that the use of a component as mentioned above between heating device and reactor tube is not only suitable for equalizing the temperature but also for protecting the heating device during drawing-out of the reactor.

(11) In the prior art, damage to the heating elements occurred as a result of the breaking-up of the reactor tube. This can be avoided by means of the present invention. The component is insensitive to breaking parts of the reactor tube and can be reused.

(12) In addition, unlike the prior art, no radiation shield is present since the component has openings and energy input into the fluidized bed thus occurs in a more economical way.

(13) The invention is illustrated below with the aid of figures.

LIST OF REFERENCE NUMERALS USED

(14) 101 Heater 102 Component 103 Inner reactor tube 104 Fluidized bed

(15) Fluidized bed 104 is located in the inner reactor tube 103. The component 102 is located between heater 101 and inner reactor tube 103. Heater 101 and component 102 are arranged concentrically around the inner reactor tube 103.

(16) FIGS. 2 and 3 show two embodiments of components which can be used; however, these are not intended to restrict the general inventive concept in any way.

(17) FIG. 2 shows a component in which 55% of the cylindrical surface is open. The openings have a rectangular shape (slits in the longitudinal direction of the cylinder) and are uniformly distributed over the cylindrical surface of the cylindrical component. This embodiment is particularly useful for arranging heating elements in the openings.

(18) FIG. 3 shows a component in which 47% of the cylindrical surface is open. A plurality of square openings are present on the cylindrical surface of the cylindrical component.

(19) The invention also provides a process for preparing granular polysilicon using a reactor according to the invention, which comprises fluidization of silicon seed particles by means of a gas stream in a fluidized bed which is heated by means of a heating device, with polycrystalline silicon being deposited on the hot seed particle surfaces by introduction of a silicon-containing reaction gas by means of pyrolysis thereof so as to form the granular polysilicon.

(20) The process is preferably operated continuously by discharging particles whose diameter has been increased by deposition from the reactor and fresh seed particles being introduced.

(21) The temperature of the fluidized bed in the reaction region is preferably 850-1100 C., more preferably 900-1050 C., and most preferably 920-970 C.

(22) The fluidizing gas is preferably hydrogen. The reaction gas is injected into the fluidized bed through one or more nozzles. The local gas velocities at the outlet of the nozzles are preferably from 0.5 to 200 m/s.

(23) The concentration of the silicon-containing reaction gas is preferably from 10 mol % to 50 mol %, more preferably from 15 mol % to 40 mol %, based on the total amount of gas flowing through the fluidized bed.

(24) The concentration of the silicon-reaction gas in the reaction gas nozzles is preferably from 20 mol % to 80 mol %, particularly preferably from 30 mol % to 60 mol %, based on the total amount of gas flowing through the reaction gas nozzles. Trichlorosilane is preferably used as silicon-containing reaction gas.

(25) The reactor pressure is in the range from 0 to 7 bar gauge, preferably in the range from 0.5 to 4.5 bar gauge.

(26) In the case of a reactor having a diameter of, for example, 400 mm, the mass flow of trichlorosilane is preferably from 200 to 400 kg/h. The volume flow of hydrogen is preferably from 100 to 300 standard m.sup.3/h. For larger reactors, greater amounts of TCS and H.sub.2 are preferred.

(27) It will be clear to a person skilled in the art that some process parameters are ideally selected as a function of the reactor size. Reactor heating power, rate of introduction of seed particles and the bed weight are preferably higher than the abovementioned values in the case of relatively large reactors, e.g. in the case of a reactor having a diameter of 800 mm.

(28) To demonstrate this clearly, the ranges of the operating data normalized to the reactor cross-sectional area in which the process described in the context of the present invention is applicable are shown below.

(29) The specific mass flow of trichlorosilane is preferably 1600-5500 kg/(h*m.sup.2).

(30) The specific volume flow of hydrogen is preferably 800-4000 standard m.sup.3/(h*m.sup.2).

(31) The specific bed weight is preferably 800-2000 kg/m.sup.2.

(32) The specific rate of introduction of seed particles is preferably 8-25 kg/(h*m.sup.2).

(33) The specific reactor heating power is preferably 800-3000 kW/m.sup.2.

(34) The average diameter of the silicon particles (seed particles) is preferably at least 400 m.

(35) The granular polysilicon preferably has particle sizes of 150-10,000 m, with a mass-based median value of a particle size distribution being 850-2000 m.

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