Method for producing granular polysilicon

Abstract

Segregation of silicon granules in the fluidized bed production of polycrystalline silicon is achieved by successively transferring granular polycrystalline silicon through a plurality of vessels designed for funnel flow of granular material. The transfers may occur prior to introduction of feed particles into the reactor, or the enlarged granules from the reactor may be thus transferred to improve product size uniformity.

Claims

1. A process for avoiding particle segregation by size in the handling of granular polysilicon after production of the granular polysilicon by a process comprising: fluidizing silicon particles by means of a gas flow in a fluidized bed which is heated to a temperature of 850-1100 C. by means of a heating apparatus, adding a silicon-containing reaction gas and depositing silicon on the silicon particles, and withdrawing granular polysilicon from the fluidized bed reactor, and before packing the granular polysilicon, transferring the granular polysilicon by means of a plurality of successive conical vessels designed for funnel flow, and wherein at least one of the plurality of vessels comprises filling distributor cones consisting of silicon of coated or lined with silicon, installed in at least one of the plurality of vessels.

2. The process of claim 1, wherein at least one of the plurality of vessels comprise an emptying funnel, an emptying tube or a binsert, each of which consists of silicon or is coated or lined with silicon.

3. The process of claim 1, wherein the silicon particles on which silicon is deposited in order to produce granular polysilicon are produced by grinding silicon and classifying by screening, wherein the silicon particles are transferred by means of a plurality of successive vessels designed for funnel flow between production of the silicon particles and supply of the silicon particles in the reactor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows, in schematic form, the transfer operations from the withdrawal of the granular polysilicon from the reactor up to the packing thereof.

(2) FIG. 2 shows the particle size of the granules as a function of transfer volume before the first transfer.

(3) FIG. 3 shows the particle size of the granules as a function of transfer volume after the first transfer.

(4) FIG. 4 shows the particle size of the granules as a function of transfer volume after the second transfer.

(5) FIG. 5 shows the particle size of the granules as a function of transfer volume after the third transfer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) The invention envisages that the bulk Si material is transferred on more than one occasion. It is preferably transferred on at least three occasions.

(7) The vessels used are designed for funnel flow. This is understood to mean that, in the course of emptying, the granular polysilicon in the center of the vesseli.e. the finesis the first to be drawn off, while predominantly coarse material is discharged toward the end of the emptying. As a result, in contrast to transport containers with mass flow, it is possible to avoid wall abrasion and hence contamination of the granular polysilicon.

(8) After a particular number of transfer operations, it is surprising that only slight particle segregation is observed over the production batch.

(9) A further reduction in particle segregation is preferably accomplished by means of filling distributor cones which are installed in the inlet of the vessels. Such internals are designed to give very low contamination, preferably in silicon.

(10) It has been found that, surprisingly, significant segregation is present after the first filling, as described in the prior art. However, if the bulk material is transferred on further occasions, backmixing takes place, so as to result in homogenization of the particle size distribution after a particular number of transfer steps. The result is a granular polysilicon having a homogeneous particle size distribution over the entire production batch, with a deviation in the median particle size of an arbitrary sample from the batch of not more than 30% from the mean particle size of the entire batch.

(11) If required, for a further reduction in particle segregation, filling distributor cones are installed in one or more vessels. These internals are designed to give very low contamination, and are preferably made of silicon.

(12) Preference is also given to the use of alternative internals such as emptying funnels, emptying tubes and binserts in order to further minimize segregation by backmixing.

(13) The internals are manufactured from low-contamination materials such as silicon or are lined or coated with these materials.

(14) The number of transfer operations in which maximum homogeneity of backmixing of the batch is established depends on the particle size distribution of the bulk material and the outflow characteristics of the vessel. The optimal number of transfer steps is determined empirically.

(15) The best way of empirically determining the transfer steps is by means of a test setup composed of two transport containers arranged one on top of the other. The containers are connected by means of a container emptying and filling station and a pipeline. Additionally installed in the pipeline is a sampling station which enables representative sampling.

(16) Before the first transfer step, the upper container is filled with a test material having a homogeneous particle size.

(17) During the transfer operation, samples are taken at regular intervals for the determination of the particle size. With the aid of the results of the particle size measurements, particle segregation is determined.

(18) The containers are exchanged before the next experiment: the full container is connected to the emptying station and the empty container to the filling station. Particle segregation is restarted.

(19) The experiments are repeated until there is maximum homogeneity of particle size over the entire batch.

(20) If required, the incorporation of a cone-shaped distributor into the vessels can reduce segregation further. In order that the high-purity Si product is not contaminated by these internals, the invention solves this problem with a very low-contamination design, preferably made from silicon. The incorporation of such a distributor reduces the formation of a large cone of bulk material and hence the segregation potential.

(21) Alternatively, the distributor cone can also be formed from the product itself, in that the formation of a cone of bulk material is enabled on a platform beneath the entry stub.

(22) Preferably, the silicon particles on which silicon is deposited in order to produce granular polysilicon are also transferred by means of vessels designed for funnel flow on more than one occasion between the production of the silicon particles by grinding and supply of the silicon particles to the reactor. These silicon particles too, i.e. the seed particles in the deposition process, have bulk material properties. For the process for producing granular polysilicon, it is advantageous when the seed particles have a homogeneous particle size distribution.

(23) Such a process for producing polycrystalline silicon granules in a fluidized bed reactor, comprising fluidization of silicon seed particles by means of a gas flow in a fluidized bed which is heated by means of a heating apparatus, with deposition of elemental silicon at the hot seed particle surfaces by means of pyrolysis through addition of a silicon-containing reaction gas, which gives rise to the polycrystalline silicon granules, can be operated continuously by removing particles that have grown in diameter as a result of deposition from the reactor and metering in fresh seed particles.

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

(25) For the fluidization of the seed particles, preference is given to using hydrogen.

(26) The reaction gas is injected into the fluidized bed by means of one or more nozzles.

(27) The local gas velocities at the exit of the nozzles are preferably 0.5 to 200 m/s.

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

(29) The concentration of the silicon-containing reaction gas in the reaction gas nozzles is, based on the total gas volume flowing through the reaction gas nozzles, preferably 20 mol % to 80 mol %, more preferably 30 mol % to 60 mol %. The silicon-containing reaction gas used is preferably trichlorosilane.

(30) The reactor pressure varies within the range from 0 to 7 bar gauge, preferably within the range of 0.5 to 4.5 bar gauge.

(31) In the case of a reactor having a diameter of, for example, 400 mm, the mass flow rate of trichlorosilane is preferably 200 to 400 kg/h.

(32) The volume flow rate of hydrogen is preferably 100 to 300 m.sup.3 (STP)/h.

(33) For larger reactors, higher amounts of TCS and H.sub.2 are preferred.

(34) The person skilled in the art will appreciate that some process parameters are ideally selected as a function of the reactor size. The reactor heating output, seed particle metering rate and bed weight are preferably higher than the aforementioned values in the case of larger reactors, for example in the case of a reactor of diameter 800 mm.

(35) In order to illustrate this, there follows a summary of the ranges of the operating data normalized to the cross-sectional reactor area in which the process described in the context of this invention is viable.

(36) The specific mass flow rate of trichlorosilane is preferably 1600-5500 kg/(h*m2).

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

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

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

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

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

(42) The granular polysilicon preferably has particle sizes of 150-10,000 m, where a mass-based median particle size distribution is 850-2000 m.

(43) 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.

EXAMPLE

(44) In the example, three transfer operations were conducted.

(45) FIG. 1 shows that the granules are withdrawn from the reactor 10 and transferred into a buffer vessel 11. They are then transferred into a transport container 12 and transported to the screening facility 13. They are then transferred again into a buffer vessel 14 and finally packed 15.

(46) The transfer from buffer vessel 11 to transport container 12 corresponds to the first transfer 1.

(47) The transfer from transport container to buffer vessel (via a screening facility) corresponds to the second transfer 2.

(48) The transfer from buffer vessel to the packing facility corresponds to the third transfer 3.

(49) FIGS. 2-5 show the particle segregation at the start (first buffer vessel after withdrawal from the reactor, FIG. 2) and after each of the three transfer operations.

(50) The segregation is illustrated using the plot of the particle parameters 10, 50 (median) and 90 as a function of the amount of bulk Si material drawn off.

(51) On the basis of the results found, the layers of coarse material (shaded with large symbols) and fine material (shaded with small symbols) are shown in FIG. 1.

(52) After the first transfer 1, significant particle segregation is present; see FIG. 3. The plot of the median (50) begins at 980 m. Up to a discharged amount of 320 kg, the value falls to 670 m. Thereafter, the value rises to 1300 m up to about 880 kg of the amount drawn off, before declining to about 720 m at a further 60 kg.

(53) After the second transfer 2, particle segregation is already less marked; see FIG. 4. The median falls from initially almost 840 to 650 m after about 160 kg of the amount has been drawn off. Within the 300 kg which then follow, the median rises again very significantly to more than 1100 m. From 460 kg, the median decreases to about 790 m. In the remaining 150 kg, the median rises once again to more than 920 m.

(54) After the third transfer 3, a relatively homogeneous particle distribution over the entire batch is present; see FIG. 5. After 100 kg, the median declines only slightly from 820 to 740 m, after which there is a rise in the 220 kg which follow to about 1000 m. By 500 kg, the value drops to 900 m and, by 700 kg, rises to more than 1100 m. In the remaining volume of Si, it decreases again and reaches a value of about 700 m.

(55) Through the funnel flow in the vessels, in which the material in the center of the vessel is the first to flow out, the upper coarse layer comes in about the middle of the transfer operation out of the vessel. Since the coarse and fine particles mix again, there is apparently partial backmixing of the production batch.