FI82232B - Process for preparing silicon halide - Google Patents

Description

t 82232

This invention relates to a process for the preparation of a silicon halide by reduction of a silicon tetrahalide in a plasma flame in a shielding gas atmosphere. In particular, this invention relates to a process for the preparation of reactive silicon fluoride and its homologues at high temperature using plasma technology.

Silicon fluoride and its homologues are useful intermediates which contain silicon in a highly reactive form and from which various silicon-based products can be prepared by mild heat treatment, such as high purity silicon for photocells and silicon chips, for example, and silicon frames.

U.S. Pat. No. 4,070,444 discloses a process for producing pure semiconductor grade silicon in which metallurgical silicon and silicon trifluoride are reacted at an elevated temperature above 1000 ° C and under reduced pressure to form silicon fluoride gas. However, since silicon fluoride gas tends to discharge back to its starting material at lower temperatures, in practice the reaction mixture must be quenched rapidly to a low temperature, thus kinetically preventing decomposition. Upon cooling to at least -70 ° C, the product polymerizes and thus becomes solid. A limitation of this method is its low yield due to the temperature limitation associated with resistance heating. The yield can be increased by lowering the reaction pressure, but then the production capacity correspondingly decreases in terms of mass flow.

The use of plasma in the treatment of silicon-containing materials is also known. Thus, for example, U.S. Patent Nos. 4,309,259, 4,377,564 and 4,439,410 disclose the reduction of silicon-containing substances in plasma to produce silicon and its compounds.

U.S. Patent No. 2,82232 to U.S. Patent No. 4,309,259 discloses a process for hydrogenating silicon tetrachloride in plasma using high pressure plasma to react hydrogen and silicon tetrachloride to trichlorosilane and other hydrogenated silicon chlorides.

U.S. Patent No. 4,377,564, on the other hand, discloses a process for producing silicon by developing a plasma in a gas stream containing a silicon compound so that the silicon compound is reduced or decomposed into silicon. The silicon compound may be silicon tetrachloride and, for example, hydrogen may be used as a reducing agent. Methods and devices for plasma generation are described in detail in this publication.

U.S. Patent No. 4,439,410 prepares silicon from a silica-containing powder by spraying it together with a reducing agent into a gas plasma together with a carrier gas. The mixture is then introduced into a reaction chamber surrounded by a solid reducing agent, where the yeast melts and is reduced to liquid silicon. Carbon has been used as a reducing agent.

It is therefore an object of the present invention to provide an even simpler and more economical process for the preparation of a silicon halide by reducing a silicon tetrahalide in a plasma flame under an inert gas atmosphere. The present invention is characterized in that, instead of using a reducing agent foreign to the intermediate or final product, the exact substance to be purified is now used, namely finely divided silicon. By further using the silicon halide obtained as an intermediate by liberating the silicon by mild heat treatment, a silicon tetrahalide is obtained at the same time, which can thus be recovered and reused in the process according to the invention. An advantage of the present invention is that no separate reducing agent is required, but a purifier can be used as the reducing agent

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3,822,232 finely divided silicon and silicon halide used as a second reagent can be recycled from the final product.

In addition, it has been found that when the silicon tetrahalide is reduced to plasma in a sea gas atmosphere at a high temperature above about 2000 ° C, it is possible to operate at normal pressure and still achieve a high reaction rate.

To prevent the resulting silicon halide from reacting back, it is preferably cooled very rapidly at a temperature of 1200 ° C to 200 ° C. Cooling should be carried out within about 1-2 seconds. As the reducing agent, a metallurgical silicon powder having a particle size of less than 10 μm is preferably used, and silicon trifluoride is fed to the plasma plate, and silicon is preferably present in a molar ratio of 0.8 to 1.5.

In the process of the present invention, a reactive silicon fluoride compound is thus prepared by feeding finely divided silicon powder and gaseous silicon trifluoride to a high-frequency excited argon plasma. The process is characterized by a substantially higher reaction temperature than previous processes, resulting in high yields. The reaction can be carried out at normal pressure, whereby a higher capacity per net power used is achieved compared to previous methods. Furthermore, due to the mode of operation of the plasma, the production is continuous and thus process-technically useful.

The silicon halide prepared by the process of the invention is a useful intermediate in the preparation of pure semiconductor silicon or silicon frames. The resulting silicon halide is a polymer that precipitates on the reactor walls as a rubbery paste and should be stored in a shielding gas atmosphere as it loses combustion in the air. The silicon halide polymer prepared by the process of the invention can be represented by the general formula Sin + jF ^ n + 2> where n is an integer (0, 1, 2, 3 ...).

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However, for simplicity, the product is described herein as monomer S1F2.

The invention will now be described in more detail with reference to the accompanying drawings, in which Figure 1 shows the reaction slurry of the reaction SiF1 + Si = 2S1F2 as a function of pressure at temperatures and Figure 2 shows a cross-sectional view of a plasma generator suitable for carrying out the process according to the invention.

As seen in Figure 1, high temperature and reduced pressure favor silicon fluoride formation. It is also seen that o if the reaction is carried out above 2000 C, it is possible to operate even at normal pressure (dashed range).

The apparatus for producing silicon halide is shown in Fig. 2, in which the water-cooled reactor is generally indicated by reference numeral 8. At its upper end there is a feed ring 6 through which silicon rafluoride is fed via line 5. Above the supply ring 6, on the other hand, is a water-cooled induction torch <3 MHz), generally indicated by reference numeral 7. At the upper end of the induction torch 7 there is a central water-cooled supply pipe 2 for the silicon powder 4, both of which are argon.

At the lower end of the water-cooled reactor 8 there is an outlet line 15 for gases with a pressure gauge 13 and a pump 14. Also inside the water-cooled reactor 8 a sampling pipe 9 protrudes vertically upwards through it.

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The length of the water-cooled reactor 8 is 530 mm and the inner diameter is 125 mm. The water-cooled sampling tube 9 made of stainless steel, on the other hand, has a length of 475 mm and an inner diameter of 3.2 mm and is lined with a Pyrex glass tube. The water-cooled supply pipe 2 of silicon powder is likewise made of stainless steel and the filter 10 contains steel wool.

Silicon difluoride polymer precipitates in the lower part of the inner wall of the water-cooled reactor 8 and can be removed by scraping. The apparatus shown in Fig. 2 easily achieves a reaction temperature of more than 2000 ° C, in which case the reaction rate at normal pressure is practically 100%. In addition, the process can be carried out continuously, and when the device was used, it was found that the silicon powder fed below the plasma torch provided with argon was almost completely evaporated.

The longitudinal temperature profile of the water-cooled reactor 8 is adjusted so that silicon fluoride gas is rapidly cooled to 1200-200 ° C within 1-2 seconds, thereby preventing silicon fluoride reaction and precipitating polymeric silicon fluoride on the reactor walls. .

The invention is described in more detail below by means of examples.

Example 1

The apparatus of Figure 2 was used to synthesize silicon fluoride from silicon powder and silicon trifluoride. Argon 3 used as plasma gas was fed at a rate of 66 l / min to the center of the induction torch 7. The water-cooled quartz wall of the torch was protected with argon shielding gas 4, which was fed at 88 l / min.

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Prior to even the ignition of the plasma, the mass spectrometer 12 was calibrated by feeding 1, 2 and 5 * silicon trifluoride 5 to the argon stream through the feed ring 6. The mass spectrometer was of the quadrupole type and was operated at an undervoltage of 19 eV in order to avoid the formation of fragment SiF2 from SiF2, which interferes with the analysis of SiF2 formed in the reaction under study. In the Multiple ion monitoring mode, the following mass numbers were recorded: m / z Ion 20 Ar / 2 40 Ar

47 S i F

66 SiF2 85 SiF3 104 SiF4

The gas sample was sucked through the sampling tube 9 to the mass spectrometer 12 carried by the membrane pump 11 by means of the pressure difference generated by the front pump of the ground spectrometer and the turbomolecular pump (not shown). The quench rate of the gas sample in the sampling tube 9 from reaction to below 200 ° C was sufficient to prevent the reverse reaction.

The SiF2 content of the gas mixture to be analyzed was calculated by comparing the mass spectrum with that of a known calibration mixture (the numbers represent the intensity of the mass in question) using the following formula: <66 mixture / 66 calibr>

SiF2 ~ pit. (*) ------------------------------------ x 100% * ®®mixture ^ ®® caliber + * 04eeos / 104kalibr *

The plasma ignited when water was sucked into the gas-tight equipment with a ring pump 14 at a pressure of 0.25 bar. The plate generator power was set to 23 kW. SiF ^ flow

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7,82232 was weighed to 13.9 g per minute, corresponding to 3 l / min and a volume fraction of 1.9% of the total gas flow.

Silicon powder 1 (d50 = 5 .mu.m) was pneumatically introduced onto the plasma reactor and fed to the center of the plasma torch by feed tube 2. The feed mass flow was varied on both sides of the SiF4 / Si stoichiometry by first feeding 2.5 g / min and gas after stabilization (about 5 min) 4.5 g / mm.

The results calculated by the calculation formula were as follows:

Si-feed Molar SiF2-pit of the feed

g / min ratio (SiF 3 / Si) (%) 2.5 1.5 62 4.5 0.8 70

The result can be further improved, for example, by increasing the concentration of the silicon-containing components in the plasma gas mixture, whereby the probability of collision of the ions causing the reaction increases. Optimization of circuit acanth input technology also improves the result.

Claims (5)

1. A process for the preparation of reactive silicon halide and homologues thereof by reduction of silica tetrahalide in a flame flame in a protective atmosphere, characterized in that finely divided silicon is used in the first 1.1. Process according to claim 1, characterized in that the reduction is carried out in min. 2000 C and under substantially normal pressure. 3. A process according to claim 1 or 2, characterized in that the obtained silica halide or its homolog is rapidly cooled in the temperature range 1200-200 C, which is preferably 1-2 seconds. 4. A process according to any of the preceding claims, characterized in that, if a reducing agent is used, silicon powder used is equal to or greater than 10 µm. Process according to any of the preceding claims, characterized in that the tila flame is measured in silicon tetrafluoride and silicon in the molar ratio of about 0.8-1.5.