JPS627620A - Reaction tube for polycrystalline silicon production - Google Patents

Abstract

PURPOSE:To obtain a reaction tube for producing polycrystalline silicon which is strong in thermal stress and is improved in the productivity by covering a base body made of ceramic with high-purity dense SiC. CONSTITUTION:The titled reaction tube is obtained by covering the surface of a base body made of ceramic with dense SiC which is high purity and zero in the porosity. As a covering method of the above-mentioned SiC film, an organic Si compd. is decomposed in a vapor phase and SiC generated therein may be stuck on the inside wall of a tube consisting of the base body made of ceramic. As the above-mentioned organic Si compd., the following organosilane or organopolysilane is exampled which is shown in a formula [at least one piece of R is H and the other is CH3, C2H5, C3H7, phenyl and vinyl, (n) is 1-4] and has H joined to at least one piece of Si in a molecule and does not contain SiX group (X is halogen atom and O). The heating of these organic Si compds. in the reactor is preferably performed at 700-1,400 deg.C to decompose them in the vapor phase.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a reaction tube for producing polycrystalline silicon, particularly a reaction tube made of ceramic used when producing granular polycrystalline silicon using a fluidized bed reactor. This relates to reaction tubes.

(Prior technology) High purity polycrystalline silicon for semiconductors is manufactured by heating a thin rod of high purity polycrystalline silicon placed in a Pel jar with electricity.
It is made using the so-called Siemens method, in which a mixed gas of chlorosilane and hydrogen is supplied and reacted, and silicon is deposited on the surface of this thin silicon rod.The Pel jar is made of quartz. .

On the other hand, for the production of polycrystalline silicon, a mixed gas of chlorosilane and hydrogen is supplied into a fluidized bed reactor, and the silicon generated in this reaction is converted into silicon particles (seeds) held in a fluidized state in the reactor. A method has also been proposed in which silicon is deposited on the surface of particles (see Japanese Patent Publication No. 35-18555), and this method is said to be particularly suitable for producing silicon as photoelectric conversion elements such as solar cells. The fluidized bed reaction tube used here is also made of quartz.

However, the quartz bell jar used in the Siemens method is susceptible to thermal stress, and if silicon with a different coefficient of thermal expansion is deposited on its inner wall, it may break or crack during cooling. A method is used to prevent silicon precipitation by cooling from the outside. This therefore has the disadvantage that the thermal efficiency of the reaction is poor and the resulting silicon is expensive. Also,
Like the Pelger used in the Siemens process mentioned above, the quartz reactor used in the fluidized bed reaction method is also susceptible to thermal stress, and has the problem of breakage and cracking during cooling due to the precipitation of silicon on the inner wall of the tube. Since the method uses external heating, the reaction tube cannot be cooled from the outside.
In order to remove silicon deposited on the tube wall, it is necessary to clean the inside of the tube with a chemical solution, which also has the disadvantage of reducing productivity.

(Structure of the Invention) The present invention relates to a reaction tube for producing polycrystalline silicon that solves the above-mentioned disadvantages.The present invention relates to a reaction tube for producing polycrystalline silicon that solves the above-mentioned disadvantages. It is characterized by being coated with a plain material.

In other words, the present inventors conducted various studies on reaction tubes mainly used for producing silicon granules by the rotating bed method, and found that when the surface of a ceramic substrate is coated with silicon carbide, silicon carbide has less thermal stress than quartz. For reasons unknown, silicon precipitation on the coated surface disappears or becomes extremely small, eliminating the need for chemical cleaning. In addition to discovering that this highly pure and dense silicon carbide coating is best carried out by a gas phase pyrolysis reaction of an organosilicon compound containing a hydrogen atom bonded to a silicon atom in its molecule. After confirming this, the present invention was completed.

The reaction tube substrate of the present invention may be made of any heat-resistant material, including ceramics such as carbon, silicon carbide, and silicon nitride, or composites thereof. Although ceramics with excellent strength are preferably used, these materials must have zero porosity and high purity when used alone as a fluidized bed reaction tube. Moreover, the coefficient of thermal expansion is similar to that of silicon, and carbon alone poses problems in terms of porosity, coefficient of thermal expansion, and purity compared to other ceramic substrates. This problem can be solved by coating with a silicon film, and it has been discovered that this prevents silicon from depositing on the vessel wall. The organosilicon compound may be vapor-phase pyrolyzed on the inner wall of the tube, and the generated silicon carbide may be attached thereto. One is a hydrogen atom, the others are monovalent hydrocarbon groups selected from methyl, ethyl, propyl, phenyl, and vinyl groups, n is a positive number from 1 to 4)
Organosilanes or organopolysilanes having at least one hydrogen atom bonded to a silicon atom in the molecule represented by, but not containing an SiX group (X is a halogen atom or an oxygen atom) or the general formula (where R is Same as above. ul is a methylene group, ethylene group, or phenylene group, and the integer is a positive number from 1 to 2). These may be used alone, or as a mixture of two or more of them. Furthermore, these are dimethylpolysilanes represented by the formula (X is a positive number) heated at 350°C.
Methylhydrodiene silanes mainly consisting of dimethylpolysilane obtained by thermal decomposition at the above temperature are preferred. These organosilicon compounds can be produced by conventionally known methods, but since they can be easily purified to a high degree through a distillation process, silicon carbide coatings obtained by vapor phase pyrolysis are also extremely difficult to obtain. This gives the advantage of high purity.

The coating of silicon carbide with this organosilicon compound is performed by applying the above-mentioned organosilicon compound to the silicon carbide film reactor using hydrogen gas, nitrogen, or helium.

An inert gas such as argon or a mixture thereof may be used as a carrier gas, and the organic gas may be charged into the reactor and thermally decomposed in the reactor. Since the rate of thermal decomposition of silicon compounds is slow, a temperature of 700° C. or higher is required to obtain the desired silicon carbide in good yield. However, if the temperature is 1,400℃ or higher, the growth rate of silicon carbide crystals will improve, but the crystal growth will become uneven and the adhesion to the reactor wall will deteriorate.
, preferably in the range of 900-1,200°C.Also, since this organosilicon compound does not contain a halogen atom, Since there is no generation of chlorine-hydrogen gas, there is no need for exhaust gas treatment equipment.

The surface of the reaction tube for producing polycrystalline silicon of the present invention obtained as described above is coated with a silicon carbide film whose thermal expansion coefficient is close to that of silicon, which prevents silicon from depositing on the vessel wall. Therefore, the thickness of this silicon carbide film must be in the range of 50 to 500 g.m from the viewpoint of strength. Note that the ceramic substrate coated with this silicon carbide film can also be used as various high-temperature reaction chambers.

Next, examples of the present invention will be given, and parts in each example indicate parts by weight.

Example 1 35 parts of α-type silicon carbide with a particle size of 5 to 10 JLm, 17 parts of natural graphite with a particle size of 3 to loom, 10 parts of silicone resin
and 60 parts of toluene were mixed in a ball mill, the toluene was removed by volatilization, and the resulting powder was molded using a rubber press at a pressure of 1.0 t/cm' to form a 70 mmφX
A tubular molded body of 5 mm x 500 mm was made, then heated at 1,000°C for 30 minutes to decompose the binder and form a calcined body, and then the molten silicon at 1,000°C was reacted with the carbon in the raw material. It was made into a molded body.

Next, after thoroughly cleaning the inner surface of the silicon carbide tube (density 3.10) with a diameter of 70 mm, a length of 500 mm, and a wall thickness of 5 mm obtained above, it was poured into a quartz tube. 0
After heating to 50°C, the pressure was reduced, and a mixed gas of equal amounts of hydrogen gas and argon gas containing 5% by volume of tetramethyldisilane was introduced at a rate of 200 c, c-7 min for 4 hours to produce tetramethyldisilane. When the silicon carbide tube was taken out after pyrolysis and cooling, it was found that the inner surface of the tube was uniformly coated with β-type silicon carbide to a thickness of about 30 μm.

This was then repeatedly heated in air to 1,300° C., but the coating did not change, confirming that it was strong, uniform, and free of pinholes.

Next, using the silicon carbide tube thus obtained as a reaction tube, a fluidized bed reactor was assembled, with a reaction gas outlet and a silicon particle inlet in the upper part, and a gas inlet and a silicon granule outlet in the lower part. Particle size 350-500p in reactor
500 g of silicon fine particles of 500 m were charged, and trichlorosilane gas was fed into the reactor at 5 kg/hour and hydrogen gas at 2.4 kg/hour from the gas inlet at the bottom, and the inside of the reactor was heated to 1. , 200°C was maintained to carry out a fluidized reaction, and during this reaction, 10g of silicon fine particles were added at 7 hours, and the operation was continued for 48 hours.
11,326 g of high-purity silicon granules of 2.3 mm in diameter were obtained, and when the inner wall of the reaction tube was examined after the reaction was completed, no silicon was observed to adhere to them, and no change was observed in the silicon carbide coating layer. I couldn't.

However, for comparison, the diameter is 70 mm and the length is l.

A fluidized bed reactor was assembled using a 500 mm, 5 mm thick quartz tube and a reactive sintered silicon carbide tube that was not coated with silicon carbide, and a fluidized reaction was carried out under the same conditions as above. In order to remove this, it was necessary to flow a mixed gas of cyclized hydrogen and hydrogen gas for 7 hours. When the reactor was cooled without removing the silicon, the reaction tube was destroyed.

In addition, when the thermal decomposition reaction of tetramethyldisilane was carried out under the same conditions except that the reaction for forming the silicon carbide coating on the silicon carbide tube was set at 1,400°C, in this case, the temperature was too high. Therefore, the silicon carbide coating formed inside the silicon carbide pipe is non-uniform with large β-type silicon carbide crystals.

Irregularities were observed on its surface.

Example 2 82.5 parts of silicon nitride with a particle size of 0.3 to lILm, 11 parts of aluminum nitride. Oia! , 8.5 parts of yttrium oxide, 15 parts of paraffin, and 200 parts of toluene were mixed in a ball mill, and the toluene was evaporated to dryness. ') and
This was covered with a silicon nitride powder bundle and sintered at 1,750°C for 30 minutes.

Next, this sintered body (density 3.18 g/c, c,) was cut into a size of 70 mm φ x 5 mm x 500 mm using a grinder.
The inner surface of this reaction tube was coated with a silicon carbide film under the same conditions as in Example 1, and a flow reaction was carried out using this reaction tube in the same manner as in Example 1. 1 Silicon was deposited on the inner surface of the reaction tube. No cracks were observed, and no cracks occurred even after cooling.For comparison, when a tube without silicon carbide coating was used, the purity of the product obtained was poor, and there was a small amount of silicon on the inner surface of the tube. Precipitation was observed.

Claims (1)

[Claims] 1. A reaction tube for producing polycrystalline silicon, characterized in that the surface of a ceramic substrate is coated with dense silicon carbide having high purity and substantially zero porosity. . 2. Dense silicon carbide coating has at least one hydrogen atom bonded to silicon atom in the molecule, but SiX bond (
The reaction for producing polycrystalline silicon according to claim 1, wherein tube.