JPS63225516A - Production of high-purity granular silicon - Google Patents

Abstract

PURPOSE:To obtain the dense silicon having a low void coefficient and a smooth outer surface, by evenly supplying a fluidization gas to the horizontal cross section of a fluidized-bed reactor packed with silicon crystal grains to fluidize the grains, and supplying silicon hydrides, etc., from the center part. CONSTITUTION:Silicon crystal grains are packed as seeds from a line 4 into the fluidized-bed reactor 6 made of the SiC, quartz, Si3N4, etc., with the contact part with the grains and gases coated with a high-purity silicon layer. A fluidization gas consisting of gaseous H2 and/or an inert gas such as Ar is introduced from a line 1 at the >=1.2 times supply velocity for the minimum fluidization velocity, and evenly supplied through a gas diffusing plate 7 to the horizontal cross section vertical to the gravitational direction to fluidize the grains. The grains are heated to 550-1,000 deg.C by a heater 10, silicon hydrides such as monosilane and/or disilane or the hydrides and gaseous H2 and/or an inert gas are supplied from a line 2 at the center part at the velocity higher than the terminal velocity of the largest silicon crystal grain, a reaction is initiated, and the titled silicon having 500-1,500mu grain diameter is obtained.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method for producing high-purity granular silicon, and more specifically, it can be melt-processed to form polycrystalline silicon or monocrystalline silicon for use in solar cells and other products. The present invention relates to a method for producing high-purity granular silicon used as a raw material for semiconductor devices.

(Prior Art) Conventionally, as a method for manufacturing high-purity polycrystalline silicon, a mixed gas of chlorosilanes and hydrogen or monosilane gas is supplied to a Pelger type reactor, and silicon is deposited and grown on rod-shaped silicon heated with electricity. (hereinafter referred to as Pelger reaction method) has been used industrially. Although high-purity silicon can be easily produced using this method, since rod-shaped silicon is used, the surface area per unit reaction volume of the rod-shaped silicon, which is the reaction surface, is small and productivity is low. Heat radiation from the Pelger reactor surface is large. Power consumption is large.

Since the silicon product is rod-shaped, it has to be manufactured in batches, resulting in poor manufacturing efficiency, and it also has various drawbacks, such as the need to crush it when melting it into a single crystal.

In recent years, research and development efforts have been actively conducted to develop new methods for producing inexpensive, high-purity polycrystalline silicon that eliminates the various drawbacks of these conventional methods. One of the typical methods is to apply silicon to the surface of silicon crystal particles maintained in a fluidized state using hydrogen gas or an inert gas and a precursor gas such as chlorosilane gas or monosilane gas through a reduction reaction or thermal decomposition reaction of the precursor. There is a fluidized bed reaction method for depositing silicon crystal particles to grow silicon crystal particles. For example, this method is shown in US Pat. No. 3,012,861 and US Pat. No. 3,012,862. According to this method, since the reaction surface is made of granular silicon, the surface area per unit reaction volume is significantly increased and the productivity is significantly improved compared to the conventional Pelger reaction method. Furthermore, by continuously supplying small-sized silicon seed particles and continuously extracting grown large-sized silicon particles, continuous operation becomes possible and production efficiency is significantly improved. Furthermore, since the produced silicon is granular, it has the advantage that when it is melted for single crystallization, it can be used as is without requiring a crushing step that may cause contamination. As described above, the production of granular silicon by the fluidized bed reaction method is expected to have many advantages, and therefore various companies are actively researching and developing it, and numerous patent applications have been filed.

(Problems to be Solved by the Invention) As mentioned above, the production method of granular silicon using the fluidized bed reaction method has many advantages compared to the already industrialized Pelger reaction method, so it is possible to produce polycrystalline silicon at low cost. expected as a law.

The present inventors conducted research to develop a method for producing granular silicon using a fluidized bed reaction method using silicon hydride as a precursor gas, and found that depending on the production conditions, the silicon layer deposited on silicon crystal particles becomes porous. I discovered that it can become something. When observing the particle cross section with a scanning electron microscope, it is found that the particle size ranges from several ILm to several 1O.
Numerous pores of μm size were observed. The condition was particularly severe near the outer surface of this particle. A close examination of the inner surface of the pore reveals that it is covered with numerous microparticles ranging from submicron to about 2 pm in size. If the product is manufactured in this state, the following problems may occur.

(1) Since the product has a large surface area per unit weight (hereinafter referred to as specific surface area), when the product is stored, substances present in the atmospheric gas are adsorbed to a large extent, resulting in contamination of the product.

(2) If the product is stored in an oxygen atmosphere such as in the air, the surface area that comes into contact with oxygen will be oxidized, so the larger the specific surface area, the more oxygen will be taken into the product.

(3) The mechanical strength and abrasion resistance of the product particles decrease.
Cracks occur when handled, and the generation of fine powder due to wear increases (
Product contamination due to l) and (2) increases.

When this polycrystalline silicon product is melt-processed to produce polycrystals or single crystals, various problems occur, such as not being able to obtain high quality products or poor product yields.

In view of the fact that solving this problem is a very important issue when manufacturing high-purity granular silicon using a fluidized bed, the present inventors conducted extensive research and arrived at the present invention.

(Means for Solving the Problems) That is, the present invention deposits silicon on the surface of silicon crystal particles while maintaining them in a fluid state and grows the silicon crystal particles using hydrogen gas or/and an inert gas. Gas is supplied evenly to the cross section perpendicular to the direction of gravity to maintain the silicon crystal particles in a fluid state, and silicon hydride or a mixed gas of silicon hydride and hydrogen gas or/and inert gas is supplied from the center. The present invention provides a method for producing high-purity granular silicon characterized by the following.

By the method of the present invention, it is possible to produce granular silicon with fewer pores in the deposited silicon layer and with a smooth outer surface.

In the present invention, when supplying silicon hydride or a mixed gas of silicon hydride and hydrogen gas or/and inert gas from the center, the hydrogen gas or/and inert gas is distributed evenly in a cross section perpendicular to the direction of gravity. The reason why the silicon crystal particles are maintained in a fluidized state by supplying them is to maintain the entire reaction system in a stable fluidized state and to facilitate the circulation of the particles. If this is not done, particles outside the jet phase will solidify together, resulting in an inoperable operation. The supply rate of hydrogen gas or inert gas is at least 1.2 times the minimum fluidization rate.

In the present invention, it is preferable that the velocity of the silicon hydride or the mixed gas of silicon hydride and hydrogen gas or/and inert gas supplied from the center at the blowing part is equal to or higher than the terminal sedimentation velocity of the maximum silicon crystal particle.

The terminal sedimentation velocity of the largest silicon crystal particle will be explained below.

The silicon crystal particles in a fluidized bed reactor generally have a particle size distribution. For example, when high-purity granular silicon is continuously produced using one fluidized bed reactor, the particle size distribution will be from the seed crystal particle size supplied to the product crystal particle size extracted. The final sedimentation velocity is the final sedimentation velocity of the silicon crystal particles under gravity under a stationary state of gas supplied from the center, and its maximum value is the maximum final sedimentation velocity of the silicon crystal particles. In other words, when comparing particles having the same shape factor, the final sedimentation velocity is that of the largest particle.

By supplying gas from the center at a speed higher than the terminal settling velocity of the largest silicon crystal particles, a jet phase is formed in the center, and particles with the largest size are carried by the airflow, causing violent collisions between the particles and the gas. contact occurs. Vigorous collision between particles has the effect of peeling off the weak porous layer adhering to one particle and depositing only a dense layer.

The reaction mechanism for depositing silicon onto particles by thermally decomposing silicon hydride is thought to basically consist of two routes. One is a reaction in which silicon is directly deposited from the gas phase on the particle surface (hereinafter abbreviated as particle surface reaction). This reaction forms a dense deposited silicon layer without pores. The other route is that the silicon fine particles generated by gas phase decomposition adhere to the particle surface and form a deposited silicon layer in cooperation with the particle surface reaction.The silicon fine particles that are not captured on the particle surface are discharged from the reactor. be done.

It is believed that the porous silicon deposit layer is formed by the latter route. If we closely observe the silicon fine particles discharged from the reactor, we can see that the primary particle size ranges from submicron to about 21 Lm, and the shape is a single particle or a floc of several to dozens of particles weakly aggregated. It has become. It is determined that the floc-like fine silicon particles adhere to the particle surface depending on the operating conditions of the fluidized bed, forming a porous silicon deposit layer. They realized that if they were prevented, a dense silicon deposit layer could be formed, and as a result of various experiments, they were able to arrive at the present invention. In other words, gas is supplied from the center at a speed higher than the terminal sedimentation velocity of the largest silicon crystal particle to form a jet phase in the center to cause intense particle collisions and remove the floc-like silicon particles attached to the surface of each particle. This is a method of breaking and peeling off to form a dense deposited layer.

If this method is adopted, the contact efficiency with silicon fine particles floating in the gas phase will further increase, and the silicon fine particles generated by gas phase decomposition can be captured on the particle surface at a stage when they have not increased significantly due to particle surface reactions, thus doubling the effect. . It is thought that a similar effect can be obtained by providing a strong stirring blade in the fluidized bed reactor and violently stirring the silicon crystal particles, but silicon may deposit on the stirring blade, making continuous operation impossible. Depending on the selected material, there is a risk of contamination of the silicon crystal particles due to infiltration of components from the stirring blade, which is not practical.

When the velocity of the gas supplied from the center is lower than the terminal sedimentation velocity of the largest silicon crystal particle, the effect rapidly decreases as the velocity decreases. This is interpreted to be because the disturbance state of the particles decreases significantly after reaching the terminal sedimentation velocity of the largest silicon crystal particle.

In addition, the largest silicon crystal particle here refers to the largest one among the product granular silicon produced after reaching a steady state.

The silicon hydride used in the present invention is monosilane, disilane, or a mixed gas thereof. Further, as the inert gas, both helium and argon can be used, but argon is preferable because it is inexpensive.

Embodiments of the present invention will be specifically described with reference to the drawings.

FIG. 1 is a schematic diagram of the configuration of an apparatus according to the present invention. 6 is a fluidized bed reactor, which is usually cylindrical in shape, but there are no particular restrictions on the shape, and it may be square.Also, an enlarged part may be provided at the top to prevent particles from flying out. I'll come. In addition, the fluidized bed reactor is made of silicon whose parts that come in contact with particles and gas are coated with a high-purity silicon layer to prevent product contamination.

silicon carbide, glassy carbon, quartz, or silicon nitride. Reference numeral 7 denotes a gas distribution plate which is cooled to a temperature below the decomposition temperature of the silicon hydride with a coolant such as cooling water in order to prevent silicon solids generated by thermal decomposition of the silicon hydride from being deposited on the gas distribution plate. As the gas distribution plate, a perforated plate made of metal such as stainless steel, a sintered plate, or a wire mesh can be most easily used, but it is preferable to cover the particle contact area with a high-purity silicon perforated plate to prevent product contamination. Also? :! A packed bed of I-purity silicon particles can also be used instead.

In the figure, lO is a heating heater used to heat the fluidized bed reactor to a predetermined temperature. Reference numeral 9 denotes a raw material gas supply nozzle, which is installed at the central axis of the fluidized bed reactor. FIG. 2 shows an example of the structure of the raw material gas supply nozzle 9 in an enlarged manner.

In the figure, 14 is a raw material gas conduit, and M is introduced from line 11.
Feed gas (silicon hydride) passes through. A blowout nozzle 18 having a hole 19 is attached to the tip of this raw material gas conduit. The raw material gas conduit 14 and the blow-off nozzle 18 are constructed so that they can be cooled with cooling water to prevent the silicon hydride from being decomposed in these parts. The example in FIG. 2 shows a structure in which cooling water is introduced from line 12 into cooling water introduction pipe 15 and discharged from line 13 for cooling.

A plurality of blowing nozzles 18 can be attached as necessary. Further, the direction in which the gas is ejected is not limited to the upward direction, and the blowing nozzle 20 may be inclined with respect to the source gas conduit 14 as shown in FIG. The portion of the raw material gas supply nozzle 9 that comes into contact with the particles is covered with a protection tube 17.

This protection tube 17 is made of the same material as the fluidized bed reactor, and is installed for the purpose of preventing product contamination. For the source gas conduit 14 and the cooling water conduit 15, metal materials such as stainless steel and copper are used because they are robust, have good thermal conductivity, are easy to work with, and are inexpensive. Although not shown in FIG. 2, it is convenient to fill a heat insulating material such as quartz wool between the protection tube 17 and the cooling water outlet tube 16 to reduce the amount of heat entering from the fluidized bed.

In FIG. 1, the tip of the raw material gas supply nozzle 9 is installed above the gas distribution plate 7. In order to improve the movement of particles, a position of 2 cm to 10 cm is preferable. 8 is a particle extraction tube made of the same material as the reactor.

In FIG. 1, l is a hydrogen gas or inert gas supply line, 2 is a raw material gas supply line, 3 is a reaction exhaust gas discharge line, 4 is a seed crystal silicon particle introduction line, and 5 is a supply line for hydrogen gas or inert gas.
is the line for taking out the product granular crystalline silicon particles.

Next, the manufacturing method of the present invention will be explained in more detail with reference to the drawings.

After filling the fluidized bed reactor 6 with seed crystal particles, the fluidized bed reactor 6 is heated using the heating heater 10 while supplying hydrogen gas and/or inert gas from the line 1 to fluidize the particles. When a predetermined temperature is reached, silicon hydride or a mixed gas of silicon hydride and hydrogen gas or/and inert gas is supplied from line 2 to start the reaction. In the present invention, the reaction temperature is usually 550°C to 1ooo''c, preferably 600°C to 900°C.

If the temperature is lower than 550°C, particles tend to solidify together, making it difficult to maintain a stable fluid state. Moreover, if the temperature exceeds 1000°C, the energy required for heating will increase, which is economically undesirable.Although there is no particular limitation on the 0 reaction pressure, in order to carry out the reaction easily, atmospheric pressure or higher is used, and preferably normal pressure to 5 atm. .

More pressure than that is undesirable as it increases equipment costs.

The gas supply rate entering from line 1 is required to be at least 1.2 times the minimum fluidization rate for the reasons mentioned above.

Further, the total amount of gas supplied from fin 1 and fin 2 is adjusted to be 2 to 10 times the minimum fluidization speed. If it is less than 2 times, particles tend to clump together, making it difficult to maintain a stable fluid state. Moreover, if the amount exceeds 10 times, the amount of fine powder generated increases, which is undesirable.
In order to reduce the amount, it is preferable that the gas velocity be as low as possible.The gas velocity supplied from line 2 is preferably adjusted to be equal to or higher than the terminal sedimentation velocity of the largest silicon crystal particle at the blowout section. There is no particular limit on the upper limit, but
If it is too large, there is a fear that the particles may crack due to collisions between particles. Generally, 1 to 100 times the terminal sedimentation velocity of the largest silicon crystal grain is preferred.

As the reaction progresses, the particles grow and the height of the particle layer increases, so the particles are extracted from line 5 to keep the height of the particle layer constant. When the target product particle size is reached, seed crystal particles are continuously supplied from line 4 while keeping the particle layer height constant.
Also, product particles are extracted from the inlet 5 and steady operation begins.

Reaction exhaust gas is discharged from line 3 through particle extraction pipe 8
When extracting particles through a particle, by passing hydrogen gas or inert gas in the opposite direction to the flow direction of the particles and adopting a known wind sieving method, only particles larger than the target particle size can be extracted. The lower limit of the particle size of the seed crystal particles is one that will not be blown away by the reaction exhaust gas under the operating conditions of the fluidized bed reactor. That is, particles having a terminal settling velocity exceeding the gas velocity at the top of the fluidized particle bed of the fluidized bed reactor are used. As the seed crystal particles, either crushed product silicon crystal particles or particles obtained by melt-spraying, cooling, and granulation can be used. The average particle size of the product is 500gm
A length between 15,004 m and 15,004 m is recommended.

(Effects of the Invention) By using the method of the present invention, it is possible to produce high-purity granular silicon that has a small void ratio, is dense, and has a smooth outer surface.

(Example) The present invention will be specifically described below based on Examples and Comparative Examples.

Examples 1 to 12 and Comparative Examples 1 to 3 Particulate silicon was manufactured using the reaction apparatus shown in FIG. The fluidized bed reactor 6 was manufactured using a reaction tube made of silicon carbide having an inner diameter of 40 mm and a height of 1000 mm, the inner surface of which was coated with high-purity silicon, except for the particle extraction tube 8. The raw material gas supply nozzle had the structure shown in FIG. 2, and the distance between the tip of the raw material gas supply nozzle and the gas distribution plate was 2 cm.

Further, the blowing speed was changed by replacing the raw material gas supply nozzle with a different nozzle diameter.

Particulate silicon was produced in a batch reaction using a fluidized bed reaction method. The manufacturing conditions are listed in Table 1. The symbols used in Table 1 are as follows.

ut: Terminal sedimentation velocity of the largest silicon crystal particle (cm/sec) uN: Gas blowing velocity from the nozzle (017 seconds) uD:
Gas velocity of the gas supplied from the gas distribution plate directly above the gas distribution plate (C-7 seconds) μ■f: Minimum fluidization speed of particles (c1/second) uR: Gas velocity directly above the particle layer (017 seconds) ) Please note that the particle size increases over time during manufacturing, so
Continuously increasing the gas supply rate from the nozzle and gas distribution plate, J /ut, u□ /umf, and H/u
vf was kept approximately constant.

When the surfaces and fractured surfaces of the silicon particles produced were observed using a scanning electron microscope, the surfaces of the particles produced in Examples 1 to 12 had very few irregularities, and the deposited silicon layer was smooth and dense to the inside. Against.

The surfaces of Comparative Examples 1 to 3 had numerous irregularities;
It was observed that the inside of the particle was porous. The pores (voids) existing in the deposited silicon layer can be determined by measuring the area of the deposited silicon layer and the area of the pores (voids) from a photograph of the fracture surface, and calculating the void ratio (%) = [Area of pores / Deposited silicon Layer area] x 100. The measurement results are shown in Table-1.

Comparative Examples 1 to 3 were produced with a smaller uN/dt and the particles were measured.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a manufacturing apparatus used in the method of the present invention, FIG. 2 is an explanatory diagram of a raw material gas supply nozzle, and FIG. 3 is an explanatory diagram of the other side of the raw material gas supply nozzle. "Explanation of symbols 1...Hydrogen gas or inert gas supply line 2...
Raw material gas supply line 3...Exhaust gas discharge line 4...Seed silicon particle introduction line 5...Product take-out line 6...Fluidized bed reactor 7...Gas distribution plate 8... - Particle extraction pipe 9... Raw material gas supply nozzle lO... Heating heater 14... Raw material gas conduit 15... Cooling water introduction pipe 16... Cooling water introduction pipe 17... Protection tube 18. ...Blowout nozzle 19...hole Patent applicant Mitsui Toatsu Chemical Co., Ltd. Figure 1 Figure 2 Figure 3

Claims (4)

[Claims] (1) In growing silicon crystal particles by depositing silicon on the surface of the silicon crystal particles while maintaining them in a fluid state,
Hydrogen gas or/and inert gas is uniformly supplied to the cross section perpendicular to the direction of gravity to maintain the silicon crystal particles in a fluid state, and silicon hydride or silicon hydride and hydrogen gas or/and inert gas are supplied from the center. A method for producing high-purity granular silicon, which comprises supplying a mixed gas of active gas. (2) A patent characterized in that the velocity of silicon hydride or a mixed gas of silicon hydride and hydrogen gas or/and inert gas supplied from the center at the blowing part is equal to or higher than the terminal sedimentation velocity of the maximum silicon crystal particle A method for producing high purity granular silicon according to claim 1. (3) The method for producing high-purity granular silicon according to claim 1, wherein the silicon hydride is monosilane, disilane, or a mixed gas thereof. (4) The method for producing high-purity granular silicon according to claim 1, wherein the inert gas is argon.