DE2919086A1 - METHOD FOR PRODUCING POLYCRYSTALLINE SILICON - Google Patents

Description

2919085

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S 1759

"Process for the production of polycrystalline silicon"

US Pat. No. 4,084,024, issued April 11, 1978 to Joseph C, Schumacher, to the assignee of the present application, describes and claims a process for the production of semiconductor silicon by hydrogen reduction at relatively high temperatures, for example in the temperature range 900-1200 ⁰ C. The method according to the invention, on the other hand, relates to the production of semiconductor silicon by thermal means

Dismantling at lower, economically more favorable temperatures of the range from approx. 500 - 90 ° G.

As stated in the above-mentioned patent, the latter Developments in the field of semiconductor technology create a strong demand for inexpensive silicon monocrystals that are very high Purity, the so-called semiconductor silicon, which is used to manufacture semiconductor components and silicon solar cells is used. There have therefore been a greater number of methods of manufacturing semiconductor silicon including the method indicated in the above patent. The known methods are broken down into the following six groups.

1. The Siemens process described in DD patents 1 066 564, 1 102 117 and 1 233 815 and the process described in British patent 904 239, according to which practically all of the polycrystalline semiconductor silicon is produced and which are characterized by the express the following chemical reaction: _{copper kata} _

Si (MG) + HCl * SiCl. + SiHCl, + other minor

lyser 4;> Products

SiHCl, + H ₉ -I ⁰ - ⁰ ^ - £ U Si + SiCl- + HCl + explosive Si-HX-

² ■ ■ * polymer by-products

This is a high-temperature process for the batch-wise production of heterogeneously grown together

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Silicon filaments, with large amounts of SiCl, and explosive polymer by-products arise that must be eliminated. Because of these by-products, the process is not closed Perform cycle. There is also a 2Ό: 1 excess of Hp needed beyond the stoichiometric ratio.

2. In some cases the reduction of silicon tetrachloride with hydrogen is used because SiCl is available as a by-product of the Siemens process. An alternative SiCl ₄ production reaction is given as it can serve as a source for SiCl:

Si + 2Cl ₂ **

1 ones p
₄ + 2H ₂ '££! ■ - * ► Si + 4HCl + polymers.

This is again a high-temperature process for batch production that cannot be carried out in a closed cycle, in which heterogeneous growth on germs takes place on a heated support surface, with a large H _? -Excess is needed.

3. In the DuPont process described in Uü patents 3,012,862 _{and 4,084,024, SiX 4} or SiHX (where X = Cl, Br or I) is reduced by H ₂ , Zn or Cd in a liquid or mobile bed. The chemical processes can be expressed as follows:

I. Production of the supplied material:

(MG) Si + 3HX ► SiHX ,. + SiJL. + H ₉ etc.

with SiX, as a by-product

or (MG) Si + 2X ₂ ► SiX ₄

II. Production of high purity silicon:

Drinkable ₊ H ₂ Si ₊ HX ₊ SiZ ₄ + polymers for X = Cl
or

SiX ₄ + H ₂ * · Si ₊ HX ₊ polymers

or

SiHX ₅ + M, _S i ₊ H ₂ + MX ₂ (with M = Zn, Cd) or

SiX ₄ + M ► Si + MX ₂ (M = Zn, Cd)

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These processes can be carried out at moderately high temperatures in non-closed circuits; the by-products depend on the chemistry of the process. hydrogen must be used in large excess.

4. The iodide process described in U.S. Patent 3,020,129 can be expressed as follows:

Si + 2J _O > - SiJ.

c 4

with subsequent thermal decomposition for the production of Si:

_{S1 + 2Y}

_{S1 + 2J2 m}

This process of batch production produces polycrystalline at moderate temperatures in a closed circuit Silicon or silicon single crystals on particles acting as nuclei or heated threads.

5. The Union Carbide process is as follows represent:

Trichlorosilane production ■ ο ■ Si + 2H ₂ + 3SiCl ₄ £ 22— £ - + 4

Redistribution to silane by ion exchange according to

^ ^1HC1 3 ^SiH 2 ^C1 2 ^{+ 23iH} 2 ^C1 2 ^SiH 3 ^{C1 + SiHG1} 3

^ ⁱⁿ 3 ⁰¹ " ^5SiH 4 ^{+ SiC1} 4

with suitable recycling of the by-products and subsequent thermal decomposition of the silane:

SiH ₄ 5 °° - 900 ° C _{> Si +} 2JJ ₂

The process can be carried out at low temperatures in a closed circuit and includes intermediate redistribution by ion exchange. A product with a homogeneous structure around germs is obtained "

6. Thermal decomposition of trichlorosilane according to

_{31 +}

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2919085

is described in U.S. Patents 2,943,901 and 3,012,661. The trichlorosilane is produced according to

Si + HCl ► SiHCl ₅ + SiCl ₄ + other substances

so that there is a process that cannot be carried out in a closed cycle. Batch operation is suggested to be heterogeneous To encourage accumulation of material and to avoid homogeneous training which is considered harmful.

Numerous other methods and minor modifications of the methods described above are also known; none of these However, the method is fundamentally related to the present invention.

An important feature of the present invention is that it is a continuous process that is differs from the batch processes 1, 2, 4 and briefly described above. A continuous process is well known Way an improvement over batch processes Manufacturing because there is a reduction in capital and operating costs per unit of product manufactured result.

Another important feature of the invention The process is that it can be carried out at low temperatures in a closed circuit, while those indicated above Processes 1, 2, 3 and 6 are high temperature processes that cannot be carried out in a closed cycle. The known Processes require because of the high energy consumption and the necessary final storage of corrosive and dangerous By-products higher operating costs.

Another characteristic of the method according to the invention is that the direct thermal decomposition of trihalosilane with high yields compared to the low yield thermal decomposition according to US Patents 2,943,918 and 3 012 861 is carried out, it also not being necessary to redistribute by ion exchange according to the known Perform Procedure 5 to obtain a thermal decomposition Material. The for the inventive method typical simplicity results in a reduction in com-

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complexity and operating accounts and an improvement in yields.

Another important feature of the invention The process is that the thermal decomposition reaction does not lead to the deposition of material on the walls, since a critical temperature difference between the bedding and the surrounding walls is maintained.

Figure 1 is a schematic representation of an embodiment of the method according to the invention.

In the first process step of the present invention, metallurgical silicon metal with a purity of 357 ° or more is reacted with hydrogen (Hp) and the correct amount of silicon tetrahalide (SiX-) in a conversion vessel 10 for raw silicon to produce trihalosilane, the reaction

Si + SiX ₄ + 2H

_{4 2}

takes place. The reaction vessel can be the first stage of a liquid bed be in the temperature range of 400 - 65O C is kept under atmospheric pressure or higher pressure. The reaction vessel may, for example, be of that described in U.S. Patent 2,993 described type.

The metallurgical silicon can be produced locally in the electrothermal silicon generator described in US Patent 4,084,024 or obtained from common commercial sources. The metallurgical silicon preferably has a particle size of 50-500 microns to ensure good flowability. Liquefaction is achieved with hydrogen gas containing tetrahalide vapors; these are introduced into the reaction area through an inlet evaporator 12. The conversion in the reaction vessel proceeds with an efficiency of 30 ^c /> or more of the stoichiometric ratio.

A mixture of Trihalogenailan (SiIIX,) and unreacted Hydrogen (Hp) and tetrahalosilane are in the vapor phase from the top of the reaction vessel 10 into a condenser 14

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convicted. Contaminant metal halides are removed from the bottom of the reaction vessel 10.

The hydrogen and the trihalosilane are in the separation vessel 16 separated from one another, the hydrogen being returned to the reaction vessel 10. The trihalosilane is converted into a Transferred cleaning system 18, in which it is cleaned in the second method step of the invention. In the second process step, the silicon tetrahalide that has not reacted is used recovered and the feed system of the reaction vessel 10 fed via a drum 20.

An important point is that which takes place in the reaction vessel 10 in the first step of the method erfindungogemäßen transformation takes place in an imbalance, that is taking place at 4-00-650 ⁰ C in the reaction vessel reaction

Si + 2H ₂ + _{4 5}

has a positive free energy of 5-10 Kcal per mole and an equilibrium constant that is less than 1, since df = - RTInK The reaction products must therefore be continuously removed from the reaction vessel. The manufacture of trihalosilane (SiHX.,) Thus takes place on the basis of the law of mass action takes place under imbalance conditions.

In the second process step, the trihalosilane is purified in the purification device 18 by distillation before the further conversion into extremely pure polycrystalline silicon takes place. Cleaning device 10 can be a simple Be a distillation column with several plates and is used to separate the supplied trihalosilane into a mixture of less than £ 5 tetrahalosilane in trihalosilane with a metallic or organic impurity content of less than 100 parts per billion; also in a mixture of trihalosilane and tetrahalosilane with significantly more than 100 parts per billion of metallic and organic impurities as a residue. The residue is tumbled into the first stage reaction vessel in the manner described above 20 fed back. The vapor stream from the column is the reaction vessel 2b via a capacitor 22 and a separation stage 24

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is supplied, and also passes through an inlet evaporator 26, so that the third process step can be carried out. As in the previous step, the separation step 24 becomes originating hydrogen is fed back to the feed system for the reaction vessel 10 of the first stage. The cleaning device 18 can be derived from the method described in the Adcock et al. issued U.S. Patent 3,120,128.

In the third step of the method the trihalosilane is decomposed in a reaction vessel 28 at a temperature of 600-800 ⁰ G under atmospheric pressure or reduced pressure. The thermal decomposition takes place according to the equation

₃ ► Ui η 3 ^ iX ₄ + 2H ₂

instead of.

The reaction product, namely high-purity semiconductor silicon, arises in the reaction vessel ?: Ö together with the by-products hydrogen and tetrahalosilane. The by-products are separated off in a condensation stage 30 and a separation stage 32 and fed back to the reaction vessel 10 of the first process step in the manner shown, so that a closed circuit is created.

The recirculated hydrogen is in a well-known activated carbon filter 34 cleaned and compressed with a compressor 3b. The purified, compressed hydrogen is in one Jet mixer 38 supplied, in which the hydrogen with that of Drum 20 coming silicon tetrahalide mixed and then fed to the reaction vessel 10 of the harvesting process step will. In the manner shown, there can also be a lack of hydrogen be admitted.

Reaction vessel 28 may be a movable bed reaction vessel of the type described in detail in UJ Patent 4,084,024 Be kind; it can also be a reaction vessel with a liquid layer according to US Patents 3,012,861, 3,012,862 or 3 963 838 act.

It is an important characteristic of the method according to the invention, that in the reaction vessel 28 high-purity semiconductor silicon

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is produced at a relatively low temperature of, for example, 500-800 ⁰ C without introducing hydrogen into the reaction vessel; this is in contrast to the high-temperature hydrogen reduction (at 900-1500 ⁰ G) in the reaction vessel, as described in US Pat. No. 4,084,024. The method of the present invention is based on the assumption that the reaction

► Si + 3SiX ₄ + 2H

► Si + 3SiX ₄ + 2H ₂

takes place in the temperature range of 600-üOO ⁰ C and thereby results in a high (80-100 ^) yield of purified semiconductor silicon, which is deposited on a 'i' carrier surface as fine particles of purified silicon.

At temperatures of more than 900 ^° C., the mode of action of the reaction vessel 2b changes and the silicon yield drops to a low value, namely about 1Ο '/ ό, as described, for example, in US Pat. No. 3,012: 61 is. Hydrogen must be added in the high temperature range above 900 0, as described in US Pat. No. 4,084,024, in order to obtain high-purity silicon in the reaction

HSlX ₃ + H ₂ > Si ₊ 3HX

to surrender.

Another important feature of the invention Process is the "closed loop" through which the process becomes economically feasible. The procedure yields practically no by-products that cannot be recycled for reuse. Daa only in the process actually "used" material is impure silicon, which is converted into high-purity silicon Semiconductor silicon is converted.

In the process according to the invention, silicon is obtained from the thermal Trihalosilane decomposition as a mixture of a homogeneously accumulated fine powder and a heterogeneously grown one

Silicon obtained on the liquid or mobile bedding particles in the reaction vessel 28. The ratio of homogeneous to heterogeneous accumulation depends on various factors which include (although not exclusively): 1. starting material (tribromosilane or trichlorosilane), 2. temperature and

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Pressure during dismantling, 3. size distribution of the particles of the bedding layer (of the carrier material) and surface pretreatment, 4. Flow velocity of the bedding layer and relative velocity of steam and particles, and 5. depth of bedding and residence time.

It is important to control the ratio of homogeneous to heterogeneous nucleation in order to 1. be self-sustaining To create bedding in which carrier particles grow in place rather than from the product in a separate process to be generated, and 2. to bring the efficiency and speed of conversion to the highest possible values ". Efficiencies More than $ 80-90 of theoretical values are routine achieved in the conversion.

In the practical implementation of the method according to the invention, the wall temperature of the reaction vessel is maintained at more than 900 C 28, whereas the temperature of the bedding layer between 700 and BOO ⁰ C. This avoids deposits on the walls and clogging of the reaction vessel. In most reaction vessels with flowable or movable bedding layers, the heat of reaction for endothermic chemical reactions is obtained by heating the vessel walls, which are heated with gas or heating resistors, inductive heating or other devices. The rate and extent of the reaction are generally directly proportional to the temperature, so that there is quite a lot of reaction and deposition on the walls, as described in US Pat. No. 3,963,838. These deposits usually appear on the portion of the reaction vessel which is at the highest temperature. Deposits on the walls develop and not only lead to time-dependent heat transfer properties but also to reduced heat transfer and sooner or later to blockage of the reaction vessel. It has been 'found that the deposition of silicon from the thermal decomposition of the trihalosilane stops at temperatures of 900-1000 ⁰ C. Deposits on the walls are avoided in the method according to the invention in that

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ORIGINAL INSPECTED

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is the wall temperature is maintained at values of 9OO-1OOO ⁰ C, whereas the temperature of the bedding layer in the range of 700-850 ⁰ C held, which also leads to the maximum deposition rate of the silicon and the highest yield.

An example of the conversion of tetrabromosilane to tribromosilane in reaction vessel 10 for the first process step is as follows.

A gas stream consisting of hydrogen and tetrabromosilane was introduced into the reaction vessel 10, which contained a heated layer of silicon particles. The composition of the gas stream was 2.23 moles of hydrogen per mole of tetrabromosilane. The silicon bed had a cross-sectional area of 4.45 square centimeters and a length of 40 centimeters. The layer was kept at 650 ^° C. and the average residence time of the gas stream was 5> 1 seconds. The particles placed in the reaction vessel 10 consisted of metallurgical silicon. The condensation of the gas stream emerging from the reaction vessel in the condenser 14 and the subsequent distillation of the condensate in the column 18 used for cleaning resulted in a 36% conversion of the tetrabromosilane into tribromosilane. Conversion is defined here as the number of moles of tribromosilane obtained from the reaction divided by the initial number of moles of tetrabromosilane introduced into the reaction vessel.

The following example relates to the decomposition of tribromosilane into silicon in the reaction vessel 28.

A gas stream of argon and tribromosilane with a composition of 7.7 moles of argon per mole of tribromosilane was in a Introduced into the reaction vessel with a flowable layer of Betturigs. Although the argon is not essential for carrying out the method according to the invention, it was used in the present case Case as a carrier for the tribromosilane and as a flow of the silicon particle layer within the reaction vessel evoking gas. The layer of silicon particles consisted of particles with a mesh size of 50; the total weight of the layer was 258 grams at the start of the reaction. The shift was at

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the reaction at a temperature of 786-800 ⁰ G kept.

A total of 1.57 moles of tribromosilane were added to the reaction vessel in one manufacturing process. After removing the silicon particles at the end of the reaction, it was found that the layer had a weight of 268.1 grams, that is to say that a weight increase of 10.1 grams had taken place. This corresponds to a yield of 91 ^ due to the decomposition reaction 4HSiX _x - * Si + 3SiX. + 2H _O.

The invention thus creates a continuous low-temperature process with a closed cycle for the economical spontaneous generation of high-purity semiconductor silicon. As above described, the inventive method is based on the direct thermal decomposition of trihalosilane at relatively low temperatures (below 900 C) to produce high-purity silicon. The method according to the invention prevents the accumulation of silicon on the walls of the reaction vessel due to a temperature difference between the bedding layer and the surrounding walls are maintained so that the lowest temperature of the walls is above the temperature threshold is, in which a deposition of silicon takes place.

A particular embodiment of the invention has been shown and described, but changes are possible. The claims cover the amendments made within the scope of Invention can be made.

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Claims (7)

2919035 1 BERLIN 33 8 MUNICH Aufluite-Viktoria-StraBa 85 p. nucAUl / CO DANTMCO PienzenauerstraBe 2 Pat.-Anw. Dr. Ing. Ruschke «-» Γ. KUbOrmfc &. KAK I NtK paf Anw oiaf-ÄDki, pl-rng · PATENTANWÄLTE Ηϊ "· ε ·" '; Telephone: 030 / β ü 441? BERLIN - MÖNCHEN Telefonl Talagrunm-Adressa: Quadratur Berlin "Telearamm-Adraeia: τ ρ ι ρ χ. -J 83 786" '' Quadratur Munich TELEX: 522767 S 1759 JC SCHUMACHER COMPANY Π <ai ^ 9 580 Airport Road Oceanside, California V St. A. Claims 1. A process for the production of high-purity silicon, characterized in that trihalosilane is thermally in the Tempe- 'temperature range of 600-800 ⁰ C is decomposed in order to obtain high-purity' semiconductor silicon. 2. A method for the production of high-purity silicon according to .Anspruch 1, characterized in that prior to the dismantling step to prepare silicon with silicon tetrahalide and reacting hydrogen to produce trihalosilane and purifying the trihalosilane. 3. The method according to claim 2, characterized in that j Hydrogen and the silicon tetra halide which has not reacted are fed back into the reaction so that a ! continuous, self-contained process results. 4. The method according to claim 1, characterized in that the thermal decomposition is carried out in a reaction vessel is and that the wall of the reaction vessel is kept at a temperature above 900 C to prevent the deposition of Avoid silicon on the wall of the reaction vessel. 5. The method according to claim 1, characterized in that bromosilane is used as the halosilane. 6. The method according to claim "1, characterized in that Chlorosilane is used as a halosilane. 7. The method according to claim 2, characterized in that the silicon, the silicon tetrahalide and the hydrogen - 1 030010/0608 ORIGINAL INSPECTED _ 2 - a temperature in the range 400-650 ^° C. are brought to reaction. 030010/0608