ISSN 1063 7397, Russian Microelectronics, 2011, Vol. 40, No. 8, pp. 559–561. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © V.M. Ivanov, Yu.V. Trubitsin, 2010, published in Izvestiya Vysshikh Uchebnykh Zavedenii. Materialy Elektronnoi Tekhniki, 2010, No. 4, pp. 10–13.
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INTRODUCTION
The world polysilicon industry is currently in a complicated situation. The pressing need for alterna tive energy sources has encouraged demand for solar energy technologies, which rely heavily on polysili con, but actual sales of the material have declined as a result of the recession global recession. Cutting pro duction costs ems to be crucial for dealing with the crisis successfully , becau it should stimulate the demand for photovoltaics from business and indi viduals. Above all, this strategy means adopting resource efficient technologies, in which byproduct recycling plays an important part [1].
This paper is concerned with one such byproduct,silicon tetrachloride (STC). The importance of the top
ic is illustrated by the fact that producing each kilo gram of polysilicon also yields the following amounts of STC, depending on process technology:
(1) 2–5 kg with silicon hydrochlorination in mak ing trichlorosilane;
黑眼豆豆经典(2) 11–14 kg with hydrogen reduction of trichlo rosilane;
(3) 22–27 kg with trichlorosilane disproportion ation as part of the silane process technology .
The current practices in STC utilization involve its hydrogenation to trichlorosilane (TCS) that is then recycled to produce polysilicon.
APPROACHES TO HYDROGENATION OF SILICON TETRACHLORIDE
Historically , STC hydrogenation technology has evolved through a number of stages, in line with the progress in electronics grade silicon manufacturing and in technology as a whole. The main problem lies in the fact that the silicon–chlorine bond dissociation energy is as high as 377 kJ/mol [2]. Initially , it was dealt with by using reducing agents to lower the bond energy , whereas recently , we have en the emergence of plasma process and of catalysts capable of direct ing the hydrogenation along an energy saving route.Today , the wide variety of methods for hydrogena tion of STC to TCS
may be divided into four groups,each with its advantages and disadvantages:(1) reducing agent hydrogenation;(2) high temperature hydrogenation;(3) catalytic hydrogenation;(4) plasma hydrogenation.
Reducing agent hydrogenation allows one to choo from a wide range of agents and is compatible with conventional process equipment. On the other hand, it consumes much energy , requires byproduct recycling, and contaminates the desired TCS with reactants. The drawbacks keep the technique from being employed on a commercial basis.
High temperature hydrogenation relies on the equilibrium that is attained by a homogeneous system of Si, H, and Cl under certain conditions. Hydrogena tion of STC,
SiCl 4 + H 2 SiHCl 3 + HCl,is accompanied by the dehydrochlorination of the desired TCS,
SiHCl 3 SiCl 2 + HCl,with the resultant silicon dichloride undergoing dis mutation,
2SiCl 2
4,and hydrogen reduction,
SiCl 2 + H 2The are equilibrium reactions, the position of equilibrium in each reaction depending on the tem perature. Their respective equilibrium constants for different temperatures [3] are listed in the table.
Approaches to Hydrogenation of Silicon Tetrachloride
howdoyoudoin Polysilicon Manufacture
V. M. Ivanov and Yu. V. Trubitsin
Classical Private University, Zaporizhia, Ukraine普通高中课程标准实验教科书
Abstract —A review of approaches to hydrogenation of silicon tetrachloride is prented. A byproduct of po lysilicon manufacture, silicon tetrachloride is thus converted into trichlorosilane, which is then recycled to reduce the production cost of polysilicon. Catalytic hydrogenation is identified as the most commercially promising line of rearch and development.
Keywords: polysilicon, silicon tetrachloride, hydrogenation, trichlorosilane, conversion efficiency , activa tion, reducing agents, converter, plasma source, plasma chemical reaction.DOI: 10.1134/S1063739711080099
MATERIALS SCIENCE
AND TECHNOLOGY: SEMICONDUCTORS
560RUSSIAN MICROELECTRONICS V ol. 40
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出国考试IVANOV , TRUBITSIN
The process offers a relatively high yield of TCS,becau the reactions proceed concurrently . The TCS content is highest at the temperatures from about 877to 1087°C, with the conversion efficiency being clo to 30% at an STC vapor pressure of 0.02 MPa. At the lower temperatures, the equilibrium is displaced to the left, with the TCS content decreasing. The back reac tion is prevented by rapid cooling of the products.Investigations into hydrogen reduction of TCS showed hydrogenation of STC to be thermodynami cally possible in polysilicon reactors, proceeding at an adequate rate [4, 5]. The process was implemented in modified hydrogen reduction of a hot rod type.Today , reactors of similar design and STC converters are being produced by Siemens, GT Sol
ar, SolMic,the Krasnoyarsk Machine Building Plant, etc. [6]. In the converters, silicon rods (rving as substrates) are replaced with graphite tubes or flat siliconized lamel las, through which an electric current is pasd to maintain the desired temperature, and a graphite shield is mounted to reduce heat loss. Hydrogenation takes place at a hydrogen to STC molar ratio of 3 : 1,a pressure of 0.6 MPa, and a heater temperature of 1250°C. The STC flow rate can reach 3000 kg/h. The output TCS content is 20–22%. A converter consumes about 175 kW h of electricity per kg of TCS.
There exist high temperature hydrogenation tech niques that are well understood and widely imple mented in hardware, which allows their commercial u. They have the following disadvantages:
(1) The high operating temperature makes it diffi cult to find a corrosion resistant material for the equipment.
(2) There is a risk of the desired product being con taminated with the contacting materials.(3) The process consume much energy .
Hot rod reactors em to be the most popular type of hydrogenation reactor in current u. However,their heavy consumption of electricity casts doubt on their future.
Catalytic hydrogenation of STC is bad on the fact that the position of equilibrium can be shifted to the right by means of the reaction:
xyl3SiCl 4 + 2H 2 4SiHCl 3
to remove the products from the reaction zone (as sug gested by Le Chatelier’s principle), which requires a catalyst becau uncombined silicon is almost inert to hydrogen chloride. In addition to increasing the rate of a reaction, catalysis enables one to rai the yield and to reduce the temperature and energy consumption of production process. With STC, catalytic hydrogena tion proved to be a viable method of making TCS,leading to a whole range of process technologies. The catalysts were uncombined copper or copper com pounds in powder form, nickel, ruthenium, platinum metals or their compounds, and other metals. The mass of a catalyst introduced was 0.1–5% of that of the silicon load. Since the surface of a catalyst is the site where catalysis takes place, a catalyst is applied to a material with a large surface area, such as silica gel,alumogel, aluminosilicate, or activated carbon. For the same reason, silicon feedstock is powdered to a fineness modulus of 0.07–0.25 mm, and the process is carried out in a fluidized bed reactor. Raising the pres sure increas the rate of hydrogenation further.
A Lewis acid capable of making STC more reactive reprents a promising choice of a catalyst, providing a TCS yield of up to 98%, though sacrificing TCS purity .Examples are the halides of aluminum, copper, tita nium, vanadium, or chromium.
However, only one method for catalytic hydroge nation of STC has so far been adopted by the industry .Developed by Union Carbide, it employs a fluidized bed reactor fed with a mixture of STC and hydrogen preheated to 520°C, the fluidized bed consisting of sil icon or ferrosilicon with 0.1–5 wt % copper, each powdered to a fineness modulus of 0.1–0.2 mm, with the molar ratio of STC, hydrogen, and silicon being 2:2 : 1 [7]. The process takes place at 500°C and 3.0–4.0 MPa. The output contains up to 30% TCS, 0.5%dichlorosilane, and STC, which is parated by rectifi cation and fed to the reactor again.
Catalytic hydrogenation allows one to lower the temperature and, hence, the energy consumption of the process. It reprents an efficient and easy way of increasing the efficiency of the conversion of STC to TCS on a commercial basis. On the other hand, the process requires a high pressure; the desired product is contaminated with the catalyst and its impurities, as well as with ones from the equipment; and waste prod ucts have to be recycled.
4级报名
Plasma hydrogenation came with industrial plasma sources using hydrogen or a hydrogen–argon mixture as
Equilibrium constants of the reactions involved in high temperature hydrogenation of STC
APPROACHES TO HYDROGENATION OF SILICON TETRACHLORIDE561
the parent gas. It employs the interaction between STC molecules and hydrogen atoms that form in a plasma:
H2 2H*,
SiCl4 SiHCl3 + HCl.
Pressure as high as 101 kPa brings a plasma to ther mal equilibrium in which the electrons and the
gas molecules have almost the same temperatures, lying between about 2725 and 4725°C. The conditions allow a certain amount of ionization, electronic state excitation, energy transfer to molecular vibrational or rotational degrees of freedom, and molecule dissocia tion in the plasma as a result of a change in the distri bution of electron energy. An adequate understanding of the plasma process is esntial for achieving the maximum yield of TCS. Account must also be taken of the specific features of the plasma source employed.
With an arc discharge plasma source, a parent gas is converted into a plasma as it cross an arc discharge generated between a cathode and an anode in the form of a nozzle. A special device is ud to inject STC and gaous hydrogen into the plasma in the vicinity of the nozzle edge. The products of hydrogenation are cooled with a cold mixture of hydrogen, argon, and STC. The process offers a yield of up to 50% but con sumes as much as 4.5 kW h of electricity per kg of TCS, even with recuperation.
An inductively coupled plasma source us a radio frequency (RF) generator to convert hydrogen gas into plasma, to which STC and hydrogen are added in a specific molar ratio. Hydrogenation takes place at a temperature between 2725 and 4725°C. As an exam ple, a mixture of 41.3 vol % TCS, 8.6 vol % dichlorosi lane, and 50.1 vol % STC was produced with 1.7 kW of RF power, the pro
cess consuming 7.94 kW h of elec tricity per 1 kg of TCS [8].
Hydrogenation with a capacitively coupled plasma source was recently reported, using an RF generator and a mixture of hydrogen and STC as the parent gas [9]. This approach to plasma generation made it pos sible to create a nonequilibrium plasma with a gas temperature of about 625°C, a level well below the electron temperature. The design of the reactor enabled stable hydrogenation at the atmospheric pres sure with a yield approaching 60% and an electricity consumption of 2 kW h per 1 kg of TCS.
布局英文In summary, plasma hydrogenation techniques can offer adequate efficiency and a yield of up to 60%, but consume much energy, may give silane chlorides (an explosion/fire hazard) as byproducts, and suffer from the inadequate reliability of plasma sources. The drawbacks keep them from being widely adopted by the industry.
CONCLUSIONS
Of the four approaches, only high temperature and catalytic hydrogenation have found u in the industry. The former is currently more popular due to process equipment considerations, providing a yield of up to 37%. However, its heavy consumption of electricity (100–200 kW h per kg of TCS) rais
es significant doubts over its future. Catalytic hydrogenation is far more efficient (consuming about 19 kW h per 1 kg of TCS) and is easy to implement on a commercial scale with a yield of up to 30%. The advantages outweigh the problems arising from a high process pressure and waste products, encouraging investments in this pro cess technology.
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滑铁卢大学排名
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