A collection of articles "Methods and technology of modern petrochemical and oil-and-gas production" was issued in the scope of the 2nd Scientific and Technical Conference dedicated to the 70th anniversary of Omsk State Technical University. An article "Especially pure trichlorosilane production technology" written by Yaritsa V. A., Cryogaztech LLC employee, was published in this collection. Especially pure trichlorosilane is used for production of polycrystalline silicon (PCS) of high purity (99.999 %, 99.9999 % and higher). PCS is used for production of electronic components, photoelectric transducers, solar cells and panels.
Yaritsa V. A., Post-Graduate Student (Cryogenics Department, Institute of Refrigeration and Biotechnology, St. Petersburg National Research University of Information Technologies, Mechanics & Optics)
Research Adviser: Borzenko E. I., Doctor of Engineering, Professor, Cryogenics Department Chairman
ESPECIALLY PURE TRICHLOROSILANE PRODUCTION TECHNOLOGY
Polycrystalline silicon from which monocrystalline silicon is produced in its turn is used for production of electronic components, photoelectric transducers, solar cells and panels. Especially pure polycrystalline silicon (PCS) of high purity (99.999 %, 99.9999 % and higher) is required for that.
The great majority of polycrystalline silicon in the world is produced from trichlorosilane. Tetrachlorosilane and monosilane are used more rarely.
The so called "Siemens process" using especially pure trichlorosilane as a raw material is the most common method for polycrystalline silicon production. In this process polysilicon dummy bars heated to high temperature are placed in Siemens reactor with a cooled domed housing. Gaseous trichlorosilane is supplied to the reactor. Trichlorosilane passing through the reactor is degraded on the surface of heated dummy bars with formation of polysilicon .
Currently active developments of various alternative technologies are conducted. For example the process of polysilicon production in fluidized bed reactors and direct purification of technical silicon with production of enhanced metallurgical silicon that meets "solar quality" requirements. By-products is formed at production of semiconductor quality trichlorosilane. Such by-products can be used for production of commercial products being in demand at the market:
- silicon tetrachloride – for production of ethyl silicates and aerosil;
- technical trichlorosilane, A grade trichlorosilane – for
production of silanes from which silicone resins, heat-resistant lacquers, oils etc. can be produced in its turn.
Due to fast growth of demand in polysilicon for production of microelectronics components and solar energy photoelectric transducers the creation of wide polysilicon production in Russia is a pressing task and not for economic reasons only. The production of especially pure polycrystalline silicon will allow tailoring domestic effective production of this strategic material on an industrial scale.
1. Polycrystalline silicon production technology
The polycrystalline silicon production technology includes the following stages:
- production of technical (metallurgical) silicon;
- synthesis and purification of trichlorosilane SiHCl3;
- production of especially pure polycrystalline silicon in hydrogen reduction reactor;
- treatment of solid, liquid and gaseous wastes;
- production of monocrystalline silicon;
- alloying of monosilicon.
Let's consider the stage of synthesis and purification of trichlorosilane SiHCl3 in detail.
A variety of methods and their combinations were proposed to use for trichlorosilane purification. However this problem still exists currently. It is explained by steady severization of requirements for purification degree (especially from carbon-containing admixtures). In addition increase in purification process efficiency is required i.e. reduction of energy costs, increase of target product output, etc. 
A technology for especially pure trichlorosilane extraction by fractionating in five columns has been developed. However this extraction method leads to high energy/output ratio of the purification process and use of large number of equipment . A method for especially pure trichlorosilane production has been developed. This method includes interaction of silicon tetrachloride with technical silicon and hydrogen chloride with subsequent unmixing of chlorosilanes and recirculation of unreacted silicon tetrachloride that is distinguished by the fact that the interaction is conducted at 380-4500С and chlorosilanes unmixing is carried out on diffusion membranes . The disadvantage of this method is low degree of trichlorosilane purification from boron-containing admixtures .
The considered trichlorosilane production method includes:
- trichlorosilane synthesis;
- "dry" or "wet" preliminary purification of vapor-gas mixture obtained in the result of trichlorosilane synthesis;
- condensation and separation of vapor-gas mixture components with production of liquid chlorosilanes and extraction of hydrogen and hydrogen chloride;
- fractioning and purification of chlorosilanes condensate including "deep" chlorosilanes purification with production of especially pure trichlorosilane.
1.1 Trichlorosilane synthesis is usually carried out using three methods:
• Hydrochlorination of ground metallurgical silicon.
The process is conducted using hydrogen chloride in a fluidized bed reactor with production of trichlorosilane in vapor-gas mixture containing silicon tetrachloride, hydrogen and hydrogen chloride. Synthesis at increased pressures (up to 2.0 MPa) that allows reducing energy costs for vapor-gas mixture condensation by means of increasing trichlorosilane synthesis reactor capacity should be referred to promising trichlorosilane production methods .
• Catalytic hydrogenation of silicon tetrachloride with addition of ground silicon.
The catalytic hydrogenation of silicon tetrachloride is carried out in a fluidized bed reactor with production of vapor-gas mixture containing trichlorosilane, silicon tetrachloride, hydrogen chloride and admixture chlorides. Copper serves as a catalyst in this case.
• Thermal hydrogenation of silicon tetrachloride.
The thermal hydrogenation of silicon tetrachloride is carried out in a reactor (convector) with heated bars with production of vapor-gas mixture carbon composite material. 1.2 "Dry" or "wet" vapor-gas mixture preliminary purification unit is designated for sequential purification of vapor-gas mixture obtained in the result of trichlorosilane synthesis. In addition "dry" purification is designated for removal of fine solid particles and it is performed using cyclones. "Wet" purification is carried out in a column with a bubbling box sprayed with liquid chlorosilanes.
1.3 Vapor-gas mixture condensation and separation unit designated for chlorosilanes condensation and extraction of hydrogen and hydrogen chloride from vapor-gas mixture.
A purely condensation method carried out in heat exchangers using coolants at various temperature levels from +300С (recirculating water) to minus 1940С (liquid nitrogen) is one of methods for production of liquid chlorosilanes from vapor-gas mixture and extraction of hydrogen and hydrogen chloride . However this method is rather expensive as it requires deep cold. Moreover it is near-impossible to achieve complete separation of hydrogen and hydrogen chloride from condensing chlorosilanes using this method. Extraction of hydrogen and hydrogen chloride together with mass-exchanging condensation processes (absorption and desorption) in columns is one of the most useful methods of liquid chlorosilanes production. In this case the required degree of purity is achieved at temperature level from 300С to minus 450С.
1.3 Chlorosilanes condensate fractioning and purification unit.
It is designated for fractioning fine purification of chlorosilanes condensate from chlorides of metal and boron admixtures, condensate division to trichlorosilane and silicon tetrachloride, trichlorosilane "deep" purification.
Fractioning purification of trichlorosilane and silicon tetrachloride is performed on several fractioning units.
In this case a fractioning unit to which still liquor (SiCl4) from trichlorosilane and silicon tetrachloride splitter is used for purification of silicon tetrachloride from polysilanechlorides and other high-boiling admixtures.
The distillate from this column (purified silicon tetrachloride) is fed to silicon tetrachloride hydrogenation reactor for trichlorosilane synthesis. These units feed the still liquor to "squeezing" fractioning.
The distillate formed in "squeezing" fractioning column is fed to spray the column to "wet" purification system. The use of fractioning allows deriving vapor-gas mixture separation products including hydrogen, hydrogen chloride and raw trichlorosilane.
Raw trichlorosilane contains admixtures purification of which is required to derive the end product of respective quality (polycrystalline silicon).
Raw trichlorosilane is fed to "deep" purification unit with the following parameters:
light admixtures + SiH2Cl2 (dichlorosilane) – 0.1 %mas;
heavy admixtures + SiCl4 (silicon tetrachloride) – 0.1 %mas;
SiHCl3 (trichlorosilane) – 99.80 %mas.
End product SiHCl3 (trichlorosilane) – 99.9999 %mas.
The flow chart of raw trichlorosilane purification is shown in Fig. 1.
Fig. 1 Flow chart of raw trichlorosilane purification.
Raw trichlorosilane purification from light and heavy admixtures is performed in fractioning columns and the process is continuous.
Light admixture separation column consists of two series columns with a common boiler and a dephlegmator. High-efficient counterflow regular packings can be used as mass-exchanging devices. That allows columns to work effectively in the required range of loads by liquid and gas. Light low-boiling admixtures contained in trichlorosilane are concentrated in the top part of the column and removed. High-boiling admixtures are concentrated in the still part of the fractioning column designated for trichlorosilane purification from heavy admixtures.
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