11. Very high quality product can be achieved.Luque, Antonio, and Steven Hegedus, eds. Handbook of Photovoltaic Science and Engineering. 2003. Knovel. John Wiley & Sons, 2008. Web. 7 Dec. 2009. <http://www.knovel.com/web/portal/basic_search/display?_EXT_KNOVEL_DISPLAY_bookid=1081&_EXT_KNOVEL_DISPLAY_fromSearch=true&_EXT_KNOVEL_DISPLAY_Page=1&_EXT_KNOVEL_DISPLAY_searchType=basic>. Othmer, Kirk. Silicon. Encyclopedia Of Chemical Technology. 4th ed. Vol. 21. John Wiley & Sons, 1997. 1084-102. Print.
Editor's Notes
Introduce yourself to the audience. Brief description of what you are going to talk about.
Silicon is an element with atomic number 14.Generally there are two types of common manufacturing process involved with silicon. Production of High purity semiconductor grade silicon and metallurgical grade silicon. The High grade is used for the production of semiconductors for electronic devices and metallurgical grade is used for production of silicones, silica ,casting alloys etc. The most widely used process to produce high grade silicon is based on thermal decomposition of Trichlorosilane and involves Siemens reactor and hence also called Siemens process. For the production of high grade silicon main feed stock is metallurgical grade silicon. The complete process involves 3 reactors and 2 separation processes Main separation and reaction processes are1.Reaction of metallurgical grade silicon with Hydrochloric acid to produce Trichlorosilane2.Purification of Trichlorosilane by distillation3.Decomposition to elemental silicon in Siemens reactor4.Gas recovery separation and purification
This process is based on thermal decomposition of trichlorosilane at about 1100 0 c. First of all metallurgical silicon(feed stock) is reacted with HCl in a fluidized based reactor to produce Trichlorosilane .This reaction occurs at 350 0 c. This process also produces some unwanted SiCl4 Then this stream is purified to acquire SiHCl3 using distillation. Some of the HCl and H2 is recycled and used for reaction at the reaction chamber. The left by-product is fed to bypass streams. The pure SiHCl3 is the vaporized, diluted with hydrogen and sent to deposition reactors also known as Siemens reactor. The gas is then decomposed on the silicon rods to get highly purified silicon.(refer reactions above for Siemens reactor).Some of the by-product Tetrachlorosilane from the reactor is distilled to get electronic grade Tetrachlorosilane and some left a it is to get technical grade tetrachlorosilane. Other by products such as HCL and H2 is recycled.
The First and foremost step of the production is the process of production of trichlorosilane .Trichlorosilane is prepared by hydro chlorination of metallurgical grade silicon in a fluidized bed reactor. This reaction generally occurs at3500 C. The overall Reaction is Si(s) + 3HCl = HSiCl3 + H2Here in the reaction Metallurgical grade silicon reacts with hydrochloric acid to form trichlorosilane and hydrogen.This Process not only produces Trichlorosilane but also Tetrachlorosilane. Si(s) + 4HCl = SiCl4+ 2H2 The Siemens Reactor:The highly pure Trichlorosilane coming from the distillation units are converted and deposited in the Siemens Reactor. High-purity SiHCl3 is vaporized, diluted with high-purity hydrogen and introduced into the deposition reactors. The gas is decomposed onto the surface of the heated rods at 11000 C. These rods of silicon are hyper pure. The involved reactions are.2SiHCl3 = SiH2Cl2 + SiCl4 SiH2Cl2 = Si + 2HClH2 + HSiCl3 = Si + 3HClHCl + HSiCl3 = SiCl4 + H2The third reaction process involved with the purification is the purification of Silicontetracholride obtained from the Siemens reactor.There are two basic processes for this:1.First one involves the high temperature reduction of silicon tetrachloride with hydrogen.SiCl4 + H2 = SiHCl3 + HCl With this process about 20 to 25 % molar SiHCl3 can be obtained from the gaseous mixture.The trichlorosilane obtained is of very high quality because bothbecause both silicon tetrachloride and hydrogen are basically electronic grade2. The second one involves hydrogenation of silicon tetrachloride in a mass bed of metallurgical silicon.3SiCl4 + 2H2 + Si = 4SiHCl3This hydrogenation reaction produces approximately 20% trichlorosilane through a mass bed of metallurgical gradesilicon in a fluidized bed reactor.The Separation Processes:The production of pure silicon involves different separation processes. The first separation process is the separation of Trichlorosilane from Tetrachlorosilane and other impurities using distillation. This process utilizes the principle of separation by phase addition or creation. Here trichlorosilane is also purified of impurities of different volatility. Because of the high deposition rate and low boiling point and comparatively high volatility of trichlorosilane it is easily purified to a high grade. Other silanes have higher boiling points like SiCl4 has boiling point of 57.6◦ C making it possible for distillation.The other separation process is fractional distillation which follows the above mentioned distillation process. This process also relies on the fundamental principle of separation by phase addition or creation .Here the suitable trichlorosilane undergoes double purification through fractional distillation fractional distillation, the firstly removing the heavier components resulting from the direct synthesis and secondly removing the components lighter than trichlorosilane.
In conclusion, Siemens process is most widely used industrial process .Pure silicon has very important use in semiconductor based electronic devices. This process involves various reactors and separation processes like Siemens reactor and distilation. The Siemens process uses metallurgical grade silicon as feed stock and reacts it with HCl to form trichlorosilane and tetrachlorosilane which is distilled to get only trichlorosilane which is the decomposed in reactors to give pure silicon.
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