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Poly-Silicon Process for Photovoltaic Industry Mr.Rushil shah - Managing Director SHAVO Technologies in Association with TESCOM, USA.
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ith increasing fossil fuel prices and advanced production technologies. Alternative energy sources like Photovoltaic (PV) becomes more attractive to investors. Experts expect that PV-energy and fossil fuel prices will reach the same level but will be coming from different methods in the year 2014. The two main ingredients for a Solar CeIl seem to come from unlimited resources, sunlight and sand (Quartz = SiO2). Like with many other things in life, it’s the quality that counts not the quantity. The purity of natural Quartz would not allow any solar cell or computer chip to work. Natural Quartz can be converted to “Metallurgical Grade Silicon” with 99% purity (Log 2) by a reduction process at 2000°C [Si02 +2C->Si+2CO]. Solargrade Silicon is 4-5 magnitudes “cleaner” than the original metallurgical grade Silicon (Log 6-7). Electronic grade is even 2-3 magnitudes cleaner than Solar grade (Log 9) 99.9999999% purity. There are several ways from metallurgical grade Silicon to Monocrystalline or Polycrytalline Silicon. One process is the fabrication purification of Monosilane SiH4 and later Pyrolysis at 800°C in a tube reactor. Monosilane breaks down into elementary Silicon and Hydrogen gas [SiH4 -> Si + 2 H2].
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For this process, Bulk Silane is drawn from trailers with high flow rates from 35 Nm3/h to 100 Nm3/h. The biggest challenge for this process is the cool down of Silane due to the high Joules Thompson coefficient. TESCOM has designed special bulk regulators 449-301 (1st stage) plus 44-3200 (2nd stage) with optional 400 Watt internal heating for each stage. The more common process is the Siemens type reactor for Polychrystalline Silicon deposition. Approximately 75% of global Silicon production is made with this technology. Metallurgical grade Silicon powder (99% purity) and HCI are mixed in a Fluidized Bed Reactor at high temperature. In this CVDprocess, TriChloroSiiane (SiHCI3) is formed besides other by-products - [Si + 3HCI = SiHCI3 + H2] and also [Si + 4HCI = SiCI4 + 2 H2]. Hydrogen gas can be separated
and recycled. TCS (SiHCI3) and STC (SiCI4) are clear, highly flammable and colourless liquids with a boiling point of 32°C for TCS and 57°C for STC and both having an acrid odour. They are both known to the Semiconductor Industry for wafer Epitaxy. Ultra-High Purity (UHP) TCS must be distilled to achieve the
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