Solar Grade-Silicon, Ingot, Wafer Technology and Market Trend

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Solar Grade-Silicon, Ingot, Wafer Technology and Market Trend (2008~2012) Figure 2.19. Tube Reactor in Joint Solar Silicon (JSSi) Process Source: Joint Solar Silicon All Contents of this report remain the property of Displaybank The traditional Siemens process decomposes silane in silicon seed (U-rod), whereas the Joint Solar Silicon reactor decomposes silane in reactor’s interior space that it is called “free space reactor method”. The method supplies hydrogen and silane to the reactor’s interior and enters heat to walls to decompose silane away from the walls. Like in the Siemens process, the free space reactor method does not require equipments for silicon seed preheating and is able to prevent pollutions from graphite which composes the preheating equipments. The method uses monosilane rather than chlorosilane for raw materials because the monosilane enables low temperature processes and decomposes well that it converts gasses to silicon in reactors with single process. However, the monosilane may possibly explode when it is in contact with oxygen that reactors must be designed carefully to prevent such outcome. It is also more expensive than trichlorosilane. The thermal decomposition reaction of silane is like the following. SAMPLE SiH4 -> SiH2+H2 SiH2 + SiH4–> Si2H6 SiH2 +Si2H6 -> Si3H8 The silicon powder manufactured through reactors undergoes high density process for easy handling, delivery, and loading. Here, the density increases from 50g/L to 500g/L. The figure below shows silicon powder that finished the high density process. The powder is melted and produced into ingot to be manufactured into final products. Jan’09
Solar Grade-Silicon, Ingot, Wafer Technology and Market Trend (2008~2012) Figure 2.5.1 Impurities in MG-Si: Equilibrium Distribution Coefficient Most impurities, except for phosphorus, boron, and carbon, have extremely small equilibrium distribution coefficient. This indicates that the impurities are concentrated in liquid phase silicon rather than in solid phase silicon in mushy zone. Hence, impurities tend to concentrate in areas where they are solidified at last. As shown in the figure below, impurities show the maximum solubility near melting point and the solubility radically declines as temperature drops. Hence, the metal impurities that exceed solubility during solidification processes are efficiently extracted through crystal boundaries or cracks in general. Silicon is insoluble to acids, whereas the metal impurities dissolve well in acids. Pickling process selectively eliminates the extracted metal impurities with acid. Figure 2.5.2 Metal Impurities Employment within Silicon All Contents of this report remain the property of Displaybank SAMPLE Jan’09
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Solar Grade-Silicon, Ingot, Wafer Technology and Market Trend

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