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

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Solar Grade-Silicon, Ingot, Wafer Technology and Market Trend (2008~2012) 3.2.2 Floating Zone Method (FZ) Floating zone method was developed by Keck and others in 1952. It manufactures silicon with the highest purity. Its basic principle includes partially melting long bar to produce liquid phase region. It moves the region following after the bar to grow monocrystal. Most impurities within silicon usually have equilibrium distribution coefficient less than 1 that the impurities are separated in the liquid phase region with CO/K concentration. The liquid phase region is cornered to move impurities towards the end of the bar simultaneously. Figure 3.2.3. Floating Zone Method Equipment As shown in the figure, the FZ method has no parts in contact with silicon liquid unlike the CZ method that it can fundamentally prevent pollutions from crucibles and obtain silicon with high quality. On the other hand, it requires a liquid phase to remain in-between solid phases as a surface tension that it has a limitation in a diameter of ingot to be manufactured. The method is generally used to manufacture silicon bars of several tens of mm. The maximum diameter of ingot using the currently developed floating zone method is about 150 mm. Therefore, the FZ method is improper to be used commercially due to low productivity. Instead, it is used in special occasions when silicon with high purity is needed. All Contents of this report remain the property of Displaybank SAMPLE Jan’09
Solar Grade-Silicon, Ingot, Wafer Technology and Market Trend (2008~2012) Figure 3.3.5. Comparison between Conventional Solidification Process and Electromagnetic Casting Method Unlike the ingot obtained by cooling in the conventional graphite crucible or a quartz crucible located within the graphite crucible, the FZ method obtains silicon with high quality by minimizing contacts with crucibles and hence preventing pollutions from the crucibles. Moreover, it enhances productivity by reducing replacement expenses spent in crucibles, which are disposed after a single or few uses, and enabling continuous operations (productivity in same time frame is about 5 times higher than HEM method and 7 to 15 times higher than Czochralski method). However, this technology has a difficulty in silicon application. Silicon has difficult electromagnetic induction due to low electric conductivity in solid phase. It is also difficult to expect processes with high efficiency since it melts silicon with high latent heat and melting point in water-cooling crucibles and loses a lot of heat. Therefore, the industry conducts research studies in order to resolve such issues and the method is sometime used in combination with the conventional refining process like the figure below. The figure shows a method that melts silicon through plasma melting to create liquid phase and enters it to a crucible. Silicon mostly displays nonconductive property in solid phase, but shows conductive property in liquid phase. 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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