| Summary: | 碩士 === 國立中央大學 === 機械工程學系 === 105 === Directional Solidification System (DSS) is the main method for growing the large-sized multi-crystalline silicon. The advantages of this method are lower production cost, simple manufacturing process, good quality and large crystal size. At present, the eighth generation (G8) furnace, developed by the US equipment maker GT advanced technologies, has been used to grow crystal with 1600 kg silicon feedstock capacity. However, enlarging the furnace to grow the large-sized ingot with the high quality still faces to some challenges
In order to improve crystal quality measured by the crystal-melt interface shape, impurities, dislocation density and thermal stress, the furnace geometry and crystal growth conditions should be optimized. The heating power ratio defined as the power on the top heater to the side one is one of key parameters affecting crystal quality and energy saving. By adjusting the heating power ratio, the power between top and side heater can be controlled. Different heating power ratios will cause different effects on the solidification process. The growth rate is increased at the early stages and reduced at the end of the solidification process when heating power on the top heater is higher than the side one. As a result, the crystal-melt interface will be modified and hence, the formation of defects can be controlled.
In addition, thermal stress is one of the factors affecting the crystal quality significantly. During the solidification process, the side insulation of furnace will be lifted up to take heat out. For large furnace, it is difficult to release heat quickly. Hence, higher raising velocity of the side insulation is designed to maintain the kinetic energy of solidification process. However, when the side insulation is upwardly moved too fast at the early stages, the thermal stress of silicon ingot will get larger. This agrees with the experimental growth of G8 provided by Sino-American silicon Products (SAS).
In sum, the opening of the insulation cage should be re-designed to obtain the lower thermal stress and a better process yield. Besides the improvement of crystal quality, shortening the growth time is also a very important part in this study.
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