Research on the reusable polycrystalline silicon ingot casting with the seed assistant solidification process

In the production of solar grade polysilicon, there are many wastes such as head-end, side wastes, and other wastes. The study of the repurification of these wastes was important to reduce production costs. The effect of ΔT,which was the temperature difference between the top and bottom, was studied during the ingot casting process. The seed assistant solidification process parameters were studied based on ΔT. The experimental results showed that the casting process could be divided into three stages according to the variation of ΔT. The first stage was from heating start to fully-melting. The ΔT increased to maximum first and then decreased until the ΔT increased slowly, after which it decreased to the minimum value. In the second stage, the ΔT＿D-Eincreased from the previous minimum value to the crystallization ΔT. The last stage was the annealing stage which decreased ΔT to 0. The larger or smaller ΔT＿B-C in the first stage could affect the melting degree of the seed layer. The larger ΔT＿B-Cmeant insufficient melting of the seed layer, which increased the defect density. The smaller ΔT＿B-Cmeant over melting of the seed layer, which increased the red zone. The ΔT＿B-Cbetween 168.4 ℃ and 191.8 ℃ yielded the best silicon ingot performance with lower red zone and defect density. During the crystallization stage, lower defect density ingot was obtained by reducing ΔT＿D-E. Through controlling and optimizing the process ΔT, the erosion of the seed layer was reduced, the height of the red zone and the defect density were lowered, and the quality of the reusable silicon ingots was enhanced.