Influence of different vacuum conditions on the distribution of insoluble inclusions in a multi-crystalline silicon ingot

The distribution of insoluble inclusions in multi-crystalline (m-c) silicon was investigated by directional solidification under different vacuum conditions using recycled scraps. We found an abnormal peak (weight ratio of 1.19%) of SiC and Si3N4 with an average size of 100 μm located at the bottom of the silicon ingot obtained under low-vacuum conditions. At the top of the ingot, the impurity phase featured a weight ratio exceeding 10.62% and a size of 20 μm. Results showed that large particles sank to the bottom of the ingots whereas small particles floated upward. Further investigation suggested that the dominant factor influencing inclusion distribution is related to the presence of metallic impurities (e.g., Ca, Al, and Na) and their corresponding microstructure. A mechanism of migration was proposed to reveal that bonding of metallic impurities and insoluble inclusions contribute to the abnormal distribution of impurities. The results provide guidance for controlling the distribution of insoluble inclusions in recycled m-c silicon.