The casting long crystal process is to realize the directional solidification of liquid silicon from the bottom and gradually to the top by controlling the temperature of the heater in the heat field of the ingot furnace and the heat dissipation of the heat insulation material. The solidification of the liquid silicon is 0.8~1.2cm/h. At the same time, in the directional solidification process, the segregation effect of metal elements in the silicon material can be realized, and the purification of most metal elements can be realized, and the homogeneous polysilicon grain structure is formed.
Casting polysilicon needs intentional doping in the production process. The purpose is to change the concentration of acceptor impurities in silicon melt. The main dopant of P cast polysilicon in the industry is silicon boron master alloy, where boron content is in a certain proportion. The doping level is determined by the target resistivity of the silicon wafer. The optimum resistivity is 0.02 ~0.05 Omega cm, and the corresponding boron concentration is about 2 * 1014cm-3. However, the segregation coefficient of boron in silicon is 0.8, which shows a certain segregation effect during the directional solidification, that is, the gradient distribution of boron in the vertical direction of the ingot, which leads to the gradual decrease of the resistivity from the bottom to the top of the ingot.