What is the casting method?

The main difference between the crucible descending method and the casting method is that the melting and crystallization of the raw materials of the former are carried out in the same crucible; the melting process of the latter is carried out in the first crucible, and the crystallization is carried out in the second crucible, as shown in Figure 1. Show. The silicon raw material is melted in the first uncoated quartz crucible, and then the molten silicon is poured into the second quartz crucible coated with silicon nitride (Si 3 N 4) film on the inner wall to crystallize the molten silicon. In addition, the crucible descending method is to pass the crucible containing molten silicon through the hot zone of the heating coil to crystallize; while the casting method is to control the temperature by adjusting the power of the heating wire. During the crystallization process, the crucible itself does not move. The liquid interface is buried under the melt, so the influence of temperature fluctuations and mechanical instability can be minimized. These two solidification methods from liquid to solid are called directional solidification (directional solidification), which is easy to cause columnar crystal growth. Therefore, chips cut from the same polysilicon crystal block will Have the same defect structure, such as grain boundaries and dislocations.

Figure 1 Polysilicon is manufactured by casting method. The silicon raw material is melted in the first quartz crucible, and then the molten silicon is poured into the second quartz crucible coated with silicon nitride (Si3N4) film on the inner wall. Compared with the crucible descending method, the casting method has shorter crystallization and cooling time.

Figure 2 is a schematic diagram of the front and cross-section of the molten silicon crystallization process in the casting method. Adjusting the power of the heating coil can change the temperature gradient of the molten silicon. The bottom gradually solidifies upwards; then the grain boundaries will grow laterally in a dendritic crystalline manner (lateral growth), forming a polysilicon layer (see Figure 2(b)). Eventually, the product grain boundaries will gradually grow from the polysilicon film to the liquid surface, forming polysilicon bulk materials, and the dendritic crystals will also become a wide range of grain boundaries, as shown in Figure 2(c).

At present, Deutsche Solar GmbH in Germany and Куocerа in Japan both use the casting method to grow polycrystalline silicon bulk materials, the mass can reach 250~400 kg, and the length, width and height are about 70 cm × 70 cm × 30 cm. German Deutsche Solar GmbH uses the casting method to grow Polycrystalline silicon bulk material, from 1997 to 2004, the mass of the bulk material has increased from 180 kg to 330 kg. Compared with the traditional method, the new type of casting method developed by Deutsche Solar GmbH can save about 30% of the growth time, and the mass of polysilicon bulk material can reach 400kg.