Most of the metal impurities in industrial-grade silicon cannot be effectively removed through oxidative refining, slagging, etc., but silicon has physical properties that can be used for effective impurity removal, that is, segregation and impurity removal. The solubility of most impurities in solid silicon is very low, but the solubility in liquid silicon is relatively high. This property can be used to further purify molten silicon. Since directional solidification can better control the movement of the solid-liquid interface and the shape of the solid-liquid interface, the directional solidification method is mostly used in the segregation and purification of silicon. The segregation coefficients of B, P, C, Al and Cu in industrial silicon are relatively high, which are 0.5, 0.35, 0.05, 2.8×10-3 and 8×10-4 respectively, which are not suitable for segregation refining and removal, and other impurities can be removed by this method. Remove impurities.
There are two main processes for casting polycrystalline silicon. One is the casting method, that is, the silicon raw material is melted in one crucible, and then cast in another preheated crucible for cooling. By controlling the cooling rate, the directional solidification technology is used to prepare large grains degree of casting polysilicon. Among them, the smelting is carried out in a preparatory crucible in an induction furnace lined with quartz sand, and the molten silicon liquid is poured into the solidification crucible, which is placed on a lifting table and heated by resistance around it. By controlling the resistance heating source, the temperature at the bottom of the solidification crucible is minimized, so that the silicon melt starts to crystallize gradually at the bottom of the solidification crucible, and the temperature gradient of the solid-liquid interface is controlled at the same time, so that the solid-liquid interface rises in parallel, because melting and crystallization are not in the same crucible This method can realize semi-continuous production, and its melting, crystallization and cooling are located in different places, which can effectively improve production efficiency and reduce energy consumption. But using different crucibles for melting and crystallization will lead to secondary pollution. In addition, because of the crucible turning mechanism and the ingot mechanism, its structure is relatively complicated. The other is the direct melting directional solidification method, referred to as the direct melting method, that is, the polycrystalline silicon is directly melted in the crucible, and then the melt is cooled by means of heat exchange at the bottom of the crucible, and the directional solidification technology is used to manufacture polycrystalline silicon. During the directional solidification process, under the influence of segregation effect, impurity elements will gradually enrich to the top of the ingot. The directional solidification purification process requires increasing the interface temperature gradient as much as possible and slowing down the solidification rate. It can reduce the metal impurity content in industrial silicon by more than two orders of magnitude. The quality of polysilicon grown by the direct melting method is better. The temperature gradient in the direction makes the solid-liquid interface as straight as possible, which is conducive to the growth of columnar polycrystalline silicon ingots with good orientation. Moreover, this technology requires less labor, and the crystal growth process is easy to control and automate. , has been maintained at a high temperature, and the “in-situ” heat treatment of the polycrystalline silicon body is carried out, resulting in a reduction of thermal stress in the body, and ultimately a reduction in the dislocation density in the crystal. When growing cast polysilicon, the main problems to be solved include: as uniform solid-liquid interface temperature as possible; thermal stress as small as possible; grains as large as possible; contamination from the crucible as little as possible. Because clean grain boundaries have little or no effect on the lifetime of minority carriers, and high density of dislocations is particularly detrimental to the photoelectric conversion of materials, especially when metal impurities and oxygen precipitation are deposited on the dislocations, which increases The minority carrier recombination ability of dislocations, especially metal and oxygen, is easy to segregate in dislocations. In the high-density dislocation region of polysilicon, the agglomeration of metal impurities will cause high minority carrier recombination.