Impurity Removal Of Polysilicon Powder By Electron Beam Processing

With problems such as global energy shortage and environment pollution etc. becoming more and more severe, solar photovoltaic electricity generating has become an emerging industry that is commonly concerned and developed with some emphasis by the whole world due to its preponderant characters of cleaning, safety, convenience, high efficiency etc. As the main raw material, its yield of solar-grade polysilicon (SOG-Si) for solar cells has been being in a state of urgent shortage. The requirement is estimated to be exceeding180000tons in2012. Despite the fact that Siemens Process is the main method for producing SOG-Si, a great-leap-forward development is restricted due to the negative factors existed during the producing process, such as its high energy consuming, high cost and environmental pollution. Because of it, technology research and development of producing SOG-Si using metallurgical method is staring to be more and more commonly focused by the whole world, being a core field in domestic and overseas.The metallurgical method is an integration method that mainly removes phosphorous, boron and metal impurities in turn by several different techniques based on the characteristics of these impurities.Among the different steps, electron beam melting is considered to be effective to remove volatile impurity in silicon because of the character of high temperature, high vacuum and no pollution etc. Polysilicon purification technology by electron beam melting has been applied to industrial production, but its further development has been restricted due to its still problem of high energy consumption, high cost and discontinuous melting.In this paper, based on using the electron beam, high efficiency, low-cost and continuous melting new purification technology by EB are explored.Those technologys contain the electron beam injection into silicon powder to remove the metal impurity and the electron beam melting silicon powder to remove the volatile impurity.It is found that in the process of electron beam injected into the polysilicon powder, injected electrons take injection and penetration effect on polysilicon powder, so as to form a micro self-electric field around the powder and rise the powder temperature. Because of the micro-electric and thermal effects caused by the electron beam injection, most of the impurity elements are removed in a certain.The removal rate of the metal impurity is a little large, about10%to59%.Boron’s chemically similar impurity elements, Ga and Al have been removed in a certain persent, while the removal efficiency of B element is not obvious. During the same conditions of electron beam injection, the removal rate of the metal impurity elements is related to their diffusion coefficient. The removal rate increases with the increase of the diffusion coefficient of the impurity elements. The removal rate of impurity elements is related to the time of the electron beam injection. The removal rate increases with the time of the electron beam injection lengthened.During the process of electron beam melting silicon powder, the method that silicon pool is formed before silicon powder melted by electron beam is considered to be effective and feasible. By numerical calculation, in the process of electron beam melting silicon powder, melting time of silicon powder t熔is much smaller than dropping time of silicon powder t下, that is, silicon powder will be melted quickly as soon as it drops into the molten pool surface rather than reach the lowest point. It provides a favorable prerequisite for silicon powder melting and impurity removing. Through the experimental comparison of the electron beam powder melting with the electron beam bulk melting, it is shown that electron beam melting silicon powder is more effective for dephosphorization than electron beam melting silicon bulk.