Removal And Segregation Of Metal Impurity In Polycrystalline Silicon During Directional Solidification Process

In recent years,solar grade silicon is rapid growth.The polysilicon preparation method with low cost and environmental protection technology is in urgent need.Metallurgical method is the effective way to remove the impurities such as B,P,and metal impurities in silicon.Directional solidification technology,as one of the metallurgical method in the process,can be used to remo’ve the metal impurities.The metal impurieis can be enriched to the top of the ingot casting.As a result,impurity content of bottom and middle of silicon ingot is low,which form purification area as production.To raise the purity and production of purification area,reduce the production cost has become a focus problem in the process of industrialization of poly silicon directional solidification.In this paper,impurity segregation behavior coupling with crystal growth,vacuum and temperature gradient during directional solidification,to collaborative control impurity removal and improve the efficiency of impurity removal.Using seed crystal impurities segregated coupling with seed induced technology,to producte high power conversion efficiency of silicon ingot casting with the multisilicon by metallurgical method.Different grain morphology leads to metal impurities show different segregation behavior.Substitution impurities such as B and Al are not controlled by grain morphology and have stable segregation effect.For the interstitial impurities such as Fe、Ti、Cu、and Ni,which have a small diffusion coefficient,and the crystal structure is controlled by crystal morphology.When the grains are irregular grains,the solid-liquid interface is a paste-like region,and the impurities in the silicon melt of the diffusion is affected by the paste-like region,impurities are enriched at the solid-liquid interface and through the "short-circuit diffusion" to promote the back diffusion of impurities to the solid phase,resulting in segregation coefficient is unstable.If the grains are a columnar grain,the solid-liquid interface is stable,and the metal impurities are rapidly transferred to melt to reduce the content of the impurity at the front of the solid-liquid interface,thereby reducing the effective segregation coefficient of the impurities.In the process of directional solidification of polysilicon,the content of metal impurities increases,leading to the composition of the solid-liquid interface is supercooling,affecting the crystal morphology.During the solidification process,the temperature gradient of the solid-liquid interface is improved,and avoids the constitutional supercooling,the formation of paste-like region is inhibited,and the effective segregation stability of the impurities can be promoted.Under the industrial conditions,the temperature gradient of the solid-liquid interface is adjusted by adjusting the temperature gradient of environment.By increasing the temperature gradient of environment,the flow of the silicon melt is accelerated to promote the transmission of the solid-liquid interface front impurity.Thereby the thickness of the impurity difusion layer is reduced.The thickness of diffusion layer of Fe、Cu、Ni、Ti were reduce to 3.35ppmw、1.21ppmw、1.85ppmw、5.81ppmw respectively.The impurity content of the polysilicon in the purified region was reduced from 2.69ppmw to 0.96ppmw,the impurity content was reduced by 64.3%,and the solidification time was reduced by 21.4%.In the later stage of directional solidification of poly silicon,the higher temperature gradient was used,the height of columnar crystal was increased to 95.2%,and the proportion of polysilicon in the purified region was increased to 90.0%,which effectively reduced the production cost under industrial conditions.In the reverse solidification technique,the segregation of solid and liquid is introduced into the silicon melt,which effectively restrains the back diffusion of the impurity during the directional solidification process,and the proportion of the poly silicon in the purified region was increased to 97.3%.In the process of directional solidification,the segregation and evaporation of impurity were controlled synergistically,and the transmission of impurity in the gas-liquid-solid three phase is controlled,and the volatile impurities such as Na,Al,Ca,Mg and Cu were removed.Under the industrial conditions,polysilicon was directional solidified at low vacuum(0.6atm),and the Na and Mg impurities with original contents of about 1038.22 ppmw and 17.10 ppmw were removed to 0.10 ppmw and 0.06 ppmw respectively.Because of the segregation of Al impurities,A1 was reduced from 494.5 ppmw to 2.85 ppmw.The Al and Ca impurities can be reduced from 1.40ppmwand 190 ppmw to 1.67 ppmw and 1.85 ppmw respectively,during directional solidification of polysilicon under the vacuum(0.1 Pa).The segregation and evaporation of impurity were controlled synergistically under the directional solidification process of poly silicon.The effective segregation coefficient of Au and Ca were keff=1.21×10-3 and keff= 5.69×10-3 respectively,content of A1 and Ca in the purifited region were 0.60 ppmw and 1.75 ppmw respectively.The limits of impurity removal by segregation was breaked.The polysilicon by the metallurgucal method was used as raw material to produce polysilicon ingot.The efficient conversion of 18.65%was achieced by using impurity segregation and seed crystal induced crystal growth.Through nucleation induction by a flat substrate nucleation and the concave angle raised the rate of crystal nucleation to control the crystal growth,30%more than(100)crystal plane.Grain size in the bottom,middle and top of silicon ingot were 3.47 mm,6.64 mm and 10.23 mm,respectively.Even small grain to avoid production of dislocation.The existence of the twin can reduce the proportion of grain boundary,effectively reduce the dislocation of the silicon wafer.The even small grain and twin dislocation generation improve the photoelectric conversion efficiency of ingot casting.Based on collaborative control of crystal growth and impurities segregated during directional solidification process,the red zone of polycrystalline silicon ingot casting ratio was made to reduce to 10%.