Numerical Study Of Hot Zone Upgrade And Heat And Mass Transfer During Silicon Directional Solidification Process

Directional solidification technology of crystalline silicon is one of the main technologies for the industrial production of solar grade silicon.At present,expanding the size of silicon ingot can effectively improve production efficiency and reduce production costs,which has become the trend of crystalline silicon production.However,size upgrade will affect the growth conditions of silicon,such as thermal field,flow field,thermal stress field and impurity field.Thus,considering the large size and high quality of silicon ingot becomes one of the challenges in research and production.In this paper,the heat and mass transfer of GT G6 and ALD G7 furnaces before and after size upgrading are studied by numerical simulation combined with experiments,which provides theoretical guidance for improving the growth quality of large size silicon.The main research contents and conclusions are as follows:（1）According to the literatures about crystalline silicon growth at home and abroad,the silicon growth mechanism,heat and mass transfer mechanism and thermal stress generation mechanism of crystalline silicon,as well as the research status of heat and mass transfer in the growth of crystalline silicon are introduced in detail.These lay a foundation for fully understanding and crystalline silicon growth and analyzing the results of numerical simulation.（2）Based on the principle of directional solidification growth,2D global unsteady heat and mass transfer numerical models of GT type G6 and ALD type G7 are established by Fluent simulation software,in which heat conduction,convection,radiation and phase change and oxygen and carbon impurity transport are considered.The reliability of the numerical heat transfer model is verified by comparing heaters power and TC2 temperature of G6 and G7 with the simulated values.In addition,the thermal stress model of G6 and G7 silicon ingots is established by Comsol simulation software.（3）Based on the G6 and G7 heat transfer numerical models,the effect of hot zone change on the growth parameters of silicon ingot is studied.The results show that the radial temperature difference of G7 silicon melt is greater than that of G6,which makes the flow intensity of G7silicon melt greater than that of G6.Compared with G6,the center of the G7 silicon ingot at the early stage of crystal growth is too cold,which leads to the problem of overconvex center at the melt-crystal interface and obvious edge warping problem melt-crystal interface in the middle and late stages of crystal growth.The temperature gradient of the G7 silicon ingot at crystal growth process is greater than that of the G6,which increases the crystal growth rate and thermal stress.In addition,by comparing the V_cr/G_c ratios of G6 and G7,it is found that the V_cr/G_c parameters of both are higher than the critical value,and the higher V_cr/G_c ratio of G7 makes G7 grown silicon ingots more likely to form defects.（4）Based on the heat and mass transfer models of G6 and G7,oxygen and carbon impurities distributions in argon and silicon melt at the full melting and crystal growth stages are analyzed.From the full melting period to the crystal growth stages,the argon on melt free surface changed from a pair of vortex cells to two pairs of vortex cells,and the concentration of Si O and CO above the free surface of the melt changed from a single peak to a double peak.The flow structure and strength of silicon melt have an important influence on the distribution of oxygen and carbon impurities.The flow strength of silicon melt at the melt-crystal interface of G6 and G7 is weak,which is easy to cause the accumulation of C impurities.The flow of G6 and G7 silicon melts flows along the melt-crystal interface from the center to the side wall of the crucible,which is beneficial to reduce the accumulation of impurities in the center of the melt.As the solidification fraction continues to increase,the oxygen impurity concentration of G6 and G7 silicon melts gradually decreases,and the carbon impurity concentration gradually increases.At the end of the growth process,the distribution of oxygen and carbon in G6 and G7 ingots show that the oxygen concentration decreased gradually and the carbon concentration increased gradually from bottom to top.（5）In the early stage of crystal growth in the G7,the melt-crystal interface is too convex,which is not conducive to the vertical growth of crystal grains.The problem is improved by adjusting the bottom hot door movement rate,and analyzes the changes of the G7 melt-crystal interface and heat and mass transport of silicon area under different bottom hot door movement rates.It is found that with the gradual decrease of the popular movement speed of plan 1 to plan 3,the flow structure of the silicon area has not changed significantly,but the convexity of the melt-crystal interface has been significantly improved.The convexity from plan two to three has been reduced by 55%and 44%,respectively.Compared with plan 1,plan 2 and 3 increase the heat flux of silicon sidewall by 38%and 60%,and decrease the heat loss of silicon bottom wall by 18%and27%,respectively.The temperature gradient in the axial directions of the plan 1 to plan 3 is reduced,the maximum thermal stress of plan 2 and plan 3 are reduced by 20%and 27%,and the minimum thermal stress is reduced by 31%and 43%,respectively.Because the changes of the silicon melt flow are small,the distribution of O and C impurities in the silicon area from plan one to plan three is similar.The reduction of the hot movement rate improves the melt-crystal interface and the heat flow distribution of the silicon ingot,but has a smaller impact on the transport of impurities.