改良西门子法制备多晶硅过程的理论分析及建模

【作者】 侯彦青；

【导师】 谢刚； P.A.Ramachandran； 陶东平；

【作者基本信息】 昆明理工大学 ， 有色金属冶金， 2013， 博士

【摘要】 随着全球范围内传统能源的枯竭以及石油价格的不断攀升,太阳能作为环境友好能源受到全世界的广泛关注。多晶硅是作为光伏转换器最好的材料之一,95%的太阳能电池都是以硅作为基材。改良西门子法是制备多晶硅的主要生产工艺,然而如何提高硅产率、增大硅沉积速率、降低能耗是目前遇到的最大挑战,并且副产物SiCl4的处理也是影响多晶硅生产成本的主要原因之一。因此,研究了改良西门子法制备多晶硅过程中SiHCl3(TCS)合成过程的热力学,确定了TCS合成过程的最佳操作条件：550K-600K,2MPa-3MPa, H/Cl>100,计算结果与实际常用操作条件基本一致。在上述条件下,计算得到TCS的选择率为81.21%-83.21%,比实际生产中的65%-75%略高。通过研究Si-H-Cl三元系热力学得到：做为硅沉积原材料,TCS作为原材料时得到的硅产率要比SiCl4(STC)高。虽然SiH2Cl2的硅产率是最高的,但是由于SiH2Cl2不稳定,因此并不常用来作为硅沉积原材料。基于Si-H-Cl三元系热力学,研究了以TCS、STC、TCS和STC混合以及SiH2Cl2为原料氢化制备多晶硅过程热力学。以TCS为原料时,硅沉积过程中的最佳操作条件为：1425K、0.1MPa和进料配比(nH2/nsiHCl3)为15,与实际生产过程中通常所采用的操作条件基本一致。在最佳生产条件下,硅的理论硅产率为35.58%,而实际生产中不足20%。以STC为原料时,硅沉积过程的最佳操作条件为：温度为1400K、压强为0.1MPa、Cl/H比为0.05(即H2与STC的摩尔比为40：1),此时硅的沉积率为34.84%。以STC和TCS的混合物为原料时,确定硅沉积过程的最佳温度为1400K,压强为0.1MPa。通过研究还确定当得到合理的硅产率时,Cl/H比与原料配比的关系。当进料为纯SiH2Cl2时,可在低温条件下进行,硅产率可达到可观的45%。为了进一步提高硅产率,可用氢气作为还原剂。然而最佳温度范围为1400K到1450K,这势必会增大多晶硅的生产能耗。所以当以SiH2Cl2为原料制备多晶硅时,以纯SiH2Cl2是较优的生产多晶硅的方法。发展了电流加热硅棒的热传递模型,包括直流电加热和交流电加热。通过研究发现：由于交流电的表面效应,当硅棒用交流电加热时,硅棒中心的温度低于直流电加热时的温度。并且随着交流电频率的增大,硅棒中心的温度明显降低,但是硅棒两端所施加的电压明显增大。当交流电频率较低时,随着硅棒半径的增大,硅棒中心的温度明显升高。但是随着交流电频率的增大,硅棒中心与表面的温度差随着硅棒半径的增大而变化的越来越平缓。通过交流电加热增大生产过程中硅棒的增长半径,从而达到降低能耗的目的。建立了西门子反应器动力学-传递模型,该模型同时研究了传递现象和表面化学反应两个方面对硅沉积速率的影响。并应用该模型研究了西门子反应器中,进料气流速度、进料气体组分、硅棒表面温度、进料气体温度和压强对硅沉积速率、生产单位质量多晶硅的能耗和硅表面HCl摩尔浓度的影响。通过研究发现：(1)随着气流速度的增大,硅的沉积速率开始明显增大,然后曲线开始变得非常平缓；硅沉积效率降低；生产单位质量的多晶硅的能耗明显升高。(2)随着进料中氢气的摩尔分数的增大,硅沉积速率先增大后减小；硅表面的HCl摩尔浓度也是先增大后降低,高H2摩尔分数有利于HCl从硅表面解析；生产单位质量多晶硅的能耗先降低后增大。当硅表面温度为1423K,进料气体温度为573K,硅棒半径为5cm时,最佳的氢气摩尔分数为89%。此时硅沉积速率达到最大(5.54μm/min),并且生产lkg多晶硅的能耗也最低(21.58kWh),总能耗为55.9kWh,远低于生产过程中西门子反应器的能耗120kWh。(3)随着硅棒表面温度的升高,硅沉积速率明显增大,硅表面HCl摩尔浓度增大,但是生产单位质量的多晶硅的能耗明显降低。(4)尽管随着进料气体温度的增大,硅沉积速率增大,硅表面HCl摩尔浓度增大,但是变化很小；生产单位质量的多晶硅的能耗降低。(5)随着压强的增大,硅沉积速率明显增大,硅沉积速率达到最大时的氢气摩尔分数也增大。硅棒表面与气体之间因对流传递的能耗明显增大,并且因高压会增大反应器的设计和操作的难度。反应器内部的最佳压强为0.1MPa-0.2MPa。研究了STC氢化过程中的热力学,包括STC高温氢化和STC低温氢化过程。STC高温氢化过程的最佳操作条件：温度为1000℃,压强为0.3MPa,进料配比xH2/xsicl4为4,与实际生产过程中所选用的操作条件一致。在此条件下,SiCl4的氢化率为25.78%,远高于实际生产中的低于20%。STC低温氢化过程的最佳操作条件为：温度为850K,压强为1.5MPa,进料配比H/C1为10。计算结果和实际生产条件(温度、压强和H/C1比)基本一致,但是通过计算得到的STC转化率为58.89%,远远高于实际生产中的15%-30%。研究了锌还原法制备多晶硅的热力学,通过研究确定最佳操作条件为：温度控制在1200K左右、2atm、进料配比nZn/nSiCl4=4,与实际生产条件基本一致。在此条件下硅的理论产率是90.3%,而实际生产中仅为67%左右。最后利用二阶Moller-Plesset方法和密度泛函理论中的广义梯度近似(GGA)对Si(100)面上SiCl4锌还原过程进行了研究,结果表明：锌原子倾向于与因顶位吸附解离的Cl原子结合；当体系中的氢气过量时,能耗降低且硅产率升高,从而可进一步降低硅的生产成本；最后确定了SiCl4锌还原的最佳反应途径。更多还原

【Abstract】 Alternative energy source for electricity production is a subject of considerable value globally in view of the ever-increasing cost and diminishing supply of petroleum fuels. Polysilicon is presently one of the best materials for application in photovoltaic energy conversion and the polysilicon is the base material in95%of solar cells. The modified Siemens process is the mian process for manufacturing polysilicon. However, how to find the best methods to increase the silicon yield, increase silicon growth rate and decrease energy consumption is the challege in present. Furthermore, the recycle of the by-production SiCl4is also one of the reasons which affect the cost of polysilicon.Based on the thermodynamic data of related species, the thermodynamics of TCS synthesis in Siemens process for manufacturing polysilicon has been studied. The optimum conditions have been obtained as550K-600K,2MPa-3MPa, H/Cl>100, which is accord to that in the actual operation. Under the above conditions, the calculated TCS selectivity is81.21%-83.21%, which is a little higher than65%-75%in industrial production.The interesting results can be obtained by investigating the thermodynamics of Si-H-Cl system. As a precursor for silicon deposition in Siemens process, SiHCl3is preferred for deposition of silicon to SiCl4.By the same reasoning SiH2Cl2would be even better but it is not as stable a gas and hence it is less frequently used. The interesting results show that the mixture of SiCl4and SiHCl3as the precursor can be fed into Siemens reactor. It is worthwhile to study the implication of this in the overall system by a process simulation model in a future study. The thermodynamics, therefore, have been studied when TCS, STC, the mixture of TCS and STC and SiH2Cl2as the precursors are fed into Siemens reactor for manufacturing polysilicon. The conversion, yield and product distribution have been calculated and the optimum conditions have been obtained and compared with limited published data in the open literature. When TCS is as the precursor, the optimum conditions are1425K,0.1MPa and the ratio of H2to TCS of15, which are good agreement with that reported by experimental studies. Under these conditions, the calculated silicon yield is35.58%, which is much higher than20%that is the average silicon yield in industrial production. It is an interesting method to deal with the main by-production STC that the mixture of TCS and STC as the precursor are fed into Siemens. The optimum temperature is1400K and the best pressure in the reactor is0.1MPa. The higher silicon yield can be obtained at even higher Cl/H ratio when the higher molar fraction of TCS is fed into the reactor. The relations of silicon yield, then, have been studied. From the results, we can find the range of Cl/H ratio which should be kept to obtain reasonable silicon yield when the fixed molar fraction of TCS is fed into the reactor and can also determine the range of molar fraction of TCS fed into the reactor to obtain reasonable silicon yield when the Cl/H ratio is fixed. When the SiH2Cl2without H2is fed into the reactor, the temperature has slight influence on silicon yield. The silicon yield can be up to be45%, which is much higher than that obtained with TCS as the precursor at lower temperature. In order to increase the silicon yield, the H2can be inject into reactor together with SiH2Cl2. However, the gas in the reactor should be heated to be1400K-1450K, which cause the power increase significantly. The SiH2Cl2without H2is, therefore, the best method when the precursor is SiH2Cl2.The heat transport model of rods heated by DC and AC has been developed in Siemens reactor. The results show that the temperature inside the rods is more homogenous when the rods is heated by AC than DC and the temperature inside the rods becomes more and more homogenous with increasing the frequency of AC. The temperature in the centre of the rods decreases when the surface temperature is fixed as a constant when the temperature inside the rods becomes more homogenous. When the radius of the rods growth because of silicon deposition on the silicon rod surface, the central temperature of the rods increases significantly when the rods are heated by AC with lower frequency but become flat when the rods are heated by AC with higher frequency. The central temperature of the rods almost keeps as a constant when the radius increases to be a somewhat value. The diffrence of the temperature between the center and the surface of the rods decreases by AC heating. Therefore, the radius of the rods can grow to a bigger value. The energy consumption can reduce as a result.The kinetics-transport model of Siemens reactor has been developed. The model is employed to investigate the effect of transport rate of gas and reaction rate on the silicon surface on the silicon growth rate. The results show that the silicon growth rate firstly increases significantly and becomes flat with increasing gas flow rate. But the silicon deposition efficiency decreases and power loss for manufacturing1kg plolysilicon increases when the gas flow rate increases. With increasing fraction of hydrogen in the feed gas, both of the silicon growth rate and HCl concentration on the silicon surface increase firstly and then decrease with a maximum value while the power loss for manufacturing lkg polysilicon decreases firstly and then increases. The silicon growth rate is up to maximum (5.54μm/min) and the power loss is the lowest (21.58kWh) at the optimum molar fraction of hydrogen (89%) in the feed gas. The total energy consumption is55.9kWh, which is much lower than120kWh which is the result in the inductrial production. With increasing the surface temperature on the rods, both of the silicon growth rate and HCl concentration on the silicon surface increase significantly and power loss for manufacturing lkg polysilicon decreases. Furthermore, the higher surface temperature on the rods can increase the desorption rate from silicon surface. Both of the silicon growth rate and HCl concentration on the silicon surface increase with increasing the temperature of the feed gas in the inlet, but the change is slight. However, the power loss for manufacturing1kg polysilicon decreases. Although the higher pressure can increase the silicon growth rate significantly, the power loss for manufacturing lkg silicon increase and the difficulty of design and control of the reactor increase because of the higher pressure in the reactor. The optimum pressure in the reactor should be0.1MPa-0.2MPa.The thermodynamics in STC hydrogenation process has been studied, including direct hydrogenation (hydrogenation process at higher temperature) and hydrochlorination process (hydrogenation process at lower temperature). The optimum conditions for direct hydrogenation of STC at high temperatures are studied and found to be in the range of1323K temperature,0.3MPa pressure and a flow ratio of4for hydrogen to STC. The conversion value is around25%. The actual SiHCl3conversion ratio of less than20%is much less than25%calculated under these conditions. The optimum conditions in hydrogenation of SiCl4process at lower temperature in presence of silicon (hydrochlorination process) have been obtained as850K,1.5MPa and H/Cl ratio of10. Under these conditions, the SiCl4conversion ratio is58.89%against the actual conversion ratio of15%-30%. Compared to hydrogenation of SiCl4process at higher temperature, the hydrogenation of SiCl4process at lower temperature is operated at lower temperature but the pressure and H/Cl ratio is much higher. The power assumption for the process at lower temperature is much lower but operation is difficult and complicated.By the study of thermodynamics in zinc reduction process for manufacturing polysilicon, the optimum conditions have been obtained as1200K, about2atm and the molar ratio of Zn to STC of4, which is accord to these reported in the open literature. Under these conditions, the calculated silicon is90.3%against about67%in experimental results.The study of heterogeneous reaction mechanisms of zinc reduction of SiCl4for manufacturing polysilicon is reported in this paper. The results show that the zinc tends to combine with atomic chlorine which dissociated in dissociation absorption. And the excessive gaseous zinc is necessary in the process. When there is enough gaseous zinc in the system, the energy consumption can be reduced and polysilicon yield rate increase which can reduce the cost in actual manufacturing production. Finally, the optimum reaction mechanisms have been obtained.更多还原