电子束提纯多晶硅过程中杂质的传输与去除机制研究

发布时间：2018-03-29 23:06

本文选题：电子束熔炼　切入点：多晶硅　出处：《大连理工大学》2017年博士论文

【摘要】：多晶硅是太阳能电池的主要原材料,急需开发具有高效率、低成本、工艺稳定的多晶硅绿色制造方法。冶金法是针对制造太阳能级多晶硅而提出的一种专属提纯技术,该方法以冶金级硅为原料,根据硅和杂质物理化学性质的差异,优化集合酸洗、造渣精炼、真空冶炼、电子束熔炼、定向凝固等方法,分步降低硅中杂质含量,实现杂质高选择性、梯度式分离,最终获得太阳能级硅材料。硅中杂质与硅物理化学性质的差异主要表现在蒸发、分凝和氧化特性上,目前的冶金法各单元技术都是基于杂质的某一种特性,某些杂质无法通过一次熔炼过程被去除到目标值,需要经过两次或多次熔炼去除。同时,这些单元技术之间相互独立,缺少有机耦合,每个环节都在重复熔化-熔炼-凝固的过程,存在流程长、效率低、产品品质不稳定等问题,增加了能耗和成本。电子束熔炼是冶金法制备太阳能级多晶硅的关键技术,其特点是具有高温、高真空的熔炼环境,并且熔池内的温度分布可以通过功率大小、扫描方式、扫描频率等进行精确控制。本研究提出冶金法去除硅中杂质的本质是固态硅中的杂质通过冶金过程完成在硅的三态(固相、液相、气相)或其它相(渣相或合金熔体等)中的再分配,并考察了杂质在蒸发、分凝和氧化组成的多个维度的性质,发现杂质在多个维度上具有特征,可通过多个特征的结合深度去除杂质。在此基础上,针对不同类型的杂质,利用电子束构建特征熔炼环境,研究杂质的传输和去除的机制。基于电子束熔炼状态下高温、高真空的特征环境,建立了多晶硅熔炼过程中蒸发性杂质在硅液相-气液界面-气相传输的全域控制模型,解析硅中杂质去除的动力学机制。研究表明,杂质从硅熔体中去除的总传质系数主要受到温度和和真空室压力的影响,温度越高、真空室压力越小,传质系数值越大。在相同的熔炼条件下,P、Al、Ca三种杂质元素的总传质系数值大小为kPkAlkCa,表明P更容易通过蒸发去除。在电子束熔炼过程中,P的去除由界面蒸发和液相边界层传质共同控制,而Al和Ca的去除由界面蒸发控制。杂质去除的同时硅也在蒸发损失,当P的去除率接近100%的时候,硅的损失率可以控制在10%以内,说明电子束熔炼是去除硅中杂质P的有效方法。但此时Al的去除率仍然较低,要想提高其去除率,需要更高的熔炼温度或更长的熔炼时间,这会导致硅损失率的进一步增加。针对Al、Ca等同时具有分凝特性和蒸发特性的杂质,通过缓慢降低电子束功率,构建熔体内部单向温度场-气相高真空的特征环境,实现了电子束诱导多晶硅定向凝固。研究表明,杂质Al和Ca在硅中的分布呈现出了分凝的趋势,其去除率取决于熔体温度和凝固速率,合理的控制熔体温度和凝固速率可以获得较高的杂质去除率和较好的均匀性。在熔炼过程中,杂质的去除受到气液界面蒸发的控制,在15kW的功率下,Al和Ca的总传质系数分别为3.43×10-5m/s和2.04×10-5m/s,熔炼1800s后去除率分别达到了98%和91%。在电子束诱导定向凝固过程中,杂质的去除受到固液界面分凝和气液界面蒸发的耦合控制,其含量进一步降低到0.7×10-4wt.%以下。与传统的电子束熔炼方式相比,在总时间相同的条件下,电子束诱导定向凝固的方式具有相同甚至更高的杂质去除率,且由于凝固时功率降低减少了能耗。在本实验中,与使用15kW的功率熔炼3000s相比,当熔炼时间为1800s、降束时间为1200s时的能耗降低了 20%。将电子束熔炼去除P杂质和电子束诱导定向凝固去除金属杂质的方法应用于产业化电子束熔炼设备上,使硅中的P含量降低到0.3×10-4wt.%以下,Fe、Al、Ca的含量降低到10-6～10-5wt.%的数量级,且这些金属杂质的分凝效果优于相同凝固速率下传统定向凝固的分凝效果。在此基础上,结合数值模拟和实验手段,从能量利用角度对现有熔炼工艺进行优化,通过在坩埚内部增加石墨衬底提高能量利用率。发现在硅和水冷铜坩埚之间增加一层石墨衬底,相当于增加了热阻,减少了热量的损失。使用石墨衬底的熔炼方式,在熔炼功率相同时,熔池温度提高,加速了 P的去除。实际熔炼过程中,在达到相同的除P效果时,可以缩短熔炼时间,使单位质量能耗从29.3kW·h/kg降低到19.5kW·h/kg。
[Abstract]:Polysilicon is the main raw material for solar battery, it is urgent to develop high efficiency, low cost, green manufacturing process stable polysilicon. Metallurgical method for manufacturing solar grade polysilicon and proposed an exclusive purification technology, the metallurgical grade silicon as raw materials, according to the differences of physical and chemical properties of silicon and impurity, optimized pickling, slag refining, vacuum metallurgy, electron beam melting, directional solidification method, step by step to reduce the impurity content in silicon impurities, achieve high selectivity, gradient separation, finally obtain solar grade silicon material. Different impurities in silicon and silicon physical and chemical properties mainly in evaporation, segregation and oxidation characteristics, metallurgical method at present each unit is a kind of technology based on the characteristics of impurities, some impurities can not be removed by a melting process to the target value, need to go through two or more times with smelting removal. When these technologies are independent of each other, the lack of organic coupling, each link in the process of melting and solidification of the repeated melting process, there are long, low efficiency, unstable product quality problems, increase the energy consumption and cost. Electron beam melting is the key technology of metallurgical preparation of solar grade polysilicon, which is characterized by high temperature, high vacuum melting atmosphere, and the temperature distribution in the molten pool through the power of the size, scanning, scanning frequency and precise control. The essence of this research put forward the method of metallurgical removal of impurities in silicon is solid impurities in silicon in three states of silicon by metallurgical process (solid phase, liquid phase and gas phase) or other phase (slag or alloy melt etc.) in redistribution, and the effects of impurities in the evaporation properties of multi dimensions of segregation and oxidation of impurities, with features found in many dimensions, through multiple features With the depth of removal of impurities. On this basis, according to the different types of impurities, construction characteristics of smelting environment by electron beam transmission mechanism research and impurity removal. Electron beam melting state under high temperature environment based on the characteristics of high vacuum, a global control model of the evaporation of impurities in silicon liquid - gas phase gas-liquid interface the transmission of polysilicon in the process of smelting, the kinetic mechanism of removal of impurities in silicon analysis. The research results show that the effect of the total mass transfer coefficient to remove impurities from the silicon melt is mainly affected by the temperature and pressure of the vacuum chamber, high temperature, vacuum chamber pressure is small, the mass transfer coefficient greater melting. In the same conditions, P. Al, the total mass transfer coefficient of size Ca three impurity elements kPkAlkCa, P showed more easily through evaporation removal. In electron beam melting process, the removal of P by interfacial evaporation and liquid phase boundary layer mass transfer co control, Al Ca was controlled by the interfacial evaporation. At the same time also in the removal of impurities in silicon evaporation loss, when the removal rate of P is close to 100% of the time, silicon loss rate can be controlled within 10%, indicating electron beam melting is an effective method to remove the impurities in silicon P. But the Al removal rate is still low, to to improve the removal rate, melting time requires higher melting temperature or longer, it will lead to further increase of the silicon loss rate for Al, Ca also has the impurity segregation characteristics and evaporation characteristics, reduce the beam power through the slow construction characteristics of environment inside the melt temperature and gas phase unidirectional high vacuum the realization of the electron beam induced polysilicon directional solidification. The results show that the distribution of Al and Ca impurities in silicon showing a segregation trend, its removal rate depends on the melt temperature and solidification rate, the reasonable control of melt temperature and solidification rate can Get a higher impurity removal rate and good uniformity. In the process of melting, removal of impurities is controlled by evaporation in the gas-liquid interface, the power of 15kW, the total mass transfer coefficient Al and Ca were 3.43 * 10-5m/s and 2.04 * 10-5m/s, melting after 1800s removal rate reached 98% and 91%. induced in the electronic beam directional solidification process, impurity removal by coupling the solid-liquid interface segregation and gas-liquid interfacial evaporation, the content is further reduced to 0.7 * 10-4wt.%. Compared with the traditional way of electron beam melting, in the condition of the same total time, electron beam induced directional solidification method has the same even higher impurity removal the solidification rate, and reduce the power consumption. In this experiment, compared with the power of melting of 3000s using 15kW, when the melting time of 1800s, falling time is 1200s when the beam energy is reduced by 20%. electron beam The removal of P impurity and electron beam melting method by directional solidification removal of metal impurities used in industrial electron beam melting equipment, the content of P in silicon is reduced to less than 0.3 * 10-4wt.%, Fe, Al, Ca content reduced to 10-6 ~ 10-5wt.% magnitude, and the effect of these metal impurity segregation is better than points coagulation effect is the same as the solidification rate of traditional directional solidification. On this basis, combined with numerical simulation and experimental methods, from the viewpoint of energy utilization of existing smelting process is optimized through the interior of the crucible increases the graphite substrate to improve the energy utilization rate. It is found that adding a layer of graphite substrate between the silicon and water cooled copper crucible, equivalent to an increase of thermal resistance and the heat loss is reduced. The use of graphite substrate melting, melting in the same power, the temperature of the molten pool increase, accelerate the removal of P. The actual smelting process, in addition to P to achieve the same effect, The melting time can be shortened and the unit mass energy consumption is reduced from 29.3kW to h/kg to 19.5kW. H/kg.

【学位授予单位】：大连理工大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TQ127.2

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