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真空定向凝固法去除硅中金属杂质和晶体生长控制的研究

【作者】 梅向阳

【导师】 戴永年; 马文会;

【作者基本信息】 昆明理工大学 , 有色金属冶金, 2010, 博士

【摘要】 目前,将主要生产电子级硅的西门子法用于生产太阳能级硅,该方法环境负荷大和生产成本高成为光伏产业的发展的瓶颈。冶金法具有成本低、工艺流程简单、设备投资少、环境负荷小等特点,是现阶段全世界研究的热点工艺。本课题组发明了冶金法制备太阳能级硅的新工艺(专利授权号:ZL200610010654.8),真空定向凝固是其中一个重要的组成部分。定向凝固可同时实现去除金属杂质和控制晶体生长这两种功能,而这两方面的研究目前还鲜见报道,本文对与此相关的科学技术问题进行了较系统的研究。本文从定向凝固和区域熔炼的相关理论出发,较系统地计算分析了定向凝固和区域熔炼去除硅中多种金属杂质的影响因素。研究表明,固液界面间的扩散层厚度、凝固速率、扩散系数等参数对金属杂质的去除有重要影响,减薄扩散层、提高扩散系数、降低凝固速率有利于提高杂质的去除效率。进而理论上给出了能较好去除硅中金属杂质的工艺参数的取值范围:在定向凝固中,温度在1471℃-1521℃之间、凝固速度控制在3μm/s~20μm/s之间比较适宜;去除硅中高含量的金属杂质Fe和Al扩散层厚度应控制在5.0×10-3cm以下较佳;在区域熔炼中,凝固速率控制在3μ/m/s~10μm/s之间比较适宜。在理论计算和分析确定的工艺参数参考范围内,采用不同的工艺条件对几种常用的硅原料进行了几十克级的区域熔炼、感应定向凝固和小型电阻加热真空定向凝固以及几公斤级的中型电阻真空定向凝固去除硅中金属杂质的实验研究。实验结果表明:几种方法都能不同程度地去除硅中的金属杂质;对比一次区域熔炼或一次定向凝固对样品中金属杂质的去除效果可知,电阻加热真空定向凝固去除杂质的效率最高,感应加热定向凝固次之,一次区域熔炼去除杂质的效果最差。感应加热定向凝固对不易挥发杂质Fe和Ti的去除效果与电阻加热真空定向凝固的相近。3N级硅粉经感应和电阻加热定向凝固后,Fe和Ti的最高去除效率均可达到99.9%。而对于易挥发的杂质Al和Ca,在以3N级硅粉为原料的电阻加热真空定向凝固实验中,去除效率最高均可达到99.7%以上,而感应加热定向凝固则稍差。在优化工艺的条件下,对以3N级硅粉为原料时,感应加热定向凝固所得样品中最低的Fe.Ti.Al和Ca含量分别为0.07ppmw、0.012ppmw、1.89ppmw和0.252ppmw;小型电阻加热真空定向凝固所得样品中最低的Fe、Ti、Al和Ca含量分别为0.07ppmw、0.016ppmw、0.63ppmw和0.084ppmw。上述结果均与理论预测吻合较好。另外,实验还发现,不同硅原料定向凝固去除金属杂质的效果不同。以纯度较高、杂质较少的硅料为实验原料时,定向凝固除杂效果较好。以3N级硅粉为原料经一次电阻加热定向凝固后,除A1外,样品中Fe、Ti和Ca的含量均在0.1ppmw以下,达到太阳能级硅的要求。从加热方式和实验规模对比杂质的去除效率可知,与小型电阻加热真空定向凝固相比,中型电阻加热真空定向凝固对Fe和Ti的去除率较高。扩大电阻加热真空定向凝固的实验规模,即可保持Al和Ca高的去除率,又可提高Fe和Ti的去除效率。在对硅锭中杂质的理论含量进行计算时,选择了三个扩散层厚度参数。对比不同方法在不同凝固速率下对不同原料中杂质的去除效果的实验值和理论计算值表明,在感应加热定向凝固中,扩散层厚度选择0.003cm-0.005cm计算较适宜;在电阻加热真空定向凝固中,扩散层厚度选择0.005cm-0.007cm计算较适宜。进行了不同方法不同凝固速率的晶体生长和缺陷控制实验研究。研究表明感应加热定向凝固获得的晶体细小、杂乱,小角度晶界相对较多;小型电阻加热真空定向凝固在凝固速率为20μm/s时,大部分晶体垂直于坩埚底部生长,获得的晶体尺寸’较大、位错密度较小、小角度晶界所占比例少、∑3晶界所占比例大;中型电阻加热真空定向凝固在凝固速率为10μm/s时,晶体垂直坩埚生长较明显,获得的晶体尺寸相比最大、大角度晶界和∑3晶界所占比例较大、小角度晶界所占比例较少,晶体质量较佳。由改变坩埚形状的晶体实验结果可知,在凹形坩埚底部放置单晶籽晶时,可明显减少晶体形核区,晶体明显垂直于坩埚底部生长。由于沿(111)晶面择优生长的晶体有利于提高太阳能电池转换效率和改善硅片的韧性,因此进行了晶体生长取向的实验。结果表明,感应和电阻加热定向凝固均可得到沿(111)晶面择优生长的硅锭,但是感应定向凝固实验得到的硅锭中晶体没有形成强的(111)晶面织构,实验结果与理论分析吻合较好。通过对不同方法和不同实验规模对不同金属杂质的去除效果和晶体生长质量综合对比分析可知,结合获得既可使多种金属杂质去除达到太阳能级硅要求又能定向生长<111>择优取向的粗大柱状多晶硅,电阻加热真空定向凝固的效果较好,而且扩大电阻加热真空定向凝固的规模对去除杂质和晶体控制生长均有利。综合考虑去除金属杂质和晶体生长,小型电阻加热真空定向最优工艺条件为:凝固速率为20gm/s和加热温度为1500℃;中型电阻加热真空定向最优工艺条件为:凝固速率为10gm/s和热温度为1500℃。为了进一步改善硅晶体结构和提高硅片的电学性能,对冶金法制备的硅片进行了热处理研究。考查了热处理温度、时间、气氛、腐蚀时间等实验参数对硅片的晶体择优取向、电阻率、位错密度和晶界缺陷的影响。结果表明,各实验参数对硅片的电阻率影响较大,在适当的实验条件下,热处理技术能大大地提高定向凝固多晶硅的电学性能。

【Abstract】 Now most polycrystalline silicon is produced by Siemens method which is used to produce electronic grade silicon, and their more environment load and high cost has blocked development of photovoltaic industry. The characteristics of metallurgy purification method is lower cost, simple process, little equipment investment and less environment load, the method is hotspot research process in the world. Vacuum directional solidification is one process of metallurgy purification method (authorized patent number:ZL200610010654.8), which is originated by our work group. The research on removing metal impurities and controlling crystal growth at the same time by vacuum directional solidification, is not discovered. In this paper, the technology problems about vacuum directional solidification are systematically studied.Influence facts of removing metal impurities of silicon by directional solidification and zone refining are systematically accounted and analyzed by solidification theory. The results indicate that experimental parameters including diffusion layer, solidification rate and diffusion coefficient, are important effect facts to removing metal impurities of silicon. Thinner diffusion layer, larger diffusion coefficient and lower solidification rate can enhance the segregation effectiveness of metal impurities. The fitting span of experimental parameters is presented. When the directional solidification experiment is carried out, the temperature should be controlled between 1471℃and 1521℃, and solidification rate should be controlled between 3μm/s and 20μm/s. When the diffusion layer is lower to 5.0×10-3cm, the removing effectiveness of Fe and Al is higher in the directional solidification experiment. When the zone refining experiment is carried out, solidification rate should be controlled between 3μm/s and 10μm/s.When the fitting span of experimental parameters is presented by theory calculation and analysis, bench-scale vacuum resistance directional solidification experiments with several decagram scale, pilot-scale vacuum resistance directional solidification experiments with several kilogram and zone refining experiments with several decagram scale were carried out. The results showed that the methods can gradely remove metal impurities of silicon. Through experiment results of once directional solidification and once zone refining were contrasted, the removal efficiency of metal impurities was the highest in vacuum resistance directional solidification experiments. The removal efficiency was higher in induction directional solidification experiments and the removal efficiency was the lowest in once zone refining experiments. The removing efficiency of Fe and Ti metal impurities which is difficult to volatilize was close in induction directional solidification experiments and resistance directional solidification experiments. When induction directional solidification experiments and vacuum resistance directional solidification experiments with 3N silicon powder as raw material were carried out, the highest removal efficiency of Fe and Ti metal impurities was 99.9 percent. When vacuum resistance directional solidification experiments with 3N silicon powder as raw material were carried out, the highest removal efficiency of Al and Ca metal impurities which is easy to volatilize was higher than 99.7 percent. But the removal efficiency of metal impurities was lower in induction directional solidification experiments than that of vacuum resistance directional solidification experiments. The concentration of Fe、Ti、Al and Ca in the samples was 0.07ppmw、0.012ppmw、1.89ppmw and 0.252ppmw in induction directional solidification experiments with optimize technical conditions. The concentration of Fe、Ti、Al and Ca in the samples was 0.07ppmw、0.016ppmw、0.63ppmw and 0.084ppmw in bench-scale vacuum resistance directional solidification experiments with optimize technical conditions. The previous experimental results were more anastomotic with theory value. In addition, the removal efficiency of metal impurities was different in directional solidification experiments with different raw materials. When the raw material was purer and metal impurities was less, the removal efficiency of metal impurities was higher. When once pilot-scale resistance directional solidification experiments with 3N silicon powder as raw material were carried out, metal impurity concentration of silicon ingots was lower than 0.1 ppmw except for Al concentration and have got the require of metal impurity concentration in solar grade silicon.Through experiment result contrast of different heating modes and different experiment scale, the removal efficiency of Fe and Ti metal impurities was higher in pilot-scale resistance directional solidification experiments than that of bench-scale resistance directional solidification experiments. When the experiment scale of vacuum resistance directional solidification experiments was expanded, the higher removal efficiency to Al and Ca was kept, at the same time, the removal efficiency to Fe and Ti was enhanced. When the theory value of metal impurities of silicon ingots was calculated, the three value of diffusion layer thickness were used. Through experiment result contrast of removal efficiency in different heating modes with different solidification rates and different raw materials, the thickness of diffusion layer should be cheese between 0.003cm and 0.005cm in induction directional solidification experiments, and the thickness of diffusion layer should be cheese between 0.005cm and 0.007cm in vacuum resistance directional solidification experiments.Crystal growth and defect control were studied in the experiments. The results showed that disordered columnar crystals, small grain size and the more small-angle grain boundaries is obtained in induction directional solidification experiments. When solidification rate is 20μm/s in bench-scale resistance directional solidification experiments, the growth direction of columnar crystals was basically perpendicular to the bottom of crucibles, and columnar crystals have some characteristics of large grain size, lower dislocation density, fewer small-angle grain boundaries, and more∑3 grain boundaries. When solidification rate is 10μm/s in pilot-scale resistance directional solidification experiments, the growth direction of columnar crystals was perpendicular to the bottom of crucibles, and average grain size was largest. The results of electron back-scattered diffraction measurements showed that the large-angle boundaries was dominating and the small-angle grain boundaries was very fewer, and the∑3 grain boundaries accounted for large proportion. These suggested that high crystal quality was obtained in pilot-scale resistance directional solidification experiments. When the bottom of crucible is concave, crystal nucleation region is decreased, and columnar crystals was markedly perpendicular to the bottom of crucibles.Columnar crystals of preferred growth along (111) crystal face was propitious to enhance conversion efficiency of solar cell and improved the tenacity of silicon wafers, so the crystal control growth experiments carried out. The results showed that columnar crystals of<111> preferred orientation were both obtained in induction directional solidification experiments and resistance directional solidification experiments, and stronger crystal texture has been formed in resistance directional solidification experiments, but done not in induction directional solidification experiments. The experiment results and theory analysis are accordant. The results of different methods and different experiment scale showed that higher removing efficiency to metal impurities and the columnar crystals with large grain size and directional growth along (111) crystal face, can be obtained. When the experiment scale was expanded in vacuum resistance directional solidification, it was favorable to enhance the removal efficiency to metal impurities and grow high quality crystals. The optimal technological condition of bench-scale resistance directional solidification experiments is that solidification rate is 20μm/s and heating temperature is 1500℃. The optimal technological condition of pilot-scale resistance directional solidification experiments is that solidification rate is 10μm/s and heating temperature is 1500℃.Heat treatment experiments were carried out to improve crystal structure and heighten resistivity of silicon wafers. The experiment factors including atmosphere, temperature, heat treatment time and acid corrosion time and the influence to crystal preferred orientation, resistivity, dislocation density of silicon wafers were research in heat treatment experiments. At the same time, the influence of the experiment factors to the performance of silicon wafers which includes preferred growth orientation, resistivity, dislocation density and grain boundaries, were studied. The results of heat treatment experiments showed that the experiment factors have rather great influence to the resistivity of silicon wafers. There is a conclusion that heat treatment can increase the electricity performance.

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