多晶硅定向凝固反向诱导泉涌效应的研究

Directional solidification is the main technology to remove metal impurities in multicrystalline silicon, which is widely used in multicrystalline silicon ingot casting and silicon purification by metallurgical route. In theory, the impurity removal efficiency is up to more than 90%. However, the existence of back diffusion phenomenon greatly reduces the yield of the silicon ingot, because the impurity concentration in the end of the ingot is much higher than that in the other area so that impurities diffuse towards purified area under the action of concentration difference. It is an urgent key common problem to restrain the back diffusion behavior during directional solidification process. This project proposes a new method to restrain the back diffusion of the impurities based on the characteristics of volume expansion during silicon solidification. At the end of solidification, the surface of the melt containing a lot of impurities solidifies firstly by changing the thermal field and atmosphere environment. A quartz tube is inserted from the top across the solidification shell into the melt, and then the pressure in the tube will be changed. The silicon melt will gush along the quartz tube under the action of pressure difference, separate from the purified area and solidify rapidly, which is called gushing effect. In this process, several scientific problems, such as interface control, melt flow and solidification, will be analyzed and discussed. This project is the extend work of the applicant's doctoral research, and it is also the postdoctoral research subject. The formation mechanism of backward induced gushing effect will be studied systematically, and it will lay the foundation for the complete solution of back diffusion phenomenon during directional solidification.

定向凝固是去除多晶硅中金属杂质的主要技术，广泛应用在多晶硅铸锭、冶金法提纯过程中。理论除杂率可达90%以上，但由于凝固后硅锭末端杂质浓度远高于先凝固区域，在浓度差驱动下杂质向已提纯区域扩散，形成反扩散现象，大幅降低了硅锭的良率。如何抑制定向凝固过程中杂质反扩散行为，是一个急需解决的关键共性问题。本项目提出一种全新的研究思路，在凝固达到预期良率、液固共存的状态下，利用硅在凝固时体积发生膨胀的特性，通过改变热场和气氛环境诱导富含杂质的液硅表面首先形核凝固，将石英管从铸锭上方穿过表面插入熔体，通过改变石英管内的压强，使熔体在压力差的作用下涌入石英管，产生泉涌效应，实现熔体与已提纯基体的分离。本项目研究这一过程中的界面控制、熔体流动及凝固提纯等科学问题。本研究是申请人博士工作的延续，也是博士后研究课题，系统研究反向诱导硅熔体泉涌效应的形成机制，将为彻底解决定向凝固的反扩散现象奠定基础。

本项目针对多晶硅定向凝固提纯过程中杂质的反扩散造成提纯良率下降的瓶颈问题，提出了在硅定向凝固过程利用反向诱导泉涌凝固抑制杂质反扩散的方法，具体开展了以下几个方面的研究：.（1）开展了多晶硅定向凝固过程的杂质分布及固液界面控制的研究，获得了硅中杂质分布与固液界面形态、凝固速度、初始浓度之间的关系，基于定向凝固理论，建立了包含杂质反扩散行为的分布模型，获得了与实验值更加接近的理论结果，并得到控制硅凝固固液界面的技术方案；.（2）开展了泉涌效应诱导多晶硅凝固实验，确定了泉涌凝固的发生条件及时间，发现通过泉涌凝固在硅锭中形成的中空区域可以将杂质富集区和已经提纯的区域分离，减小了其接触面积，有效地抑制杂质的反扩散。建立了反向诱导硅熔体泉涌凝固的模型，阐明了其形成的机制。采用反向凝固控制、外加孔径等方案，对泉涌凝固的工艺进行优化，并将该研究结果向产业转化，获得了有效提高硅锭良率10%的技术方案；.（3）将泉涌诱导凝固的工作延伸到电子束熔炼过程中，通过缓慢降低电子束功率，实现了电子束诱导多晶硅定向凝固，杂质铝和钙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，与传统的电子束方式相比，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，且能耗降低了20%；.（4）探索了电子束诱导硅定向凝固去除硅中轻质元素的方法，发现而当采用缓慢降束慢速凝固时，SiC和氮化硅杂质均随时间增加不断沉积在铸锭底部，氮化硅的存在成为了替位碳析出形成SiC的形核点。慢速凝固也促进了C在氮化硅上的形核。SiC的形核生长及沉积是由熔体对流、温度梯度和慢速冷却过程共同作用的结果。