Vacuum Directional Solidification Method To Remove Metallic Impurities In Silicon And Crystal Growth Control

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.