Research On The Detection Of Metal Impurities In Silicon Tetrafluoride Gas In The Wet Route

Silicon tetrafluoride?SiF₄?,used as an important raw material in the electronics and semiconductor industries,is also used in the manufacture of isotope silicon,polysilicon,solar cells and polycrystalline silicon thin films.Polycrystalline silicon used as solar cells not only requires its purity to reach 6N?99.9999%?,but also requires its total metal impurity content to be less than 1ppm?mass fraction?.Otherwise,it will result in the decrease of carrier concentration and lifetime,the increase of resistivity and the decrease of photoelectric conversion efficiency.As a large province of phosphate rock resources,Gui zhou has about 5 million tons of phosphate rock mining per year.The processed phosphate rock is about 4 million tons,and the fluorine content in phosphate rock is about 2%to 4%.In the process of wet processing of phosphate rock to produce phosphoric acid,SiF₄ is produced as a by-product.If it is not recycled,it is equivalent to a waste of about 1 million tons of fluorosilicon resources per year.Due to the large amount of impurities in the phosphate rock,it is unavoidable to introduce metal impurities into the SiF₄ gas.The presence of these metal impurities has an effect on the purity and application of SiF₄.At present,the analysis of metal impurities in SiF₄ gas is still infancy,and the analytical methods and detection systems are still not perfect.Therefore,it is especially important to detect and purity metal impurities in SiF₄ gas.In this paper,trace elements and rare earth elements in SiF₄ prepared by fluosilicic acid decomposition,SiF₄ prepared by sodium fluosilicate thermal decomposition,freeze-purified SiF₄ and high purity SiF₄ gas are determined.It is found that the total metal impurities in SiF₄ gas prepared by four methods are more than 10000ppm,and the total metal impurities in SiF₄ gas prepared by fluosilicic acid decomposition method are the highest,reaching 43422.78ppm.Then the content and distribution of metal impurities in SiF₄ gas prepared by different methods are studied.It is found that most of the metal impurities are below 10 ppm.At the same time,the change of metal impurity content before and after freeze-purified of SiF₄ prepared by decomposition of fluosilicic acid is analyzed.It is found that the reduction rate of total metal impurity content in the freeze-purified SiF₄ gas is 75%,and that The reduction rate of single metal impurity content ranges from 50%to 98%.It indicats that freeze-purified could reduce the metal impurity content in SiF₄ gas to some extent,and the effect of reducing different metal impurities is also different.Secondly,the maximum likelihood method is used to establish the normal distribution function model to calculate the total metal impurities in SiF₄ prepared by fluosilicate decomposition,SiF₄ prepared by sodium fluosilicate thermal decomposition,freeze-purified SiF₄ and high purity SiF₄.It is found that the actual measured values are higher than the theoretical values by four orders of magnitude,indicating that there are abnormal high concentrations of one or several metal impurities in SiF₄ gas.It is also indicated that metal impurities are contaminated in the process of preparation,absorption and digestion of SiF₄ gas.In addition,by dividing the total metal impurities in SiF₄ gas prepared by different methods into four categories and drawing a normal distribution function curve for theoretical calculation,the following conclusions are drawn:in the classification of metal impurities,alkali metals and alkaline earth metals have high concentrations,The probability that rare earth elements determine the purity of samples in a certain concentration range is less than that of alkali metals and alkaline earth metals,and the transition metal and the P type element are in the middle.Rare earth elements contribute greatly to the average concentration and total metal impurity concentration.The transition metals and P-type elements are in the middle,and the contribution of alkali metals and alkaline earth metals is the smallest.Finally,the effects of glassware,gas pipelines and the cleanliness of laboratory on the determination of metal impurities in SiF₄ gas are investigated,and the experimental conditions are optimized.All glassware should be immersed in 10%nitric acid for 12 hours before use.Pipelines,valves and joints are made of polytetrafluoroethylene.Sample transfer and digestion are carried out in super clean room of grade 1000.After optimization,the total content of metal impurities in SiF₄ gas is 12.31ppm,and the content of metal impurities below 1ppm accounts for about 96%of the total amount of metal impurities.The results show that by optimizing the cleanliness of glassware,gas pipeline and laboratory,the pollution of metal impurities can be reduced and the influence on the determination of metal impurities in SiF₄ gas can be reduced.