Brief introduction
Chinese name: poly
English name: polycrystalline silicon
Nature Features
Polysilicon gray metallic luster. Density of 2.32 ~ 2.34. Melting point 1410 ℃. Boiling point of 2355 ℃. It was dissolved in a mixture of hydrofluoric acid and nitric acid, insoluble in water, nitric acid and hydrochloric acid. Hardness between germanium and quartz at room temperature brittle, easily broken when cutting. Heated to above 800 ℃ that it is ductile, showing significant deformation at 1300 ℃. Inactive at room temperature, under high temperature reaction with oxygen, nitrogen, and sulfur. The high temperature molten state, has a large chemical inertness with almost any material effect. Having semiconductor properties, are very important fine semiconductor material, but a trace amount of impurities can greatly affect its conductivity. Electronics industry is widely used in the manufacture of semiconductor radios, tape recorders, refrigerators, color TV, VCR, computer and other basic materials. Under certain conditions, the dried silicon powder and chlorinated dry hydrogen chloride gas, and then by condensation, distillation, reduction derived.
Polysilicon materials can be used for pulling single crystal silicon, polycrystalline silicon and monocrystalline silicon differences mainly in physical properties. For example, the anisotropy of mechanical properties, optical properties and thermal properties of single crystal silicon far less obvious; in terms of the electrical properties of the conductive polysilicon monocrystalline silicon crystal is far less significant, even almost no conductivity. In terms of chemical activity, the difference between the two is minimal. Polycrystalline and monocrystalline silicon can be distinguished from the appearance, but the real identification shall be measured by analyzing the crystal plane direction, conductivity type and resistivity.
Polysilicon is the direct production of silicon raw materials, is the electronic information materials contemporary artificial intelligence, automatic control, information processing, photovoltaic and other semiconductor devices. It is known as the "cornerstone of the edifice of microelectronics."
Production method
Polysilicon production technology mainly to improved Siemens method and silane method. Siemens columnar polysilicon production by vapor deposition, and in order to improve the utilization of raw materials and environment-friendly, based on the former uses a closed loop production process that is modified Siemens process. The process will be industrial silicon powder reacts with HCl, processed into SiHCI3, let SiHCl3 reduction in H2 atmosphere get deposited polysilicon reduction furnace. Reduction furnace exhaust emissions H2, SiHCl3, SiCl4, SiH2Cl2 and after separation of HCl and then recycled. Silane is introduced into the polysilicon silane seed as a fluidized bed of fluidized particles, the silane pyrolysis and deposited on the seed, resulting granular polysilicon. And silane modified Siemens method is mainly the production of electronic grade silicon crystals can also produce solar-grade polysilicon.
Siemens fold
Siemens was invented by the German Siemens company and in 1954 applied for a patent around 1965 to achieve industrialization. After decades of application and development, Siemens continues to improve, has appeared in the first generation, second generation and third generation, the third generation of polysilicon production technology that is modified Siemens process, which adds in the second generation on the basis of Restore dry exhaust gas recovery system, SiCl4 recovered hydrogenation process to achieve a complete closed-loop production, the production of high-purity polysilicon Siemens technology, the latest technology, the specific process shown in Figure 1. Silicon in Siemens polysilicon production process internal recycling.
Silane fold
Silane is introduced into the polysilicon silane seed as a fluidized bed of fluidized particles, it is a silane pyrolysis and deposited on the seed, resulting granular polysilicon. Silane preparation methods vary, there are magnesium silicide Law of Japan Komatsu invention, the specific process shown in Figure 2, the disproportionation method US Union Carbide invention, NaAlH4 and SiF4 reaction process used in the United States MEMC.
Method is to use magnesium silicide Mg2Si with NH C1 reaction in liquid ammonia silanes. The method because consumption of raw materials, high cost, dangerous, and there is no extension, only Japan Komatsu the same method. Modern prepared using silane disproportionation method, namely the metallurgical grade silicon and SiC14 for the synthesis of silane, first with SiCl4, Si and H2 reaction SiHCl3, then SiHCl3 disproportionation reaction SiH2Cl2, and finally by SiH2Cl2 catalytic disproportionation reaction SiH4, namely: 3SiCl4 + Si + 2H2 = 4SiHCl3,2SiHC13 = SiH2Cl2 + SiC14,3SiH2C12 = SiH4 + 2SiHC13. Each step of the above conversion efficiency is relatively low, so that the material requires multiple cycles, the whole process should be repeated heating and cooling, so that power consumption is relatively high. After the resulting silane purified by distillation, which leads to similar Siemens fixed bed reactor at 800 ℃ thermal decomposition reaction is as follows: SiH4 = Si + 2H2.
Silane gas is toxic and flammable gases, low boiling point, the reaction device should be sealed and should fire, frost, explosion and other safety measures. Silane Youyi its unique self-ignition, an explosive known. Silane has a very wide range of spontaneous ignition and combustion energy strong, determined that it is a highly dangerous gas. Silane application and promotion largely because of its limited high-risk characteristics of the project in relation to the silane or experiment, improper design, operation or management will cause a serious accident or disaster. However, practice shows that excessive and inappropriate fear and can not provide security to prevent application of silane. So how safe and effective use of silane, has been the subject line and the laboratory should be of great concern.
Thermal decomposition of silane compared with Siemens, its main advantage lies: silane easier purification, high silicon content (87.5%, faster decomposition, the decomposition rate of 99%), low decomposition temperature polysilicon generate energy only 40 kW • h / kg, and high product purity. But the disadvantages are also highlighted: not only high manufacturing cost silane, and flammable, explosive, security l born poor, strong foreign silane plant explosion accident had occurred. Therefore, the industrial production, the application of the thermal decomposition of silane is less than Siemens. Although the modified Siemens currently has the largest market share, but because of the inherent disadvantages of technology - the yield is low, high energy consumption, high cost, capital investment, and slow recovery of funds, operational risks are the greatest. Only by introducing plasma-enhanced, fluidized bed and other advanced technologies, and strengthening technological innovation, it is possible to enhance market competitiveness. Advantages silane conducive for the chip industry and services, the current production safety has gradually improved its scale of production could quickly expand, or even replace the modified Siemens. Although widely modified Siemens process applications, but silane promising.
Similar methods and Siemens, in order to reduce production costs, fluidized bed techniques can also be introduced into the thermal decomposition of silane, a fluidized bed furnace decomposing SiH4 decomposition rate can be greatly improved, and the deposition rate of the Si. But the purity of the product is less than the fixed bed decomposition furnace technology, but fully meet the quality requirements of solar-grade silicon, silane additional security problems remain.
US company MEMC fluidized bed technology to achieve mass production, its NaA1H4 and SiF4 to silane raw materials for the preparation, reaction is as follows: SiF4 + NaAlH4 = Sil4 + 4NaAlF4. After purification by decomposition of silane in a fluidized bed type decomposition furnace, the reaction temperature is about 730 ℃, obtain a size of 1000 microns granular polycrystalline silicon. The method of low energy consumption, granular polysilicon production exploded electricity consumption is 12kW • h / kg or so, about the modified Siemens 1/10, and a conversion rate of 98%, but a lot of dust in the presence of micron-sized product, and granular polycrystalline silicon surface area, easily contaminated, high hydrogen content shall dehydrogenation process.