1. Air Blow method
Blow inert gas, high-purity oxygen, hydrogen, nitrogen, argon and water vapor into the silicon water. These gases rise from the silicon water. Each small bubble is equivalent to a “small vacuum chamber”. The partial pressure of H, N, and CO in the bubble is close to zero. Adsorbed to the bubble surface and carried to the melt surface of the flux. Oxygen is good for removing metal impurities, and hydrogen and water vapor are good for removing boron. Oxygenation of inert gas can carry out refining and decarburization. Continuously changing the ratio of argon/oxygen during the process can reduce the partial pressure of CO in the carbon-oxygen reaction. Under lower temperature conditions, the carbon content can be reduced without silicon being oxidized. In the past, chlorine blowing was mainly used to refine the metal silicon from the submerged arc furnace, but hydrogen blowing would cause environmental pollution. Therefore, in addition to some special purposes, it is still used, other uses oxygen blowing instead of chlorine blowing. The oxygen blowing effect can also use oxygen to further react with metal impurities such as aluminum and calcium in the silicon to generate metal oxides and become gases to escape from the silicon.
2. Slagging and standing clarification method
The slag-making static clarification method is to add a slag-making agent, and keep the melt for a period of time under the condition of maintaining the refining temperature and flux coverage, so that the inclusions will float up or sink and be removed.
The low melting point flux added to the metal melt combines with non-metallic inclusions at high temperatures. This driving force mainly comes from the reduction of the interface energy. The adsorption capacity of a solvent depends on its chemical composition. The solvent is non-toxic due to the source requirement, is not easy to react with silicon, and is easy to solidify, float, and then be removed. The solvent should be heated to remove water during use, and it can be stirred at the same time. The silicon liquid is stirred during the refining process outside the furnace to homogenize the composition and temperature of the silicon liquid and promote the reaction. When the silicon liquid is in a static state, the inclusions are removed by floating upwards, which obeys Stokes’ law, and the removal speed is slow: when the silicon liquid is stirred, the removal of the inclusions is accelerated. It is best to periodically change the direction and speed of stirring to avoid forced convection caused by stirring and prevent natural convection. Every time the silicon water rises to a certain temperature, heat preservation and oxygen blowing, the temperature rise time period should maintain a low oxygen blowing pressure to ensure the smooth flow of the air inlet, so that the repeated operation will increase the temperature to 2000°C. The non-metallic impurities in the silicon water will float on the surface of the silicon water and be removed by other processes.
Hydrometallurgy refers to the method of crushing metallurgical silicon and immersing it in acid solution (or other substance solution) to remove metal impurities in metal silicon. Hydrometallurgy needs to crush industrial silicon into powder with suitable particle diameter, otherwise it will not be easy In addition to miscellaneous. Prepare a certain concentration of HCI, HF, H2SO4 or their mixture, soak the polycrystalline silicon powder in an acid solution and keep the acid solution at a suitable temperature, and filter out after a certain period of time. At this time, the concentration of metal impurities in industrial silicon It can be reduced by one to two orders of magnitude. The particle size of the pure intermediate silicon lost in the wet smelting, the acid concentration, the temperature of the pickling treatment and the length of the treatment time have an important influence on the removal of impurities. Fe, Al and Ca impurities contained in silicon are easier to remove than Mg, Ti, Zr and Ni impurities. Usually, if only acid is used, whether it is hydrochloric acid, sulfuric acid or nitric acid, the effect of removing metal impurities such as iron is better, but the effect of removing boron and phosphorus is not obvious.
4. Physical vacuum smelting
The vacuum smelting method is carried out under vacuum conditions, and the impurity removal effect is achieved through several processes of degassing, decomposition, volatilization and deoxidation. At a temperature slightly higher than the melting point of silicon (1500°C), the mu pressure of silicon is 0.5 Pa, and impurities with higher mu pressure than silicon can escape from the industrial silicon melt into the gas phase and be carried out by the working gas In the reaction furnace, the volatilized gas is pumped out of the furnace in time to prevent the volatilized impurities from colliding with the silicon melt and diffusing into the melt, so this process is irreversible. Heating molten industrial-grade silicon under vacuum conditions can enhance the volatilization effect of volatile impurities. Vacuum smelting can effectively reduce the concentration of P A1, Na, Mg, Ca and the content of volatile non-metallic impurities such as S and CI in silicon. Intermediate frequency induction heating has a strong electromagnetic stirring effect on the molten silicon, so it can accelerate the migration of impurities in the silicon melt to the surface of the hair, thereby accelerating the evaporation rate of volatile impurities, but vacuum smelting will cause the evaporation and loss of silicon.
5. Polycrystalline silicon ingot
Most metal impurities in industrial-grade silicon cannot be effectively removed after oxidation refining, slagging treatment, etc., but silicon has the physical properties that can be used for effective impurity removal, that is, segregation and impurity removal. Most impurities have low solubility in solid silicon, but high solubility in liquid silicon. This property can be used to further purify molten silicon. Because directional solidification can better control the movement of the solid-liquid interface and the shape of the solid-liquid interface, the directional solidification method is often used in the purification of silicon by segregation. The segregation coefficients of B, P, C, A1 and Cu in industrial silicon are relatively high, respectively 0.5, 0.35, 0.05, 2.8×10-3 and 8×10-4, which are not suitable for the removal of fractional coagulation refining, and other impurities can be carried out by this method. In addition to miscellaneous.
6. Czochralski single crystal method
The Czochralski method of crystal growth was invented by Polish J. Czchralksi in 1917, so it is also called the Czochralski method. In 1950, Teal et al. used this technique to grow semiconductor germanium single crystals, and then he used this method to grow Czochralski single crystal silicon. On this basis, Dash proposed the “neck” technology of Czochralski single crystal silicon growth. , G. Ziegler put forward the technology of fast necking, which constitutes the basic method of Czochralski silicon. First, put the silicon material in a quartz crucible to heat and melt, and then put the seed crystal in the molten silicon. After the solution around the seed crystal cools, the silicon crystal will adhere to the seed crystal. After the temperature and pulling speed reach the requirements Pull the crystal upwards. After the crystal is pulled up to the predetermined requirements, the tail will be drawn into a cone, so that a complete single crystal is formed. Because it has to go through a solid-liquid interface process, which is equivalent to a directional solidification process, it is also a purification process. process.
The specific method is: the raw materials are heated and melted in a crucible, and the end of a fine single crystal (called a seed crystal) cut into a specific product direction is immersed in the solution and slightly melted. Then, the temperature is controlled, the seed crystal is slowly raised vertically, and the drawn liquid is solidified into a single crystal. Adjust the heating power to get the required diameter of the single crystal rod. The furnace body of the Czochralski crystal growth equipment is generally made of metal (such as stainless steel). The seed crystal rod and the crucible rod are used to hold the seed crystal and support the crucible respectively. , And can rotate and move up and down, the crucible is generally heated by resistance or high frequency induction. The atmosphere in the furnace can be an inert gas or a vacuum.
7. electron beam vacuum melting
It is to use the huge local energy of the electron beam (103~106W/cm3) to volatilize impurities (such as phosphorus and aluminum) whose vapor pressure is higher than that of silicon (the vapor pressure of silicon at 1700K is 0.0689Pa). In addition, local overheating can remove oxides.
The basic principle of the electron beam melting furnace is: the high-speed electron beam is bombarded on the molten metal in a high-voltage electrostatic field, and the kinetic energy of the high-speed electron beam is converted into heat energy to achieve the purpose of smelting and casting ingots. Under a high-voltage electric field in a high vacuum environment, the cathode is heated to a temperature sufficient to emit free electrons, and an electron cloud is formed in the space on the surface of the cathode. Under the action of the accelerating voltage, these electrons move towards the anode at a very high speed, and the electrons are formed into beams through focusing and deflection, which accurately bombard the surface of the furnace charge and molten pool to melt and cool to form a crystalline material. Theoretical calculations and practice have proved that: within the voltage range of the electron beam melting furnace (currently no more than 40kV), the maximum loss caused by X-ray radiation does not exceed 0.5%, and the loss caused by secondary emission is also very small.
8. Plasma induction melting
Plasma induction furnace is a combination of ordinary induction furnace and plasma arc heating device. It avoids the shortcomings of ordinary induction furnaces of cold slag and no protective atmosphere, thereby significantly improving the purification ability of induction furnaces. Plasma smelting can flexibly change the working gas. Therefore, shielding gas and reactive gas beams can be introduced while smelting to achieve the purpose of removing CB elements. Europe uses plasma melting to purify metallurgical grade silicon in the ARTIST project. This technology uses high-purity metallurgical silicon as the raw material, and the silicon material is melted under the common heating of a plasma gun and an intermediate frequency electromagnetic induction heating device. Plasma gun launches plasma under heating conditions and uses inert gas as carrier to pass in H2, O2 and other reactive gases to react with non-metallic impurities such as BC on the surface of silicon melt to generate BH, BOH, BO, CO and other gases, which are evacuated. The system is discharged. An intermediate frequency induction coil is arranged outside the crucible, which generates electromagnetic stirring to the silicon melt during induction heating, which increases the reaction rate and accelerates the discharge of the generated gas. For different impurity elements in polysilicon, the corresponding reaction gas can be introduced to achieve the purpose of impurity removal, but the main elements in the gas are H and O
9. Magnetic field removal method
The magnetic field removal method is to use the action of electromagnetic field to separate impurities (mainly non-metals).