This method is carried out under vacuum conditions, and the effect of impurity removal 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 Mo gas pressure of silicon is 0.5Pa, and at this time, impurities with a Mo gas pressure higher than that of silicon can escape from the industrial silicon melt into the gas phase and be brought out by the working gas. In the reaction furnace, since the volatilized gas is pumped out of the furnace in time to avoid the volatilized impurities colliding with the silicon melt and diffusing into the melt, this process is irreversible. Heating molten industrial grade silicon under vacuum conditions can enhance the volatilization effect of volatile impurities, and 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 molten silicon, so it can accelerate the migration of impurities in the silicon melt to the surface of the muffin, thereby accelerating the evaporation rate of volatile impurities, but vacuum smelting will lead to the evaporation of silicon.
Heating is realized by the principle of electromagnetic induction. Induction heating uses the principle of electromagnetic induction and the Joule-Lenz theorem to convert electrical energy into heat energy. When there is an alternating magnetic field in the area surrounded by the circuit, electromotive force will be induced at both ends of the circuit. , If the meeting is closed, an induced current will be generated, because silicon is a semiconductor. When the temperature reaches 600 ℃ with electromagnetic heating, the resistance will rapidly drop from 2300Ω at room temperature to 5Ω, and the effect of electromagnetic induction can be realized at this time.
When the alternating current flows through the conductor, an induced current will be generated in the conductor, which will cause the current to spread to the surface of the conductor, that is, the current density on the surface of the conductor will be greater than the current density in the center, which is the skin effect, which reduces the Conductor’s conductive cross-section, thereby increasing the conductor AC by yang, the loss increases.
Because increasing the current and increasing the frequency can increase the heating effect, the induction power supply usually needs to output high frequency and large current, but the greater the frequency, the stronger the skin effect, and the more the current is on the surface. Therefore, the current frequency for induction heating can be 50Hz. In the range of ~100MHz, an important basis for frequency selection is the distribution of heating temperature. The smelting process requires uniform heating temperature, taking into account power density and stirring force.
While considering the thermal efficiency, the temperature distribution during heating should also be considered. When the cylindrical conductor is heated by induction, only the surface will heat up rapidly due to the skin effect, while the central part needs to rely on heat conduction to conduct heat from the surface high temperature area to the inner low temperature area. The induction frequency has the following relationship with the furnace capacity (see Table 1).
The frequency of electromagnetic heating is usually heated by intermediate frequency, and the frequency higher than 50Hz and less than 20000Hz is called intermediate frequency furnace. Compared with indirect heating methods such as resistance and arc, the intermediate frequency furnace has the advantages of high efficiency, fast heating, easy temperature control and guaranteed heating quality. Research in recent years has shown that with the continuous improvement of the manufacturing process and the accumulation of experience, the intermediate frequency heating device has been more and more widely used in the melting of various metals and their alloys, as well as in heat penetration and heat treatment. application.
Induction furnaces can remove heavy, dense particles by deposition methods. After vigorous stirring, when the molten pool of molten silicon is in a static state, the undissolved particles will be deposited to the bottom of the molten pool. Pure liquid silicon is poured out and impurities are left in the furnace. Impurities with high particle density, such as silicon carbide and silicon nitride particles, can be removed by depositing on the bottom of the pool. The removal efficiency depends on the deposition time to the bottom of the molten pool and the particle size of the silicon carbide. Theoretical calculations and experiments show that after 1h of deposition, only 15% of the particles below 10um remain; the particles above 20um are almost completely removed.
Phosphorus oxides volatilize as P5O10 at about 300°C in an oxidizing atmosphere, but react at lower temperatures to form P4O6 in the presence of C/CO. This compound can remain in the furnace in the presence of C/CO up to about 1250°C, when it is present mainly in the form of P2. P2 is also the main form of phosphorus when it evaporates at 907°C. Phosphorus entering the process with the reducing agent is basically converted into gaseous escaping. In the industrial silicon process, phosphorus only exists as elemental impurities without metal compounds such as Fe, Mn, and Ca.
Under vacuum conditions, after the temperature is raised, the vapor pressure of different elements is different, and the element with the larger vapor pressure volatilizes first. The vapor pressure ratios of phosphorus and silicon are 10.96, 10.22, 9.57, 8.99, and 8.01 at different temperatures of 1500K, 1600K, 1700K, 1800K, and 2000K, respectively. In this way, in the vacuum chamber, the temperature and air pressure can be well controlled, and the phosphorus can be evaporated and removed. Of course, silicon also has a certain degree of volatilization loss in this process. For boron, it can be removed by gas purging, that is, blowing argon gas bubbles or water vapor bubbles through the bottom of the molten silicon bath. Gases can be introduced through porous plugs located at the bottom of the furnace. The higher the vacuum, the better the removal: the longer the vacuum, the better the removal. However, the vacuum is too high, the time is too long, and the cost increases. Each manufacturer should find out the suitable process parameters according to their own specific conditions.
The remaining impurities can be removed by the solidification driving method, that is, the molten silicon melt is slowly solidified from the bottom of the furnace to the top of the furnace. The bottom of the furnace is driven to the top of the furnace. Finally, these residual impurities will all float on the upper surface of the silicon molten pool.