Hydrogen reduction of trichlorosilane

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Section 1: Reaction Principle of Hydrogen Reduction of Trichlorosilane I. Process Principle Purified and cleaned SiHCl3 and H2 enter the evaporator; at a temperature of 20°C and a pressure of 0.2 MPa, H2/SiHCl3 enters the reduction furnace in a molar ratio of 3.5–4:1. At temperatures ranging from 1080°C to 1100°C, SiHCl3 is reduced, and the resulting silicon deposits on the silicon core of the heating element. When SiHCl3 is mixed with H2 and heated to above 900°C, the following reaction occurs: 1-1. At the same time, thermal decomposition of SiHCl3 as well as reduction reactions of SiCl4 also take place: 1-2, 1-3. Additionally, the following reactions may occur as well: 1-4, 1-5. When there is an excess of hydrogen, SiCl2 + H2 = Si + 2HCl 1-6. There are also reduction reactions of impurities: 1-7, 1-8. All of these reactions are reversible, so the reaction process inside the reduction furnace is quite complex. During the production of polysilicon, appropriate measures should be taken to suppress various side reactions. In the reaction equations above, the first and second reactions can be considered the basic reactions for producing polysilicon; it is necessary to ensure that the reactions within the reduction furnace proceed in accordance with these two basic reactions as much as possible. Section 2: Factors Affecting the Hydrogen Reduction Reaction of SiHCl3 1. Reaction temperature: Based on the principles of chemical reaction rates and chemical equilibrium, both the hydrogen reduction of trichlorosilane and silicon tetrachloride are endothermic reactions. Therefore, increasing the temperature shifts the equilibrium toward the endothermic direction, which facilitates the deposition of silicon. According to theoretical analysis, the higher the temperature, the faster the deposition rate. However, in actual production, when silicon is deposited from the gas phase onto the solid carrier as a result of the reaction, there is a maximum temperature Tmax. When the reaction temperature exceeds this value, the deposition rate decreases as the temperature rises. There is also a minimum deposition temperature T0; only above this temperature does the reaction proceed and silicon begin to precipitate. Generally, between the lowest and highest deposition temperatures, the deposition rate increases as the reaction temperature rises. Figure 5-1 Bridge corrosion notches A: Excessive temperatures can lead to the formation of bridge corrosion notches. The so-called “bridge corrosion notches” are shown in Figure 5-1. Hydrogen chloride gas and silicon tetrachloride generated as a result of reduction processes can both cause reverse corrosion of silicon at high temperatures. At the 90° corners where bridges form, there are heat focal points, also known as hot spots; the temperature at these locations is very high. When it exceeds 1200°C, silicon corrosion occurs, resulting in the formation of notches. Therefore, by strictly controlling the surface temperature of the silicon rod to be below Tmax while keeping it at a suitable temperature close to Tmax, the surface irregularities can be eliminated. Experimental measurements show that the appropriate temperature for the hydrogen reduction of trichlorosilane is 1150°C. B. Excessively high temperatures increase the chemical reactivity of the deposited silicon, raising the likelihood that the silicon rods will be contaminated by the materials used in the equipment; therefore, their surface temperature should not be too high. Within the temperature range of 900–1000°C, thermal decomposition is the dominant reaction for trichlorosilane ； The hydrogen reduction reaction is dominant between 1100 and 1200°C. The relationship between the hydrogen reduction temperature of trichlorosilane and the actual silicon yield is shown in Figure 5-2. It should be noted that the melting point of silicon is 1410°C, which is relatively close to the reaction temperature; therefore, it is essential to strictly control fluctuations in the reaction temperature during production, to prevent the silicon rods from melting and collapsing due to excessive temperatures, which could result in significant losses. Figure 5-2 Effect of temperature on the reduction reaction C. Generally speaking, when the temperature is below the optimal level, an increase in temperature leads to larger and brighter crystals of silicon ； Below the optimal temperature, the lower the temperature, the finer the crystals become; their surface appears dark gray and lacks a metallic luster. If the temperature is below 1000°C, loose dark-brown amorphous silicon is formed (i.e., a temperature interlayer), which reduces the quality of polysilicon. When the temperature is above the optimal level, as the temperature rises the surface of the silicon rod becomes increasingly rough, resembling popcorn ； At appropriate temperatures, the silicon rod has fine and dense grains, with a smooth surface that exhibits a metallic luster ； 2. In the reduction reaction, the molar ratio of H2 to SiHCl3 (also known as the stoichiometry) has a significant impact on the deposition of polycrystalline silicon. Only under a strong reducing atmosphere can the reaction proceed sufficiently to achieve a higher conversion rate of SiHCl3. If the amount of hydrogen required for reduction of SiHCl3 is calculated on a chemical equivalent basis, no crystalline polysilicon will be obtained; only some amorphous brown powder will result, with an extremely low silicon yield. This is the result of side reactions occurring due to a lack of H2. Increasing the hydrogen ratio can significantly improve the conversion rate of SiHCl3. Figure 5-3 shows the theoretical equilibrium conversion rates of SiHCl3 under different hydrogen ratios. Generally, the actual conversion rates are much lower than the theoretical values. On the one hand, there are various side reactions during the reduction process; on the other hand, the actual reduction reaction cannot reach an equilibrium state. However, overall, the reduction conversion rate still increases as the molar ratio of H2 to SiHCl3 increases; if the molar ratio of hydrogen to SiHCl3 is even higher, the conversion rate of SiHCl3 will be even greater. However, the ratio of hydrogen to SiHCl3 cannot be too high; if it is, the following problems will arise: 1) An excessive amount of hydrogen dilutes the concentration of SiHCl3, reducing the chances of SiHCl3 molecules colliding with the surface of the silicon rod, which in turn lowers the deposition rate of silicon and thus reduces the output of polycrystalline silicon per unit time. At the same time, a large amount of hydrogen is not fully utilized, increasing hydrogen consumption and thus raising investment costs. 2) It can be seen from the hydrogen reduction reactions of BCl3 and PCl3 that another factor determining the mixture ratio is the boron and phosphorus content in the chlorosilane; an excessively high hydrogen concentration is not conducive to suppressing the precipitation of B and P. By selecting an appropriate mixture ratio, a considerable amount of boron and phosphorus can be removed; the principle behind this is the use of the law of mass action (the rate of a chemical reaction is proportional to the effective mass, i.e., concentration, of the reactants), which causes the chemical reaction to proceed in the leftward direction. 　　　　　 It is easy to see from the chemical equilibrium relationships of the above reactions that increasing the HCl concentration and decreasing the H2 concentration help to suppress the precipitation of boron and phosphorus, which is the exact opposite of the conditions for silicon deposition. To resolve this contradiction, it is necessary to increase the hydrogen chloride concentration; this HCl can come from the reaction itself or be added externally. The hydrogen chloride content has a significant effect on suppressing boron precipitation, while its impact on phosphorus precipitation is less pronounced. It can be seen that as the ratio increases, the conversion rate of SiHCl3 also rises, but the deposition rate of polysilicon decreases. Previously, domestic production often used a ratio of H2:SiHCl3 = 10:1 (molar ratio) in order to achieve a higher first conversion rate of SiHCl3 ； Lower ratios are now commonly used to increase the deposition rate of polysilicon. The impact of the lower ratio on the reduced primary conversion rate of SiHCl3 can be mitigated by recovering the unreacted SiHCl3 from the exhaust gases and reusing it in the reduction process to produce polysilicon, thereby ensuring full utilization of SiHCl3. Figure 5-4 shows the relationship between the various components after the reaction and the molecular ratio of the reactant gases. It can be seen that the yield of silicon increases as the ratio of H2 to SiHCl3 increases. The fact that the amount of SiCl4 produced decreases as this molecular ratio increases indicates that an increased hydrogen concentration helps to suppress the thermal decomposition reaction. As can be seen from Figure 5-4, which shows the relationship between the composition of various components after the reaction and the ratio of reaction gas molecules, as the content of SiCl4 increases, the amount of silicon produced also increases; in other words, adding SiCl4 can likewise suppress the thermal decomposition reaction. 3. Reaction gas flow rate and velocity: When the deposition rate reaches a certain level, a higher flow rate results in a higher furnace output. However, the flow rate is related to the structure and size of the reduction furnace, as well as the size of the carrier surface. From the perspective of molecular motion, a larger ratio of carrier area to reaction space is preferable; this increases the number of collisions between Si molecules and the deposition surface, which is beneficial for improving the yield. Furthermore, increasing the gas flow rate enhances gas turbulence – in a flowing medium, the fluid is filled with swirling vortices of varying sizes that are constantly in motion; in addition to moving along the axial direction, the fluid particles also undergo intense random movements in all directions. This helps to reduce the gas boundary layer on the surface of the heating element as well as the uneven distribution of gas within the furnace, thereby facilitating the progress of the reduction reaction. However, the flow rate of SiHCl3 cannot be too high; otherwise, the residence time of SiHCl3 in the furnace will be too short, resulting in a relatively lower conversion rate of SiHCl3. This in turn increases the amount of exhaust gas and leads to waste of some SiHCl3 ； It also cannot be too small, otherwise the HCl gas generated by the reaction will form a gas layer on the surface of the hot carrier. If the circulation of the reaction gases in certain areas around the carrier is insufficient to eliminate these gas layers, needle-like or other protrusions tend to form in those areas; silicon deposition occurs particularly readily at these needle-like points or protrusions, leading to the development of small nodules or bumps. Adjacent nodules or small tumors fuse together, with bubbles trapped beneath them, resulting in a rough surface on the deposited silicon rod. 4. As the reduction process proceeds, silicon generated is continuously deposited on the heating element, increasing its surface area. This increases the number of opportunities and frequency of collisions between reaction gas molecules and the deposition surface, which facilitates the deposition of silicon. When the deposition rate per unit area remains constant, the larger the surface area, the more polycrystalline silicon will be deposited, and thus the production efficiency of polycrystalline silicon is higher. Extending the reaction time as much as possible, that is, making the silicon rods longer and thicker as much as possible, is beneficial for improving both product quality and yield. As the reaction cycle lengthens, the silicon rods deposited become thicker, and the surface area of the carrier increases, resulting in a continuous increase in the deposition rate. As a result, the amount of impurities diffused into silicon per unit volume decreases relatively (the impact of carrier impurities on the deposited silicon rod also decreases accordingly). Furthermore, by extending the furnace operation cycle, the unit consumption of the carrier is reduced accordingly, and the non-productive time for shutting down and restarting the furnace is shortened. As the silicon rod continues to thicken, the current must be increased continuously in order to maintain a constant surface temperature of the rod. Therefore, when the capacity and current of electrical equipment are sufficient, extending the production time of polysilicon as much as possible to maximize the surface area of its heating elements is beneficial for improving production efficiency. But in reality, the furnace startup cycle is limited by factors such as the furnace structure, electrode spacing, and heating power. Therefore, the increase in reaction time always has a certain limit. In production, the amount of material fed into the reduction furnace also needs to increase as the diameter of the heating element increases; otherwise, with an increased surface area, the feeding rate cannot keep up, and the deposition rate of silicon will not increase either. There are two common methods for controlling the feed rate: one is to establish a feeding schedule (that is, a table showing the relationship between feed rate and production time), and adjust the feed rate according to time ； Another method is to control the feed rate based on the diameter of the silicon rod. Both of these methods can achieve automatic computer control. 5. Specific requirements for silicon cores in the production of carriers for deposited silicon (intrinsic quality and surface quality). Regarding the intrinsic quality of silicon cores, high purity as well as a dense and uniform structure are required ； Secondly, to meet the requirements of high-voltage starting or preheating starting, the presence of P-N junctions within the silicon core is not allowed, and a certain range of resistivity is required. Regarding product quality, the higher the purity of the silicon core, the better; however, the higher the purity, the greater the resistivity. The impact of impurity content in the silicon core on the deposited silicon is an issue that deserves consideration, especially in situations where very high requirements are placed on the resistivity of the deposited silicon. The surface quality of the silicon core includes diameter, length, surface flatness, diameter uniformity, cleanliness without contaminants, and dryness. Undoubtedly, a thicker and longer carrier is beneficial for increasing yield. However, if the carrier is too long, its stability is poor, and it tends to tilt or collapse during growth ； The slender silicon core is prone to breaking during the initial heating stage. Therefore, based on the analysis of silicon core preparation conditions, reduction equipment operating conditions, and economic efficiency, there are certain limits on the diameter and length of silicon rods. After the silicon core has reached a certain diameter, length, and resistivity, it must undergo thorough acid washing and etching treatment before being placed in the furnace; its surface is polished and cleaned, and then dried before it can be inserted into the furnace. Section 3: Process Flow Diagrams and Equipment I. Simplified flow diagram of the hydrogen reduction process for trichlorosilane SiHCl3 → Purification → H2 gas → Vaporizer → Reduction furnace → Exhaust gas treatment → Product recovery (polysilicon) → H2 recovery Figure 5-5: Simplified flow diagram of the reduction process. The mixed gas is transported from the evaporator to the reduction furnace via pipes; within the reduction furnace, this mixed gas reacts to produce polysilicon, which deposits on the silicon core. The primary conversion rate for producing polysilicon is only about 10–20%; the remaining approximately 80% is released in the form of exhaust gases. The exhaust gas emitted from the reduction furnace contains a mixture of gases including H2, SiHCl3, SiCl4, SiH2CL2, and HCl, which is sent to the exhaust gas recovery system via a main exhaust pipe. ) Reduction furnaces generally adopt a bell-jar structure, consisting of a furnace cylinder (bell jar), base, electrodes, viewing ports, and air in/out pipes. They are usually made of stainless steel to minimize contamination of the product by the material of the equipment. The inner wall of the reduction furnace is smooth and shiny; the furnace cylinder and base are secured with hinge bolts, and both have insulation layers that allow hot water to carry away the heat radiated onto the furnace walls, thereby protecting the furnace body and the sealing gaskets. The furnace top is equipped with safety explosion vents and a silicon core preheating device. The furnace body is also equipped with a viewing port, through which it is possible to observe various conditions inside the furnace. The intake and exhaust pipes are arranged separately: the intake pipes are distributed across the chassis, while the exhaust pipes are located at the center of the chassis. Additionally, a jacketed design can be used, with the exhaust pipe surrounding the intake pipe. This configuration is intended to use the hot exhaust gases to preheat the mixture entering the furnace, while also allowing the exhaust gases to be cooled initially. II. Main Equipment 1) Reduction Furnace Figure 5-6 shows a schematic diagram of the reduction furnace structure. The base of the furnace is jacketed, and a certain number of electrodes are arranged on this base; the carrier inside the furnace (silicon core) is mounted on these electrodes. The power supply of the reduction furnace supplies electricity to the carrier through electrodes, causing the carrier to heat up and providing the temperature required for the reactions within the furnace. ④The viewing port reaction furnace tube will be equipped with 3 viewing ports: Viewing port 1 displays the temperature of the temperature probe, while viewing ports 2 and 3 are used to observe and control the silicon rod as well as the deposition process. The display screen is water-cooled and consists of two layers of quartz glass. To prevent hydrolytic atomization of the inner wall, quartz glass will be flushed with hydrogen gas. ⑤Electrodes are generally made of copper. The center of the electrode is hollow and cooled with demineralized water to prevent the insulating polytetrafluoroethylene gasket of the electrode from overheating and being damaged. The electrodes are mounted on the chassis and secured to the power cables using copper set screws; the electrodes are connected to the carrier via graphite clamps. 2) Evaporator: The evaporator is also known as a vaporizer or volatilizer. It is mainly composed of a container, a heating unit, supply pipes, liquid level indicators, pressure indicators, temperature indicators, and exhaust pipes. As shown in Figure 5-8. The basic function of the evaporator is to vaporize SiHCl3 into a gas, which then mixes with H2 in a specific ratio to provide raw materials for the reduction furnace. Therefore, as long as the temperature of SiHCl3 remains constant, the saturated vapor pressure of SiHCl3 in the evaporator is fixed, and thus the partial pressure PSiHCl3 of SiHCl3 in the mixture can be determined. The pressure of the mixture is equal to the sum of the partial pressures of its constituent gases, that is: P_total = PSiHCl3 + PH2. Figure 5-8 shows the evaporator; Figure 5-7 provides a schematic diagram of the electrostatic discharge structure. Thus, once PSiHCl3 is known, it is sufficient to control the total pressure of the mixture, P_total, in order to determine the desired partial pressure of hydrogen, PH2, thereby allowing control over the desired mixture ratio. The basic function of the evaporator is to vaporize SiHCl3 into a gas, which then mixes with H2 in a specific ratio to serve as a raw material for the reduction furnace. The control of the total pressure of the mixture is achieved by adjusting the flow rate of hydrogen entering the evaporator; when the total pressure increases, the hydrogen flow rate is reduced, and when the total pressure decreases, the hydrogen flow rate is increased, in order to maintain a constant total pressure. The evaporation of a liquid is an endothermic process; heating is required for the SiHCl3 liquid in order to maintain its temperature, and hot water is usually used for this purpose. In general, in the evaporator, the liquid SiHCl3 that evaporates is replenished through the feed pipe to maintain a constant level of SiHCl3 liquid in the container. Hot water is used to heat the liquid SiHCl3 in the container, thereby providing the necessary heat of vaporization and keeping the rate of evaporation of the liquid SiHCl3 constant. Section 4: Requirements for Products and External Conditions in the Hydrogen Reduction of Trichlorosilane Hydrogen. 1. Requirements for polycrystalline silicon products: Polycrystalline silicon is silver-gray in color, with a metallic luster on its surface; its crystal structure is dense, and its cross-section contains no oxidized layers, voids, or cracks. The surface of the rods is free from any contamination, stains, or signs of oxidation. II. Requirements for external conditions: 1. Nitrogen (N2): 1) Impurity content – oxygen content, water content, oils, and other impurities. 2. Hydrogen (H2): Impurity content – nitrogen (N2), methane (CH4), oxygen (O2), water (H2O). 3. Quality requirements for graphite components: Special-grade graphite must have a density of X g/cm³ at ≥2500°C, a porosity of X%, an ash content of X ppm, and certain impurity concentrations in ppm: Al, B, Ca, Cr, Cu, Fe, Mg, Na, P, S, Si, Ti, V, Au, Ag, Pt. 4. Polytetrafluoroethylene: This material is used for electrode insulating sleeves and sealing gaskets; it should be cleaned with alcohol-soaked cotton balls before use to avoid any contamination. 5. Quality requirements for the silicon core: Φ8×2400mm; the surface must be free from contamination, and the flatness as well as diameter uniformity must be satisfactory. 6. Impurity levels in the raw material SiHCl3: boron ≤ x, phosphorus ≤ x. 7. Requirements regarding electricity: performance specifications for the high-voltage starting power supply. 8. Requirements regarding water: the water used to cool the electrodes must be deionized water, with a temperature of 90℃–100℃. Section 5: Operating conditions for the reduction of trichlorosilane using hydrogen 1. Ratio of H2 to SiHCl3 (by molar ratio): 3.5–4:1. 2. Temperature: 1080–1100℃. 3. Pressure: 0.2 MPa. 4. Gas flow rate. 5. Heating power. Section 6: Key points for process control 1. A strong sense of high-purity hygiene: Staff must have a firm commitment to high-purity hygiene standards and apply these principles throughout the entire work process. Currently, many semiconductor-grade polysilicon manufacturers install their reduction furnace systems in clean rooms (such as rooms with a cleanliness level of 100,000). To meet this cleanliness requirement, all personnel inside the rooms must wear clean work clothes. II. Strict adherence to operating procedures: To ensure production quality, it is necessary to strictly follow the operating procedures, and no one is allowed to make arbitrary changes to them. Changes to the process operation procedures are proposed by the process technology technician, discussed by the section’s technical team, approved by higher-level management, and filed with the relevant departments of the factory. III. Awareness of balanced production IV. Inspection of automatic control ● Regularly check each reduction furnace, evaporator, and supply pipeline for any signs of leakage or spills. ●Regularly check whether the circulating water pump is operating properly. ●Regularly check whether the circulating water in the reduction system is unobstructed. ●Check the operating status of the reduction furnace control cabinet, recording it once per hour; the data must be accurate and timely. ●Check the parameters of various control points in the reduction furnace on the DCS operation screen to determine whether the production operation of the reduction furnace is normal. Section 7: Process Operations for Reduction I. Cleaning and Installation of New Reduction Furnaces 1) Stainless steel furnace bodies are commonly used in production. For newly built reduction furnaces, the oil stains on the furnace walls are first wiped clean using alkali; this process continues until water flowing over the surface forms a uniform film with no hanging droplets, indicating that the oil stains have been removed. The surface is then rinsed to neutrality with water, followed by cleaning with nitric acid. After that, it is rinsed to neutrality again with distilled water, and finally passivated before being rinsed thoroughly with distilled water. For furnaces that are used continuously, it is generally sufficient to first wipe away any contaminants with distilled water, and then clean them thoroughly with high-purity alcohol. If it has been left for a long time with severe rust and contaminants, it can first be gently scraped away using sandpaper grade “00”, then cleaned thoroughly with high-purity alcohol, and finally dried using an infrared lamp before use. Under normal circumstances, the inner wall of the furnace should be cleaned as little as possible; it is best to use a vacuum cleaner to remove reaction residues such as silica, and then load a new batch of material immediately. Cleaning often introduces contaminants, reducing the quality of ultra-pure silicon; therefore, the furnace should be cleaned as little as possible unless it is absolutely necessary. II. Furnace Loading Operations A. Prepare the materials needed for furnace loading. B. Wipe the PTFE washer clean with an alcohol swab and place it on the furnace tray. C. Check whether the graphite base and graphite clamps are qualified; if so, install them on the electrode. Wear vinyl film gloves when operating. D. After the silicon core is vertically mounted on the graphite base, the graphite clamp is tightened by hand to ensure a tight, secure connection as well as good contact between the silicon core, the graphite clamp, and the electrodes. E. After the silicon core has been installed, trained lifting personnel slowly lift the furnace cylinder onto the furnace base; the lifting speed must not be too fast, to prevent the cylinder from shaking and knocking over the silicon core or damaging the PTFE gaskets. F. Secure the furnace cylinder and furnace base with hinge bolts to meet the process requirements. III. Preparations before starting the furnace A. After installing the furnace cylinder, perform a pressure test using nitrogen to check for any leaks. After the furnace achieves pressure retention, it is then purged with nitrogen, repeated three times, to dry the entire system with hot nitrogen. B. Check and ensure that the cooling water system is unobstructed, with the water flow rate and velocity meeting all requirements. Supply cooling water to the furnace barrel and sight holes. C. Check and ensure that the electrical system fully meets the conditions for driving. IV. Initiation and Operation of Reduction A. High-voltage breakdown (initiation): At lower temperatures, after replacing the gas with H2 for half an hour, the reaction exhaust gases are directed into the exhaust gas recovery system. A high voltage of several thousand volts is applied across both ends of the silicon core, causing it to break down and become a conductor, thereby allowing electricity to flow through it. B. After current is applied to all silicon cores, inform the instrumentation staff to adjust the infrared thermometer so as to keep the surface temperature of the silicon cores at around 1100°C. D. Once the temperature returns to normal, the reduction furnace will have its polarity reversed; at this time, the personnel working in front of the furnace should pay attention to any abnormalities in the silicon rods inside the furnace. E. After successful polarity reversal, it can be put into automatic current increase and automatic feeding mode as appropriate. F. During operation, the automatic control system should be inspected regularly, and parameters such as the diameter of the silicon rods, voltage, current, and feed rate should be recorded once per hour. 5. Normal shutdown and removal of the rods: 1. The reduction furnace should be shut down normally once the diameter of the silicon rods meets the predetermined requirements. Before shutting down the furnace, the feed gas mixture should be stopped first, and the flow rate of cooling water should be reduced to raise the wall temperature and ensure complete evaporation of the silicone oil. Continue to pass hydrogen (for about 1 hour) until the silicon rod becomes bright, then cut off the power and allow it to cool down. When the furnace temperature drops to 900°C, the exhaust gas is directed into the exhaust gas treatment system. Stop the hydrogen supply and introduce nitrogen for displacement; once the furnace temperature drops to room temperature, close the exhaust outlet valve and the N3 inlet valve, stop the cooling water supply, and disassemble the furnace under a nitrogen atmosphere. 2. When disassembling the furnace, the personnel involved must wear pollution-free masks, gloves, work uniforms, and special protective shoes, in order to prevent the silicon rods or blocks from getting damaged as a result of contamination or falling and causing injuries. B. Loosen the cooling water flanges, and install blind flanges at the inlet and outlet flanges of the furnace drum water to prevent burns from hot water or contamination of the reduction furnace caused by water dripping from the furnace drum onto its base. Use specialized tools to loosen the fastening bolts of the reduction furnace drum. F. Verify that all fastening bolts and connectors have been loosened completely ； Confirm that the lifting crane is in good working condition; have a dedicated person operate the crane to slowly lift the furnace cylinder and place it steadily in the designated position. G. During the lifting of the furnace cylinder, carefully check whether the silicon rods are leaning against the furnace wall; if they are, use protective barriers to prevent them from falling and damaging the furnace bottom plate and its sealing gaskets, and use manual assistance to remove the materials ； If not leaning back, no guardrail is provided. Special lifting equipment should be used during furnace disassembly, with mechanical unloading to slowly lift the silicon rods out of the furnace in pairs. Section 8: Emergency Plans I. Sudden Water Outage A. Use communication devices or verbal announcements to alert all people to the danger. B. The operator who discovers the accident shall immediately report it step by step in accordance with the accident reporting procedure. C. Upon discovering that the cooling water supply to the reduction furnace has been interrupted, the power supply to the furnace should be cut off immediately, and at the same time, the mixture of gases supplied to the reduction furnace should also be stopped. D. Then open the large hydrogen valve to use hydrogen to cool and reduce the furnace body. F. When the furnace temperature cools to 900°C, stop supplying H2 and purge with N2 until the furnace temperature drops to room temperature. E. After the accident has been resolved, in accordance with the operating procedures, the shift supervisor shall seek approval before restarting the equipment. II. Sudden power outage: A. The purge hydrogen should be turned on; immediately close the mixture control valve and the manual valve, and switch the exhaust gas to the exhaust gas scrubber tower. If power is restored suddenly, perform a reset immediately; once the reset is successful, the current will return to its value before the power outage. After that, let it run without any load for about half an hour using hydrogen, before feeding material in. If the reset is not successful, the reduction furnace operation should be stopped in accordance with the shutdown procedure, in order to prepare for a new batch of production. In the event of a prolonged power outage, cut off the hydrogen supply once the furnace temperature drops below 900°C. At the same time, the nitrogen flow rate should be increased; the minimum flow rate required is enough to compensate for the negative pressure. During the deposition process, if a power outage occurs, communication devices or verbal alerts should be used to inform all personnel of the danger involved. C. The operator who discovers the accident first shall immediately report it in accordance with the accident reporting procedures. D. Open the nitrogen valve for purging until the furnace temperature drops to room temperature. E. After the accident has been resolved, in accordance with the operating procedures, the shift supervisor shall seek permission from the dispatch manager before restarting the equipment. III. What to do in case of a sudden hydrogen supply interruption? In the event of a brief interruption in hydrogen supply from time to time, as long as the temperature of the reduction furnace and the entire system does not drop, negative pressure generally does not occur, and air will not be drawn in. When the temperature of the carrier or a certain area drops, backflow of air can cause blockages in the system’s pipes due to hydrolysis, leading to oxidation of the silicon rods and even serious explosion accidents. Therefore, when a hydrogen supply is interrupted, a large amount of high-purity nitrogen should be supplied from the hydrogen inlet; at the same time, the feed gas mixture should be stopped to allow the nitrogen to fill the entire system in excess. If a prolonged interruption of hydrogen supply is suspected, then the temperature should be reduced and the furnace shut down. It should be noted that nitrogen must be introduced first before cooling, and the flow rate of nitrogen should be increased during cooling to prevent air from entering. IV. Rod Collapse A. The operator who discovers a rod collapse incident at the scene shall immediately report it step by step in accordance with the incident reporting procedure ； B. Immediately stop supplying power and the trichlorosilane gas mixture. Increase the flow rate of H2; once the furnace temperature drops below 900°C, stop supplying H2 and introduce N2 for displacement. C. After the furnace temperature reaches room temperature, maintain the pressure with N2 and proceed with disassembling the furnace. Section 9: Basic Calculations in the Process Flow I. Calculation of Performance Parameters of Basic Materials (1) Relationship between gas mass, molar mass, and volume at standard conditions: Gas volume (L) = (Gas mass / Gas molar mass) × 22.4 L/mol (2) Conversion of gas volume between different states: In actual production, gas volume is always referenced to standard conditions; therefore conversion is necessary. The following formulas are commonly used: P0V0/T0 = PV/T or PV = nRT, where N represents the number of moles of gas. P0, V0, T0 — are the pressure, volume, and temperature under standard gas conditions. P, V, T—are the pressure, volume, and temperature of the gas under operating conditions. R is referred to as the molar gas constant, with an exact value of R = 8.314510 J·mol-1·K-1. ⑶. Calculation of gas density: The density d of a gas represents the mass of gas per unit volume; it is calculated as d = mass of gas / volume of gas (g/L). Based on the density d0 of the gas under standard conditions, it is possible to determine the molecular weight of the gas: d0 = molar mass / molar volume of the gas (g/L). II. Basic calculations: (1) Production rate of polycrystalline silicon furnaces: G = 0.785D2Ld, where D is the diameter of the silicon rod and L is the total length of the silicon rod. d: Density; Polysilicon deposition time: From the start of feeding to the end of feeding; Average polysilicon deposition rate: G/h; Average polysilicon deposition velocity: G/h·m²; Yield: Refers to the ratio of the actual amount of polysilicon produced per batch to the silicon content in the SiHCl3 used (theoretical yield). : Weight of deposited silicon: Volume of SiHCl3 consumed: Specific gravity of SiHCL3 (1.32 Kg/L): Molecular weight of silicon (28): Molecular weight of SiHCL3 (135.5): Weight of SiHCL3 consumed (3). Calculations regarding efficiency and yield in production: The values calculated based on the chemical equations are theoretical values; in actual production, due to various reasons such as incomplete reactions, side reactions, or losses of raw materials during the production process, the actual output is always less than the theoretical output. The percentage of the actual output relative to the theoretical output is known as the yield (or actual yield rate). On the other hand, in actual production the amount of raw materials used is always greater than the theoretically calculated amount; the percentage of the theoretically calculated amount relative to the actual amount used is called the raw material utilization rate. For raw materials, the conversion rate refers to the raw material utilization rate. For a product, the yield conversion rate equals the utilization rate of the raw materials, which in turn equals the yield of the product. For example, in the production of polysilicon, according to the chemical reaction equation, 4.8 kg of SiHCl3 is required to produce 1 kg of polysilicon. That is: 135.5 / 28 × 1135.5 : X = 28 : 1, so X = 4.8 kg. However, due to side reactions and the chances of unwanted chemical reactions, the utilization rate of SiHCl3 is only 13.3%; therefore, the actual amount of SiHCl3 needed is 36.1 kg (4.8 / 0.133 = 36.1 kg). 4) The relationship between the weight of the silicon rod, current, and voltage: Based on U = R·I and R = ρL/S, the cross-sectional area of the silicon rod is S = 0.785·D2. The output of the silicon rod production process is G = 0.785·D2·L·d. The relationship between the diameter of the silicon rod, current, and voltage: Section 10: Safe Production I. Properties of materials 1. A colorless, transparent liquid with volatility. Upon contact with water, it undergoes hydrolysis according to the following reaction: SiHCl3 + 2H2O = SiO2 + 3HCl + H2. Moreover, H2 is highly flammable and explosive when exposed to air ； The aqueous solution of HCI is a highly corrosive acid. 2. TCS is highly flammable in air; its ignition point is 27.8°C, its flash point is 175°C, and its explosive range is 1.2%–90.5% (by volume). The HCl and Cl2 generated during the combustion of TCS are toxic and highly corrosive. It can cause damage to human eyes and respiratory organs, as well as corrode other substances. 3. System leak detection reaction equation: SiHCl3 + 2NH3·H2O = SiO2 + H2 + HCl + 2NH4Cl↑ The silicon dioxide and ammonium chloride produced by this reaction are white particles, which, together with other gases, form white smoke. Appendix: Properties of silicon tetrachloride. Silicon tetrachloride (SiCl4) is a colorless, transparent, non-polar, volatile liquid with strong irritant properties. Upon hydrolysis, silicon dioxide and hydrogen chloride are produced. Can be mixed with benzene, diethyl ether, chloroform, and essential oils ； It reacts with alcohols to form silicates. It is highly corrosive due to its tendency to hydrolyze and produce hydrogen chloride. Properties of silicon tetrachloride: Name, Value; Name, Value. Molecular weight: 169.2; Heat of vaporization: 6.96 kcal/mol; Liquid density: 1.49 t/m³; Enthalpy of formation: -153.0 kcal/mol; Vapor density: 6.3 kg/m³; Standard free energy of formation: -136.9 kcal/mol; Melting point: -70 °C; Critical temperature: 206 °C; Boiling point: 57.6 °C. II. Safety procedures: 1. TCS is a colorless and transparent liquid that produces intense fumes when exposed to air; it is highly volatile, susceptible to hydrolysis, flammable, explosive, soluble in organic solvents, and toxic. It has a strong irritating effect on the human respiratory organs. Therefore, the equipment must be sealed to prevent air leakage. 2. The system is maintained at positive pressure using an adequate amount of high-purity N2. Strictly prevent air from being drawn back in, to avoid safety accidents and equipment failures. 3. When cleaning the equipment, the operator should wear protective gear such as safety glasses, acid- and alkali-resistant gloves, work clothes and rain boots, a gas mask, and a plastic apron. When putting them on, the gloves and rain boots should be thoroughly inspected to ensure they are undamaged and airtight before use. 4. When working with materials in place (such as replacing valves, flow meters, etc., during production), it is necessary to wear the aforementioned personal protective equipment before entering the site. After the operation is completed, the working area is inspected for leaks using NH3·H2O (ammonia water) (see Section 1.3, System Leak Detection Reaction Formula); it can only be put into use after confirmation that there are no leaks. 5. Open flames are strictly prohibited in areas equipped with TCS. When fire work is truly necessary for work purposes, an application must be submitted and approved by the superior department; only after obtaining a fire work permit may the work proceed. In the event of a fire or explosion involving TCS, the supply of material should be cut off first, the source of the fire should be isolated promptly, and the fire should be extinguished using CCl4 or CO2 fire extinguishers; in cases of minor fires, it can be extinguished by covering them with felt ； Under no circumstances should water be used to extinguish the fire, to prevent it from spreading. 6. In the event of TCS poisoning, personnel wearing protective gear such as safety glasses and gas masks should quickly transfer the affected person to a place with fresh air, keep them warm, loosen their collars and belts, and provide artificial respiration or seek medical treatment if necessary. 7. During the lifting of the furnace cylinder, carefully check whether the silicon rods are leaning against the furnace wall. If they are, use protective barriers to prevent them from falling and damaging the furnace bottom and its sealing gaskets. If they are not leaning against the wall, use specialized lifting equipment to manually remove the furnace and lift the silicon rods out of the furnace chamber one pair at a time. The dismantling personnel must wear appropriate protective equipment to prevent injury from falling silicon blocks. During lifting, follow the crane operation procedures; it is strictly prohibited to have anyone standing under or around the furnace cylinder and the silicon rods, to prevent the silicon rods from falling or other objects from causing injuries.