Poly furnace polysilicon ingot furnace furnace inspection

Since the vacuum melting process is essentially irreversible once vacuuming and heating are started, especially when the silicon material is melted, performing the furnace inspection is particularly necessary, even more important than the operation after the furnace is opened.

The purpose of the pre-furnace inspection is to discover the safety hazards and the signs of accidents in the furnace and to resolve them before starting the process. The contents of the furnace inspection can be divided into the following sections:

Cooling water system

Mainly check if the cooling water circuit is normal. From the cooling tower, to the pure water supply, to the water pump, the water valve, the water pressure and the water flow meter, all must be checked. Since there is a problem in the cooling water circuit, the furnace temperature will rise or even melt. The water temperature of the water cable may be too high, which may cause the water cable to burst. The water will enter the furnace and even the silicon liquid and cause an explosion. Therefore, the water route inspection must not be neglected. It must be checked one by one according to the furnace inspection form. After the waterway components have been inspected, conduct a water test to see if the inlet and outlet water pressure and flow of each cooling water circuit are normal. If any abnormalities are found, it must be checked again.

Heating and power supply system

Heating and power supply systems include external power supplies, switches, control boxes, power controllers, transformers, etc., as well as the inspection of water cables. In addition, special attention should be paid to electrodes and cables connected to the inside and outside of the furnace and the connection electrodes of the graphite components. Once the power supply system is open or short circuit, heating will not be able to proceed, or the arc will be generated during heating, resulting in unstable current, or sometimes no current, or complete interruption of current at high temperatures, and unbalanced three-phase current. These phenomena will lead to non-uniform temperature field, and those who are serious will lead to complete interruption of the process. Therefore, we must carefully check. When inspecting the circuit inside the furnace, pay attention to whether the insulation layer (usually silicate glass cloth) outside the water cable is in good condition and there is no loosening phenomenon, if it must be replaced or tightly packed.

Furnace part

The furnace body includes the upper and lower furnace lids and the furnace body, which is mainly whether each weld has water seepage and whether the furnace wall is clean. In addition, check for any foreign objects in the furnace, such as tools, shoe covers, or wipes. The cleaning of the furnace wall is very important. If there is dirt on the furnace wall, it will not only affect the thermal efficiency, but more importantly, because these dust dirt easily absorb the atmosphere and water vapor, it will bring a great burden on the vacuum. In addition, the dirt on the furnace wall will also contaminate the silicon material, leading to the failure of the silicon material purification. The various flanges of the furnace body will deform when they repeatedly experience high temperatures and low temperatures. These deformations may lead to poor sealing and generate problems such as air leakage. Therefore, it is necessary to check whether the flange ports are kept on a flat surface. Also check the condition of the flange ring seal, whether hardened or deformed, or if there is a crack. Cracks in the seal can cause air leaks during vacuum.

Graphite heating parts

The stone heating element in the furnace is a heating element that completes the heating of the entire furnace and is also an important part of the thermal field. Whether it is in good condition or not directly affects whether the furnace is working properly. The heating body is composed of stone grinding electrode, graphite heating rod, heating rod connector and other parts. Graphite pieces are consumables and should be replaced after a certain period of use. When installing the new stoneware, be careful not to loose or over-tighten the connection. When it is too loose, it can cause poor contact and even fall off during use. Over-tightening can cause graphite pieces to break due to thermal stress at high temperatures or when the temperature changes. Each graphite part has a certain size and size. After a certain period of use, the graphite rod will become fine because of the silicon vapor or oxidation. If the graphite rod is smaller than a certain size, it cannot be used any more.

Since the silicon vapor generated by the melting of the silicon material reacts with the graphite to form silicon carbide, the surface of the graphite often produces a green surface, sometimes falling off and sometimes clinging to the graphite surface. For a high-density stone mill, the reaction may be less, but at this time, the steam may infiltrate into the seams of the threads of the graphite connecting member, and sometimes the silicon liquid may splash in. When cooling, the silicon liquid will solidify and swell, and it is easy. Causes the graphite to break. Therefore, in all the gaps, it is best to seal it with graphite mud, which can greatly extend the life of the graphite parts.

Thermal insulation

The insulation body is usually made of carbon felt. The main inspection is whether the inner surface of the carbon felt is in good condition and whether it has a broken place.

In addition, the inspection of the insulation body should also pay attention to whether there is silicon vapor infiltration, resulting in siliceous carbon felt, resulting in silicon carbide, so that not only will the insulation body become heavy, but also lead to reduced thermal insulation, thermal insulation performance, resulting in energy consumption Larger. The stainless steel support structure on the outside of the insulation body is also checked for oxidation or loosening.

The inside of the heat insulation body is sometimes fixed with graphite bolts or molybdenum bolts to the soft glue and the external support structure. These bolts cannot be tightened too tightly or too loosely, otherwise they may cause off during production. The heat insulation body of RDS4.0 furnace type is fixed and there is no moving mechanism. At this time, it should pay attention to whether the inside of each temperature measuring hole on the heat preservation body is smooth, the position is correct, and whether the gas channel of the heat preservation body is unblocked, if there is a flow blocking element. Is it fixed already? Otherwise, these components may be blown into the silicon fluid during production to cause contamination of the silicon fluid.

The bottom insulation of RDS4.0 series furnaces needs to move up and down during production. In addition, there is a thermal switch that can be lifted up and down. Therefore, check whether the movement mechanism is normal. To check if the linear guides are clean, keep the light free from contamination, check whether the linear motion unit is normal, and whether the three motion mechanisms are balanced and synchronized. Whether the hanging hook of the heat preservation barrel is firm or not, and so on.

Graphite support structure

The graphite support structure refers to the platform that supports the crucible. In addition to supporting the crucible, this platform also plays the role of cooling and soaking at the bottom when transferring heat and crystallizing during heating. In addition, on the platform, there are also structures that allow silicon to flow from these trenches to the bottom of the furnace in the case of silicon leakage. In addition, there are institutions on the platform for loading and unloading materials forklifts. The graphite platform is usually supported by several graphite columns, so check that these graphite columns and platforms are intact. Is the support stable and the structure of the graphite column and hearth is stable? If silicon is missed, the graphite platform may react with silicon to produce silicon carbide, creating an uneven platform. Usually, the platform can be used as long as it is ground again. If the platform is too hard to use due to silicon carbide, it should be replaced.

Vacuum system

The vacuum system is the basis of vacuum melting. If there is a problem with the vacuum system, not only does the vacuum not meet the process requirements, the purification cannot be carried out, and it can easily lead to the oxidation of the graphite and oxidation of the silicon.

The polysilicon ingot casting furnace vacuum system consists of a mechanical pump (spool valve pump or rotary vane pump) and a Roots pump. Vacuum system to check whether the normal operation of the vacuum pump at all levels, whether the vacuum valve seal is intact, whether the valve action is correct, whether the inside of the pipeline is clean, whether the vacuum gauge is installed normally, whether the flange is normal, and so on. Vacuum pump cooling water must also be carefully examined.

During the process of the process, it is necessary to inflate the pressure inside the furnace. At this time, the main pump and the main valve will be closed, and the bypass valve will be used to maintain a lower vacuum level with the primary pump or the maintenance pump. Therefore, check whether the bypass valve is normal or not. For the bypass regulator, check whether the opening adjustment can be performed according to the console command.

The inside of the vacuum tube is easily contaminated with silicon vapor or silicon oxide vapor, which is often overlooked in the pre-furnace inspection due to the difficulty of cleaning. However, once it is contaminated, the vacuum pump can easily be damaged. Therefore, it must be cleaned every time. If there is a filter inside the pipe, then the filter must be cleaned every furnace.

Inflatable system

The polysilicon ingot needs to be filled with argon gas. In addition, the furnace must be filled with argon gas during cooling. When the ingot is ingot, it is necessary to protect the furnace from argon, so the inflator system should be kept intact. Inflatable system consists of gas source, trachea, valves, flow and pressure gauges. Check one by one to see if it is in good condition, especially in the furnace section of the gas line, taking care not to come into contact with other furnace components, especially heating elements, or to approach too close.

The gas source must be kept clean, because even if the pure gas is bought, it is difficult to guarantee the cleanliness at the time of bottling. Therefore, the gas source outlet should be provided with a filtering and drying device to repurify the gas, and thus also carry out the gas purification system. Inspection, filter material should be replaced regularly as required.

Instrument control system

All the furnaces in the general use automatic control. Automatic control operates according to the set process. However, the execution of the process requires sensor signals to determine how to control. Sensors for polycrystalline silicon ingot furnaces include: infrared temperature signal, thermocouple temperature measurement element, gas and water flow meter, water pressure meter, vacuum degree, insulation linear unit position signal, and so on. The vacuum melting furnace's actuators include: power controllers, switches, servo motors for lifting and insulation, valves, and more.

Meters and sensors are the eyes and ears of the control system. If the sensor signals are not accurate, the control system cannot perform proper control. In high-temperature environments, furnace inspection is particularly important because furnace sensors cannot be corrected during production.

Infrared thermometers usually need to be calibrated regularly, but under normal circumstances, the infrared channel in the furnace is more influential. Infrared channel refers to the channel from the thermometer to the surface of the liquid silicon. Infrared must be able to directly observe the liquid silicon. As it is usually necessary to go through the observation window, the infrared graphite tube, and the thermal insulation body to reach the surface of the silicon liquid, sometimes there is a lid. This channel, at high temperatures, is prone to clogging due to the severe vaporization of silicon vapor on the silicon surface. Some of the blockages are: the opening of the insulation body, infrared graphite tubes, and the observation window may be vaporized by a layer of film to cause temperature error. In the event of a blockage, the infrared thermometer will give a temperature reading that is lower than the actual value. In severe cases, all plugging will completely lose its significance. Therefore, we must observe whether the infrared port is blocked, whether it is clean, whether the infrared window is clean, and so on. Other instruments also have to be checked one by one.

In a vacuum furnace, the thermocouple is easily corroded by silicon vapor and thinned or even perforated. Therefore, after using it more than once, it is necessary to observe whether the thermowell is normal. Because the cockroach needs to be removed and placed, the sleeve of the thermocouple sometimes needs to be moved. Care must be taken to ensure that the head of the sleeve is to be attached to or in contact with the part that needs to be measured. The thermocouple wire will become brittle after it is used once. Pay attention to whether it breaks when moving.

All instruments should perform analog tests, ie give a signal to see if they are reflected on the operator interface. The inspection of the executing agency is similar to that of the instrument, and it also needs to be checked one by one, and the control signal is given through the computer interface to see the performance.

The control system can be detected by running software as well as observing signals and giving execution signals.

The above nine items should be carefully and carefully inspected by the pre-furnace checklist before each operation. Do not think that something will not be skipped and skipped, and do not omit it. Once the problem has been overlooked before the furnace inspection, there may be no chance to correct it after the furnace is opened.

Again, a good inspection of the furnace is a guarantee of the success of the furnace. Its importance is as important as the on-duty operation during production, and it can even be more important.