(1) Structural composition of polysilicon ingot furnace
The polysilicon ingot furnace is mainly composed of graphite heater, heat insulation layer, crucible and silicon material. The polysilicon production process is mainly temperature control, therefore, the structural design of the heating system of the polysilicon ingot furnace is very important. The heating methods are divided into induction heating and radiation heating. In induction heating, magnetic field induction is used to heat through the silicon material and has a stirring effect, but it is difficult to form a stable temperature gradient inside the silicon material: then radiation heating is used, which can precisely control the heat transfer in the crystallization process, which is easy to use. A straight temperature gradient is formed inside the crucible, and the radiant heating method is generally preferred in the ingot furnace.
The heating capacity of the polysilicon ingot furnace heater must exceed 1650°C, and the material cannot react with the silicon material, causing no pollution to the silicon material, or the pollution to the silicon material is within the tolerance range, and can be used in vacuum and inert atmosphere for a long time. , There are metal tungsten, molybdenum and non-metallic graphite for the heaters that meet the conditions of use. Because tungsten and molybdenum are expensive and difficult to process, graphite has a wide range of sources and can be processed into various shapes. In addition, graphite has the characteristics of small thermal inertia, rapid heating, high temperature resistance, good thermal shock resistance, large radiation area, high heating efficiency and stable basic performance, so high-purity stone black is generally used as a heating material.
(2) Design requirements for thermal insulation materials
① The heating speed of the equipment is as fast as possible, and the heat insulation effect is good;
② The amount of outgassing of the thermal insulation material in the furnace should be as small as possible, and the time of vacuum exhaust should be shortened:
③ The mass of the thermal insulation layer should be as light as possible to reduce the inertia and affect the control accuracy
(3) Requirements for the selection of thermal insulation materials
High temperature resistance, low density, low thermal conductivity, less heat storage, good heat insulation, less outgassing, light weight, and small expansion coefficient. Among many refractory insulation materials, high-purity carbon felt is the most ideal.
In addition, ways to reduce impurity pollution: choose refractory materials with stable chemical properties; do not bring impurities into the tools in contact with the material as much as possible, or protect them with paint: perform necessary cleaning treatment on the furnace in time; strengthen charge management to prevent mixing phenomenon, etc.
(4)Two intermediate frequency polysilicon ingot furnaces in China
One is a 150kg intermediate frequency polysilicon ingot furnace of a company’s foundry research institute in Beijing. Its main features are as follows: ① Output ingot size: 400mmx400mmx400mm; ② Heating speed: 160kW high-power intermediate frequency heating; ③ Operation and operation: PLC programming control, operation and monitoring are completed on the touch screen: ④ Vacuum degree, the vacuum of the furnace Degree can be determined by 3Pa: ⑤Working temperature: The working temperature of the furnace can reach above 1450℃; ⑥Maximum stroke of crucible: 425mm; ⑦Geometric size of heating zone: 700mmx700mmx600mm: ⑧Geometric size of crystallization zone: 700mmx700mmx450mm: ⑩Geometric size of furnace shell: diameter 1500mm , height 1700mm; ⑾ lower furnace cover stroke: 1000mm; 1D directional solidification speed: 1.35133mm/h. In addition, the system has a vacuum system and a chlorine protection system.
The other is an intermediate frequency induction heating polysilicon ingot furnace produced in Shanghai. Its main features are as follows. ①Power supply, three-phase 380V, 500kW, frequency: 1000Hz: Induction coil voltage: 700~750V: ②Temperature range, 1600~1800℃: heating rate: 3~5h; ③The size of the inner cavity of the heating body: 41080mmx1000mm; ④Maximum ingot Size: 680mmx680mmx500mm; ⑤ Vacuum degree: -0.09MPa; fill with N2 or Ar after vacuuming.
Equipment composition: double-layer water-cooled stainless steel furnace body, furnace cover, furnace bottom; ordinary steel platform, lifting mechanism, vacuum system, intermediate frequency power supply system, temperature measurement and control system, cooling system, heating and heat preservation system (carbon/carbon material, graphite, carbon felt) , Al2O3 fiber reinforced ceramic composite structure).
Requirements for users: ①380V, 500kW power supply: ②50t/h circulating water: ③The lower height of the workshop crane is 7m: ④The workshop crane is ≥5t; track.
In recent years, the ingot casting process has mainly developed in the direction of large ingots. Most ingots produced by companies with advanced technology are 55cmx55cm and weigh about 150kg. At present, a square silicon ingot of 65cmx65cm and an ingot weight of 230kg has also been cast. The ingot casting time is in the range of 3~43h, and the actual yield of silicon material before slicing can reach 83.8%. Large-scale ingot furnaces mostly use intermediate frequency heating to adapt to large-scale silicon ingots and industrial scale. At the same time, the quality of silicon ingots has also been significantly improved. Through process optimization and improvement of crucible materials, defects and impurity content are reduced. The shape of the solid-liquid interface during crystal growth affects the uniformity of the grain structure and the electrical properties of the material. Generally speaking, the solid-liquid interface of the horizontal shape is better. Due to the improvement of the overall quality of silicon ingots, the availability of silicon ingots has been significantly improved.
Due to the use of low-cost crucibles and mold release coatings in the ingot, the quality of the silicon ingot will still be affected. In recent years, electromagnetic method (EMC) has been used to conduct ingot casting tests. The method is that the silicon material is continuously added to the molten silicon from the upper part of the furnace, and the molten silicon is kept in contact with the bottomless cold crucible by electromagnetic force, and the solidified silicon is Pull down continuously. The cold crucible is formed with a water-cooled copper crucible. Figure 1 shows the polysilicon production scene, and Figure 2 shows the polysilicon product.