In the solar photovoltaic industry, the quality of monocrystalline silicon is directly related to the quality of monocrystalline silicon. A qualified thermal field can grow high-quality monocrystalline. Therefore, creating good thermal field conditions is a very important factor in the Czochralski monocrystalline process. Graphite is the carrier for single crystal growth. High-purity graphite and isostatically pressed graphite processing parts are mainly used in graphite Czochralski single crystal growth furnaces.
According to the needs of the actual situation, the size of the thermal field is divided according to the diameter of the quartz crucible. Currently, thermal fields in our country range from φ12" to φ28", and generally, φ18" to φ22" are the majority.
The thermal system of the single crystal furnace mentioned here is the so-called thermal field. The thermal field generally includes a pressure ring, an insulation cover, an upper, middle, and lower insulation cover, a (three-flap) graphite crucible, a crucible support rod, a crucible tray, an electrode, a heater, a guide tube, and a graphite bolt. To prevent silicon leakage in the future, protective plates and protective sleeves are installed on the furnace bottom, metal electrodes, and support rods.
1. Heater
The heater is a very important component in the thermal field. It is a direct heating element. When the temperature is high, it can reach more than 1600 degrees. Common heaters have three shapes: cylinder, cup, and spiral. Generally, heaters are processed from high-purity graphite.
2. Graphite electrode
The graphite electrodes work to hold the heater in place and deliver current through the heater. Therefore, the electrode must be thick, strong, and durable, and the surface must be smooth and stable to ensure a good contact surface and not spark when powered. The graphite electrode is also processed with high-purity graphite.
3. Graphite crucible, graphite support rod, graphite tray
The crucible support rod, crucible, and tray together constitute the support body of the graphite crucible, which must be firmly combined with the lower shaft and have good neutrality. When the lower shaft rotates, the gap between the support rod and the tray is less than or equal to 0.5mm. The height of the graphite support body can be adjusted.
4. Insulation cover
The thermal insulation cover is divided into upper, middle, and lower thermal insulation covers. The thermal insulation cover is made of graphite carbon felt wrapped around an insulation cylinder. The number of layers of graphite carbon felt depends on the situation. The lower insulation cover forms the bottom insulation system. Its function is to strengthen the insulation of the bottom of the pot, increase the temperature of the bottom of the pot, and reduce heat loss.
5. Insulation cover and guide tube
The thermal insulation cover is composed of a thermal insulation upper cover, a thermal insulation carbon felt, and a thermal insulation lower cover, that is, a layer of graphite carbon felt is sandwiched between two layers of annular graphite. The guide tube is mainly used to control the temperature gradient of the thermal field and guide the argon gas flow in the future.
6. Pressure ring
The graphite pressure ring is a circular annular graphite piece composed of several arcs. It is placed at the contact point between the cover plate and the furnace wall to prevent heat and gas from passing through the gap between the fireplace and the cover plate.