Place the coated quartz crucible on the heat exchange table, put an appropriate amount of silicon raw material, then install heating equipment, heat insulation equipment and furnace cover, and vacuum the furnace to reduce the pressure in the furnace to 0.05~0.1mbar (1bar=105Pa, the same below) and keep the vacuum, pass argon as the protective gas, so that the pressure in the furnace is basically maintained at 400~600mbar.
The furnace body is heated by a graphite heater, which first evaporates the moisture adsorbed on the surfaces of the graphite parts (including heaters, crucible plates, heat exchange tables, etc.), heat insulation layers, silicon materials, etc., and then slowly begins to melt. During the melting process, the temperature is kept at about 1500°C, and the process takes 4~5h.
Argon was used as protective gas to keep the furnace pressure basically at 400~600mbar. Gradually increase the thermal power so that the temperature in the quartz crucible reaches about 1500°C, and the silicon raw material begins to melt. During the melting process, it has been kept at about 1500 ° C until the end of the material. The process takes 9~11h.
(4) Crystal growth
After the melting of the silicon raw material, reduce the heating power so that the temperature of the quartz crucible drops to 1420~1440℃, and then the quartz crucible gradually moves down and slowly leaves the heating zone; or the heat insulation device rises, so that the quartz crucible exchanges heat with the surrounding environment : At the same time, water is passed through the cooling plate, so that the temperature of the melt starts to decrease from the bottom, so that a relatively stable temperature gradient is formed at the liquid-solid interface during the crystallization process, and the heat released during the crystallization of the silicon material is radiated to the lower furnace cavity through the directional solidification block. On the inner side, a vertical temperature gradient is formed in the silicon material, which is conducive to the growth of crystals. Its characteristics are that the temperature gradient of the liquid phase is close to a constant, and the growth rate is close to a constant controlled by the moving speed of the table and the flow of cooling water, and the growth rate can be adjusted.
To keep the solid-liquid interface basically on the same level, crystalline silicon is first formed at the bottom and grows upward in a columnar shape until the growth is completed. The speed of crystal crystallization is about 1cm/h, about 10kg/h; this process takes 20~22h . In the process of crystal growth, the growth system must be well insulated to maintain the uniformity of the temperature of the melt zone without a large temperature gradient; Variety. This avoids excessive thermal stress in the polysilicon, which can lead to more dislocation growth in the body, and even lead to cracking of the ingot. In the process of crystal growth, the solid-liquid interface is always kept parallel to the horizontal plane, which requires a special thermal field design, so that when the silicon melt solidifies, the solid-liquid interface is always kept parallel to the horizontal plane from the bottom to the top. , which is the solid-liquid interface solidification technology.
After the crystal growth is completed, due to the large temperature gradient at the bottom and the upper part of the crystal, thermal stress may exist in the crystal ingot, which is easy to cause the silicon wafer to be broken during the process of silicon wafer processing and cell preparation. Therefore, after the crystal growth is completed, the ingot should be kept near the melting point for 2 to 4 hours to make the temperature of the ingot uniform to reduce thermal stress.
After the crystal ingot is annealed in the furnace, the heating power is turned off, the heat insulation device is raised or the crystal ingot is completely lowered, and a large flow of argon gas is passed through the furnace to gradually reduce the crystal temperature to around room temperature; at the same time, the pressure in the furnace gradually rises until it reaches atmospheric pressure. , and finally remove the ingot. This process takes about 10h. Usually, the faster the growth rate of the crystal, the higher the labor productivity, but the greater the temperature gradient, which will eventually lead to greater thermal stress, and high thermal stress will lead to high density. Dislocations seriously affect the quality of materials. Therefore, when casting polycrystalline silicon crystal growth, the main problems to be solved include, the solid-liquid interface temperature as uniform as possible: as small as possible the force; as large as possible the grain; as little contamination from the crucible as possible; The size is also related to the cooling rate of the crystal: the faster the crystal cools, the larger the temperature gradient; the faster the rate of crystal nucleation, the more and smaller the grains are.
After the preparation of cast polysilicon is completed, it is a square ingot. At present, the mass of cast polysilicon can reach 250~300kg, and the size can reach 700mmx700mmx300mm. Due to the problem of heat dissipation during crystal growth, it is difficult to increase the height of polysilicon. Therefore, the main method to increase the volume and weight of polysilicon is to increase its side length. However, the increase in edge size is not unlimited, because during the processing of polycrystalline silicon ingots, it is difficult to process large-sized ingots with the cylindrical cutter or band saw currently used; moreover, graphite heaters and other graphite Devices need to be replaced periodically, and the larger the ingot size, the higher the replacement cost. Generally, high-quality polysilicon should have no macroscopic defects such as cracks and holes. The surface of the ingot should be flat and viewed from the front. The cast polysilicon is in a polycrystalline state, the grain boundaries and grains are clearly visible, and the size of the grains can be as low as about 10mm; Viewed from the side, the grains grow in columnar shape, and the main grains grow from the bottom to the top almost perpendicular to the horizontal ground.