(1) The material of the crucible
When preparing cast polysilicon, in the process of raw material melting and crystalline silicon crystallization, the silicon melt and the quartz crucible are in contact for a long time, which will cause viscous effect. Due to the different thermal expansion coefficients of the two, the volume of silicon increases by 9% when the crystal is solidified, which may cause the crystalline silicon or quartz crucible to rupture when the crystal cools; at the same time, the molten silicon can react chemically with almost all materials, so the crucible will pollute the silicon material. Must be controlled within the limits allowed by solar-grade silicon. Due to the long-term contact between the silicon melt and the quartz crucible, it is the same as the preparation of Czochralski monocrystalline silicon, which will cause corrosion of the quartz crucible and increase the oxygen concentration in the polycrystalline silicon. In order to solve the above problems, some people have proposed the following solutions.
① Use high-purity crucible. For example, use 4N grade high-purity Si crucible or high-purity Si3N4 crucible to replace the original quartz graphite crucible. These high-purity crucibles not only have low impurity content, high temperature resistance, and are not easy to chemically react with molten silicon.
②Do not use or touch the crucible. The regional suspension smelting method can be used, and the floating effect of the high-frequency electromagnetic field can be used. The crucible is not used in the process of melting and growing silicon: or the cold crucible induction melting method is adopted, the material does not contact the crucible, the crucible is not worn, and the casting can be continuously performed to reduce the infiltration of impurities.
③The inner wall of the crucible is separated from the silicon material by coating. Select materials with high temperature resistance, good chemical stability and strong impurity diffusion resistance to prepare a layer of coating on the inner wall of the quartz or graphite crucible, so that the crucible and the molten silicon will not react during the smelting process and reduce impurities in the crucible. Diffusion in silicon can effectively reduce the contamination of impurities from the crucible, and also reduce the stress generated during solidification. In the process, silicon nitride or silicon oxide, silicon amide and other materials are generally used as coatings, which are attached to the inner wall of the quartz crucible, so as to isolate the direct contact between the silicon melt and the quartz crucible, which can not only solve the problem of viscosity, but also can The oxygen and carbon impurity concentrations in the polysilicon are reduced; further, the use of the trisilicon tetranitride coating also enables the quartz crucible to be reused, thereby achieving the purpose of reducing production costs.
(2) Crystal structure
The crystal structure is controlled by methods such as adjusting the thermal field to grow unidirectional grains of appropriate size (several millimeters), and to minimize defects in the crystal, so that it is possible to make high-efficiency batteries. Therefore, columnar crystals are desirable.
During crystallization, the growth direction is parallel to the heat dissipation direction. Therefore, under the condition of unidirectional heat conduction and solidification, the temperature gradient is large and the solidification speed is small, and it is easy to form columnar crystals. The temperature fluctuations caused by convection will cause the crystals to fall off and become free, which affects the formation of columnar crystals. Applying a less strong stable magnetic field or stably moving in one direction can prevent the crystals from falling off and freeing, so it is easy to obtain columnar crystals. A complete columnar crystalline structure can be obtained by directional solidification. The key is to ensure one-way heat conduction, maintain a large temperature gradient and a small solidification rate. This temperature gradient causes the silicon liquid in the crucible to solidify from the bottom, grow from the bottom of the melt to the top, and after the silicon material solidifies, the silicon ingot is annealed, cooled, and then released from the furnace to complete the entire ingot casting process.
(3) Silicon ingot height
The growth of polycrystalline silicon ingots used to make solar wafers is a rather complex process. The goal of the ingot growth process is to produce acceptable quantities of material that can be made into silicon wafers and, ultimately, materials that can be made into high-efficiency batteries. The solar silicon material is grown in a highly controlled manner during the ingot growing process to optimize the grain structure and ensure that impurities are separated out of the melting stage before crystallizing into the polycrystalline silicon ingot. Better grain structure and fewer impurities mean higher quality ingots, resulting in higher efficiencies for solar cells and higher megawatt yields from polysilicon furnaces. The production of polysilicon ingots is a batch process, so it makes economic sense to reduce unit cost by having more material per batch.
At present, the height of silicon ingots is generally maintained at 25~26cm, and the size of silicon ingots is increasing, from about 240kg to about 450kg. While larger ingot sizes may seem like the solution to get more value from capital equipment investments, the four key economic factors that drive the greatest value are carefully considered, namely revenue per kilogram of product sold, ingot qualification rate, production capacity, and equipment prices. As ingots become wider, the heating kinetics also face more challenges, as the distance between the center point of the molten silicon and the heating components in the perimeter hot zone of the heating chamber continues to increase. The greater the distance, the more likely it is that the shape of the solid-liquid interface will not be uniform, and therefore the crystal growth from edge to edge of the ingot will not be optimal. And the silicon ingot yield may also decrease as a result, because the material quality is not ideal, so that more percentage of silicon ingots cannot be converted into silicon wafers. To solve this problem, the silicon ingot growth time can be increased, but this will reduce the production capacity. decline.
In addition, ways to grow taller ingots could be explored, with the goal of producing more wafer-ready ingots per batch. Since the height of the ingot is limited by the maximum cavity size of the furnace heating zone, the equipment had to be modified. Similar to growing wider ingots, this method sounds simple and intuitive, but producing taller ingots is not without its challenges. Because the directional solidification furnace draws heat from the bottom of the crucible, increasing the ingot height means that the heat has to travel a longer distance in the additional material. If heat is extracted too quickly, the temperature gradient between the top and bottom of the ingot increases. This situation will cause the silicon ingot to crack, thereby reducing the qualified rate of the silicon ingot. In addition, the additional time that heat experiences in higher ingots can increase batch process time, resulting in reduced productivity. There are other upstream and downstream production issues associated with producing larger ingots that impact production costs. Upstream, the crucible cost will increase with the crucible size, and the crucible preparation cost will also increase with the increasing complexity of the coating process. Downstream, wider ingots may require larger squaring wire saw equipment to process. Finally, overall wafer productivity can also decline if the line balance between ingot growth and wire dicing operations cannot maintain overall wafer productivity. Therefore, a delicate balance must be struck between all factors to achieve an increase in net production capacity.