Preparation Method Of High-purity Silicon Carbide Powder For Silicon Carbide Wafer Production

1、 Gas phase method
1. Chemical vapor deposition method (CVD method)
The CVD method obtains ultrafine and high-purity SiC powders through high-temperature reactions of gases. The Si source is generally selected as SiH4 and SiCl4, while the C source is generally selected as CH4, C2H2, and CCl4. Gases such as (CH3) 2SiCl2 and Si (CH3) 4 can provide both Si and C sources, with a purity of over 99.9999%.
The CVD method utilizes an organic gas source to synthesize high-purity SiC powder, but this method requires very high purity of the organic gas source and internal graphite components, which increases production costs. In addition, the synthesized powder is ultrafine powder at the nanoscale, which is difficult to collect and has a low synthesis rate. Currently, it cannot be used to produce large quantities of high-purity SiC powder.
2. Plasma method
The plasma method involves introducing reactive gases into a plasma container excited by a radio frequency power source, where the gases react with each other under the collision of high-speed electrons, resulting in high-purity SiC powder. The gas source used in the plasma method is the same as that in the CVD method, and the gas purity is also above 99.9999%.
The plasma method obtains high-purity SiC powder through high-energy electron collisions, reducing the synthesis temperature of SiC powder. By increasing the gas flow rate and the size of the plasma chamber, the yield of SiC powder can be improved. However, the synthesized powder particle size is too small and requires further processing to be used for crystal growth.
2、 Sol gel method
At present, only the sol gel method can synthesize high-purity SiC powder in the liquid phase method. The preparation process is to dissolve inorganic salt or alkoxide in solvent (water or alcohol) to form a uniform solution, get a uniform sol, dry or dehydrate it and convert it into gel, and then heat treat it to obtain the required ultra-fine powder. The silicon carbide powder synthesized by the sol gel method was first used for sintering silicon carbide ceramics. With the continuous improvement of the process, the purity of the synthesized powder has also been continuously improved. At present, the SiC powder prepared by the sol gel method can be used for the growth of single crystals.
Sol gel method can prepare high-purity and ultra-fine SiC powder, but the preparation cost is high and the synthesis process is complex, so it is not suitable for industrial production.
3、 Self propagating high-temperature synthesis method
The self-propagating high-temperature synthesis method belongs to the solid-state synthesis method, which involves the spontaneous and continuous chemical reaction of the reactant by adding an activator under the condition of an external heat source. However, the addition of activators will inevitably introduce other impurities. In order to ensure the purity of the product, researchers choose to increase the reaction temperature and continue heating to maintain the progress of the reaction. This method is called the improved self-propagating high-temperature synthesis method. The improved self-propagating high-temperature synthesis method has a simple preparation process and high synthesis efficiency, and is widely used in industry to produce high-purity SiC powders. This method uses solid Si and C sources as raw materials to continuously react at high temperatures ranging from 1400 to 2000 ℃, ultimately obtaining high-purity SiC powders.
At present, in the improved self propagating synthesis method, researchers can control most impurities such as B, Fe, Al, Cu, P, etc. to 1 by controlling the impurity content in the initial Si source and C source, as well as purifying the synthesized SiC powder × Below 10-6. However, in order to prepare semi insulating SiC single crystal substrates, the content of N elements in SiC powder must also be reduced as much as possible. Both Si powder and C powder are prone to adsorbing a large amount of N elements in the air, resulting in a high content of N elements in the synthesized SiC powder, which cannot meet the requirements for the use of semi insulating single crystal substrates. Therefore, the current research focus of the improved self propagating synthesis method for preparing SiC powder is on how to reduce the content of N element in SiC powder.