As one of the representative materials of third-generation semiconductors, silicon carbide is suitable for the production of high-temperature, high-frequency, anti-radiation, high-power and high-density integrated electronic devices. At present, the silicon carbide single crystal substrate material used in the production of devices is generally grown by the PVT (Physical Vapor Transport) method. Studies have shown that the purity of SiC powder and other parameters such as particle size and crystal type have a certain impact on the quality of SiC single crystals grown by PVT method and even the quality of subsequent devices. This article mainly focuses on the synthesis process of high-purity SiC powder for single crystal growth by PVT method.
SiC powder synthesis method
There are many ways to synthesize SiC powder. Generally speaking, it can be roughly divided into three methods. The first method is the solid phase method, among which the representative carbothermal reduction method, self-propagating high temperature synthesis method and mechanical pulverization method; the second method is the liquid phase method, of which the representative method is mainly sol-coagulation Glue method and polymer thermal decomposition method; the third method is gas phase method, including chemical vapor deposition method, plasma method and laser induction method.
1. Advantages and disadvantages of various methods
The silicon carbide powder synthesized by the solid phase method is more economical, with a wide range of raw materials and low prices, and is easy to industrially produce. However, the silicon carbide powder synthesized by this method has high impurity content and low quality; the high-temperature self-propagating method uses The high temperature gives the reactants the initial heat to start the chemical reaction, and then uses their own chemical reaction heat to make the unreacted substances continue to complete the chemical reaction. However, since the chemical reaction of Si and C emits less heat, other additives must be added to maintain the self-propagating reaction, which inevitably introduces impurity elements, and this method easily causes uneven reaction.
At present, the technology for synthesizing silicon carbide powder by the liquid phase method is relatively mature. The silicon carbide powder synthesized by the liquid phase method is highly pure and nano-sized fine powder, but the process is more complicated and it is easy to produce harmful substances to the human body.
The silicon carbide powder synthesized by the gas phase method has high purity and small particle size, which is a common method for synthesizing high-purity silicon carbide powder. However, this synthesis method has high cost and low yield, and is not suitable for mass production.
2. Silicon carbide powder synthesis equipment
Silicon carbide powder synthesis equipment is used to prepare silicon carbide powder required for growing silicon carbide single crystals. High-quality silicon carbide powder plays an important role in the subsequent growth of silicon carbide.
The synthesis of silicon carbide powder adopts the direct reaction of high-purity carbon powder and silicon powder, and is produced by a high-temperature synthesis method. The main technical difficulties of silicon carbide powder synthesis equipment are high temperature and high vacuum sealing and control, vacuum chamber water cooling, vacuum and measurement systems, electrical control systems, powder synthesis crucible heating and coupling technology.
Currently, major foreign manufacturers include Cree, Aymont, etc., and the purity of synthetic powder can reach 99.9995%. The main domestic units include the Second China Electric Power Research Institute, Shandong Tianyue, Tianke Heda and the Chinese Academy of Sciences Institute of Ceramics. The purity can generally reach 99.999%, and some units can reach 99.9995%.
High-purity SiC powder synthesis method
1. CVD method
At present, the synthesis methods of high-purity SiC powder used to grow single crystals mainly include: CVD method and improved self-propagating synthesis method (also called high-temperature synthesis method or combustion method).
Among them, the Si source for CVD synthesis of SiC powder generally includes silane and silicon tetrachloride, while the C source generally uses carbon tetrachloride, methane, ethylene, acetylene, and propane, while dimethyldichlorosilane and tetramethylsilane And so on can provide Si source and C source at the same time.
SashiroEzaki et al. used the CVD method, using flake graphite as the substrate, methyl chloroethane/hydrogen as the reaction gas and carrier gas, and deposited SiC film at 1250～1350℃, and then through the processes of oxidation, pickling and pulverization, the particles were obtained. SiC powder with diameter of 200～1200μm.
Although this method produces high-purity SiC powder, the subsequent process is complicated, the raw materials are expensive, and the yield is low.
W.Z.Zhu et al. used the CVD method, using silane and acetylene as the reaction gas, and hydrogen as the carrier gas, to synthesize ultra-fine and high-purity SiC powder at 1200-1400°C.
AparnaGupta et al. used hexamethylsilane as the reaction source, hydrogen and argon as the carrier gas, and also synthesized ultra-fine and high-purity SiC powder at 1050 to 1250°C using the CVD method.
The members of the above two research groups have used the CVD method to synthesize high-purity SiC powder using organic gas sources. However, the synthesized powder is nano-level ultra-fine powder. Although it has high purity, it is not easy to collect and is not suitable for large-scale high-volume production. The synthesis of pure SiC powder is not conducive to the development of later industrialization.
2. Self-propagating synthesis
The previous self-propagating synthesis method is a method of igniting the reactant body with an external heating source, and then using the chemical reaction heat of its own substance to make the subsequent chemical reaction process continue spontaneously, thereby synthesizing materials.
Most of this method uses silicon powder and carbon black as raw materials, and adds other activators to directly react at a significant speed at 1000-1150°C to produce SiC powder. The introduction of activators will inevitably affect the purity and quality of the synthesized products.
Therefore, many researchers have proposed an improved self-propagating synthesis method on this basis. The improvement is mainly to avoid the introduction of activators, and to ensure the continuous and effective progress of the synthesis reaction by increasing the synthesis temperature and continuously supplying heating.
As early as 1999, Japan’s Bridgestone Company used tetraethoxysilane as the silicon source and phenol resin as the carbon source. Using the combustion method at the range of 1700-2000 ℃, synthesized the particle size of 10-500μm, the quality of impurity content SiC powder with a fraction lower than 0.5×10-6.
However, the reactants of this method use organic substances, so the cost of raw materials is relatively high, which is not conducive to the mass production of SiC powder.
Researchers from the Institute of Silicon Research of the Chinese Academy of Sciences used Si powder and C powder with raw material mass fractions of 99.9% or more to synthesize a 99.999% mass fraction of SiC powder suitable for single crystal growth by high temperature reaction in an Ar atmosphere.
Ning Lina of Shandong University and others uniformly mixed Si powder and C powder with a molar ratio of 1:1. Using the secondary reaction method, SiC powder was synthesized at high temperature.
LiWANG and others use activated carbon (particle size 20-100μm) and flake graphite (particle size 5-25μm) as carbon source (mass fraction 99.9%), and high-purity silicon as silicon source (particle size 10-270μm, mass fraction 99.999%) ).
High-purity SiC powder was prepared in a vacuum high-temperature sintering furnace under argon atmosphere at 1900℃.
LihuanWANG et al. used silicon powder (mass fraction 99.999%, particles 5-10μm) and carbon powder (mass fraction 99.999%, particles 5-20μm) to synthesize high-purity SiC powder by an intermediate frequency heating combustion synthesis method.
Li Bin of the Second Research Institute of China Electronics Technology Group Corporation used self-propagating method to synthesize silicon carbide powder for single crystal growth. In experiments, it is found that the purity of silicon carbide powder synthesized under high vacuum conditions is better than that of silicon carbide powder synthesized under open carrier gas conditions. In particular, high vacuum conditions help reduce the N concentration in silicon carbide powder.
In addition, silicon carbide single crystals were grown using silicon carbide powder synthesized under high vacuum conditions. The results showed that the grown silicon carbide single crystals had high purity and excellent semi-insulating properties, which met the requirements of related devices for semi-insulating substrates. Electrical requirements. It can be seen that the silicon carbide powder synthesized under high vacuum conditions is beneficial to the growth of high-purity semi-insulating silicon carbide single crystals.
Prospect of Synthesis Process of High Purity SiC Powder
The improved self-propagating method for synthesizing SiC has relatively low raw materials and relatively simple procedures. It is currently a common method used in laboratories to grow single crystals to synthesize SiC powder. During the synthesis process, it is found that different synthesis process parameters have a certain influence on the synthesized product. .
In the future, research needs to be strengthened in the following areas:
1. In-depth research on the mechanism of high-purity SiC powder synthesis process, especially strengthening the basic theoretical research on the effective control of powder particle size, shape, particle size distribution, and purity.
2. Further strengthen the research on improving the specific process of self-propagating synthesis of SiC powder, in order to prepare high-purity SiC powder suitable for single crystal SiC growth with good quality and high purity on the basis of low cost and simple process Therefore, it can effectively improve the growth quality of SiC single crystal substrates and promote the development of my country's SiC-based device industry.
