At present, the third generation of semiconductors is dominated by silicon carbide. In the cost structure of its devices, the substrate accounts for 47%, and the epitaxy accounts for 23%. The two together account for about 70%, which is the most important part of the silicon carbide device manufacturing industry chain.
The commonly used method for preparing silicon carbide single crystals is the PVT (physical vapor transport) method. The principle is to make the raw materials in a high temperature zone and the seed crystal in a relatively low temperature zone. The raw materials at a higher temperature decompose and directly produce gas phase substances without liquid phase. These gas phase substances are transported to the seed crystal under the drive of the axial temperature gradient, and nucleate and grow at the seed crystal to form a silicon carbide single crystal. At present, foreign companies such as Cree, II-VI, SiCrystal, Dow and domestic companies such as Tianyue Advanced, Tianke Heda, and Century Golden Core all use this method.
There are more than 200 crystal forms of silicon carbide, and very precise control is required to generate the required single crystal form (the mainstream is 4H crystal form). According to Tianyue Advanced’s prospectus, the company’s crystal rod yields in 2018-2020 and H1 2021 were 41%, 38.57%, 50.73% and 49.90% respectively, and the substrate yields were 72.61%, 75.15%, 70.44% and 75.47% respectively. The comprehensive yield is currently only 37.7%. Taking the mainstream PVT method as an example, the low yield is mainly due to the following difficulties in SiC substrate preparation:
1. Difficulty in temperature field control: SiC crystal rods need to be produced at a high temperature of 2500℃, while silicon crystals only need 1500℃, so special single crystal furnaces are required, and the growth temperature needs to be precisely controlled during production, which is extremely difficult to control.
2. Slow production speed: The growth rate of traditional silicon materials is 300 mm per hour, but silicon carbide single crystals can only grow 400 microns per hour, which is nearly 800 times the difference.
3. High requirements for good product parameters, and black box yield is difficult to control in time: The core parameters of SiC wafers include microtube density, dislocation density, resistivity, warpage, surface roughness, etc. During the crystal growth process, it is necessary to accurately control parameters such as silicon-carbon ratio, growth temperature gradient, crystal growth rate, and airflow pressure. Otherwise, polymorphic inclusions are likely to occur, resulting in unqualified crystals. In the black box of the graphite crucible, it is impossible to observe the crystal growth status in real time, and very precise thermal field control, material matching, and experience accumulation are required.
4. Difficulty in crystal expansion: Under the gas phase transport method, the expansion technology of SiC crystal growth is extremely difficult. As the crystal size increases, its growth difficulty increases exponentially.
5. Generally low yield: Low yield is mainly composed of two links: (1) Crystal rod yield = semiconductor-grade crystal rod output/(semiconductor-grade crystal rod output + non-semiconductor-grade crystal rod output) × 100%; (2) Substrate yield = qualified substrate output/(qualified substrate output + unqualified substrate output) × 100%.
In the preparation of high-quality and high-yield silicon carbide substrates, the core needs better thermal field materials to accurately control the production temperature. The thermal field crucible kits currently used are mainly high-purity graphite structural parts, which are used to heat and melt carbon powder and silicon powder and keep warm. Graphite materials have the characteristics of high specific strength and specific modulus, good thermal shock resistance and corrosion resistance, but they have the disadvantages of being easily oxidized in high-temperature oxygen environments, not resistant to ammonia, and poor scratch resistance. In the process of silicon carbide single crystal growth and silicon carbide epitaxial wafer production, it is difficult to meet people’s increasingly stringent requirements for the use of graphite materials, which seriously restricts its development and practical application. Therefore, high-temperature coatings such as tantalum carbide have begun to emerge.
2. Characteristics of Tantalum Carbide Coating
TaC ceramic has a melting point of up to 3880℃, high hardness (Mohs hardness 9-10), large thermal conductivity (22W·m-1·K−1), large bending strength (340-400MPa), and small thermal expansion coefficient (6.6×10−6K−1), and exhibits excellent thermochemical stability and excellent physical properties. It has good chemical compatibility and mechanical compatibility with graphite and C/C composite materials. Therefore, TaC coating is widely used in aerospace thermal protection, single crystal growth, energy electronics, and medical equipment.
TaC-coated graphite has better chemical corrosion resistance than bare graphite or SiC-coated graphite, can be used stably at high temperatures of 2600°, and does not react with many metal elements. It is the best coating in the third-generation semiconductor single crystal growth and wafer etching scenarios. It can significantly improve the control of temperature and impurities in the process and prepare high-quality silicon carbide wafers and related epitaxial wafers. It is especially suitable for growing GaN or AlN single crystals with MOCVD equipment and growing SiC single crystals with PVT equipment, and the quality of the grown single crystals is significantly improved.
III. Advantages of Tantalum Carbide Coated Devices
The use of Tantalum Carbide TaC coating can solve the problem of crystal edge defects and improve the quality of crystal growth. It is one of the core technical directions of “growing fast, growing thick, and growing long”. Industry research has also shown that Tantalum Carbide Coated Graphite Crucible can achieve more uniform heating, thereby providing excellent process control for SiC single crystal growth, thus significantly reducing the probability of polycrystalline formation at the edge of SiC crystals. In addition, Tantalum Carbide Graphite Coating has two major advantages:
(I) Reducing SiC Defects
In terms of controlling SiC single crystal defects, there are usually three important ways. In addition to optimizing growth parameters and high-quality source materials (such as SiC source powder), using Tantalum Carbide Coated Graphite Crucible can also achieve good crystal quality.
Schematic diagram of conventional graphite crucible (a) and TAC coated crucible (b)
According to research by the University of Eastern Europe in Korea, the main impurity in SiC crystal growth is nitrogen, and tantalum carbide coated graphite crucibles can effectively limit the nitrogen incorporation of SiC crystals, thereby reducing the generation of defects such as micropipes and improving crystal quality. Studies have shown that under the same conditions, the carrier concentrations of SiC wafers grown in conventional graphite crucibles and TAC coated crucibles are approximately 4.5×1017/cm and 7.6×1015/cm, respectively.
Comparison of defects in SiC single crystals grown in conventional graphite crucibles (a) and TAC coated crucibles (b)
(II) Improving the life of graphite crucibles
Currently, the cost of SiC crystals has remained high, of which the cost of graphite consumables accounts for about 30%. The key to reducing the cost of graphite consumables is to increase its service life. According to data from a British research team, tantalum carbide coatings can extend the service life of graphite components by 30-50%. According to this calculation, only replacing the tantalum carbide coated graphite can reduce the cost of SiC crystals by 9%-15%.
4. Tantalum carbide coating preparation process
TaC coating preparation methods can be divided into three categories: solid phase method, liquid phase method and gas phase method. The solid phase method mainly includes reduction method and chemical method; the liquid phase method includes molten salt method, sol-gel method (Sol-Gel), slurry-sintering method, plasma spraying method; the gas phase method includes chemical vapor deposition (CVD), chemical vapor infiltration (CVI) and physical vapor deposition (PVD). Different methods have their own advantages and disadvantages. Among them, CVD is a relatively mature and widely used method for preparing TaC coatings. With the continuous improvement of the process, new processes such as hot wire chemical vapor deposition and ion beam assisted chemical vapor deposition have been developed.
TaC coating modified carbon-based materials mainly include graphite, carbon fiber, and carbon/carbon composite materials. The methods for preparing TaC coatings on graphite include plasma spraying, CVD, slurry sintering, etc.
Advantages of CVD method: The CVD method for preparing TaC coatings is based on tantalum halide (TaX5) as tantalum source and hydrocarbon (CnHm) as carbon source. Under certain conditions, they are decomposed into Ta and C respectively, and then react with each other to obtain TaC coatings. The CVD method can be carried out at a lower temperature, which can avoid defects and reduced mechanical properties caused by high-temperature preparation or treatment of coatings to a certain extent. The composition and structure of the coating are controllable, and it has the advantages of high purity, high density, and uniform thickness. More importantly, the composition and structure of TaC coatings prepared by CVD can be designed and easily controlled. It is a relatively mature and widely used method for preparing high-quality TaC coatings.
The core influencing factors of the process include:
A. Gas flow rate (tantalum source, hydrocarbon gas as carbon source, carrier gas, dilution gas Ar2, reducing gas H2): The change in gas flow rate has a great influence on the temperature field, pressure field, and gas flow field in the reaction chamber, resulting in changes in the composition, structure, and performance of the coating. Increasing the Ar flow rate will slow down the coating growth rate and reduce the grain size, while the molar mass ratio of TaCl5, H2, and C3H6 affects the coating composition. The molar ratio of H2 to TaCl5 is (15-20):1, which is more suitable. The molar ratio of TaCl5 to C3H6 is theoretically close to 3:1. Excessive TaCl5 or C3H6 will cause the formation of Ta2C or free carbon, affecting the quality of the wafer.
B. Deposition temperature: The higher the deposition temperature, the faster the deposition rate, the larger the grain size, and the rougher the coating. In addition, the temperature and speed of hydrocarbon decomposition into C and TaCl5 decomposition into Ta are different, and Ta and C are more likely to form Ta2C. Temperature has a great influence on TaC coating modified carbon materials. As the deposition temperature increases, the deposition rate increases, the particle size increases, and the particle shape changes from spherical to polyhedral. In addition, the higher the deposition temperature, the faster the decomposition of TaCl5, the less free C will be, the greater the stress in the coating, and cracks will be easily generated. However, low deposition temperature will lead to lower coating deposition efficiency, longer deposition time, and higher raw material costs.
C. Deposition pressure: Deposition pressure is closely related to the free energy of the material surface and will affect the gas residence time in the reaction chamber, thereby affecting the nucleation speed and particle size of the coating. As the deposition pressure increases, the gas residence time becomes longer, the reactants have more time to undergo nucleation reactions, the reaction rate increases, the particles become larger, and the coating becomes thicker; conversely, as the deposition pressure decreases, the reaction gas residence time is short, the reaction rate slows down, the particles become smaller, and the coating is thinner, but the deposition pressure has little effect on the crystal structure and composition of the coating.
V. Development trend of tantalum carbide coating
The thermal expansion coefficient of TaC (6.6×10−6K−1) is somewhat different from that of carbon-based materials such as graphite, carbon fiber, and C/C composite materials, which makes single-phase TaC coatings prone to cracking and falling off. In order to further improve the ablation and oxidation resistance, high-temperature mechanical stability, and high-temperature chemical corrosion resistance of TaC coatings, researchers have conducted research on coating systems such as composite coating systems, solid solution-enhanced coating systems, and gradient coating systems.
The composite coating system is to close the cracks of a single coating. Usually, other coatings are introduced into the surface or inner layer of TaC to form a composite coating system; the solid solution strengthening coating system HfC, ZrC, etc. have the same face-centered cubic structure as TaC, and the two carbides can be infinitely soluble in each other to form a solid solution structure. The Hf(Ta)C coating is crack-free and has good adhesion to the C/C composite material. The coating has excellent anti-ablation performance; the gradient coating system gradient coating refers to the coating component concentration along its thickness direction. The structure can reduce internal stress, improve the mismatch of thermal expansion coefficients, and avoid cracks.
(II) Tantalum carbide coating device products
According to the statistics and forecasts of QYR (Hengzhou Bozhi), the global tantalum carbide coating market sales in 2021 reached US$1.5986 million (excluding Cree’s self-produced and self-supplied tantalum carbide coating device products), and it is still in the early stages of industry development.
1. Crystal expansion rings and crucibles required for crystal growth: Based on 200 crystal growth furnaces per enterprise, the market share of TaC coated devices required by 30 crystal growth companies is about 4.7 billion yuan.
2. TaC trays: Each tray can carry 3 wafers, each tray can be used for 1 month, and 1 tray is consumed for every 100 wafers. 3 million wafers require 30,000 TaC trays, each tray is about 20,000 pieces, and about 600 million are needed each year.
3. Other carbon reduction scenarios. Such as high-temperature furnace lining, CVD nozzle, furnace pipes, etc., about 100 million.
Post time: Jul-02-2024