Silicon nitride (Si3N4) ceramics are a non-metallic material made from silicon nitride powder through processes such as forming and sintering, and they possess excellent comprehensive properties. Silicon nitride ceramic materials have outstanding physical and chemical properties that are difficult for general metal materials and polymer materials to match, including wear resistance, corrosion resistance (resistant to all acids and bases except hydrofluoric acid and concentrated NaOH), high-temperature resistance (withstands high temperatures of over 1800°C, with a maximum operating temperature of 1400°C), high fracture toughness (far higher than other ceramic materials), and high thermal conductivity. They can withstand harsh working environments that metal or polymer materials cannot handle, so silicon nitride ceramic materials can replace many traditional materials and are widely used in multiple industrial fields. High-quality powder is the foundation for preparing high-performance silicon nitride ceramics, and low-cost powder preparation technology is the key to the widespread application of silicon nitride ceramics. The main preparation methods for silicon nitride powder include direct silicon powder nitridation, self-propagating synthesis, carbothermal reduction, and thermal decomposition.
Direct silicon powder nitridation. Silicon powder directly reacts with nitrogen, ammonia, or other nitrogen-containing atmospheres to form silicon nitride powder. This method has a relatively mature process, can achieve large-scale production, and has relatively low production costs, and is adopted by most domestic and international enterprises.
Self-propagating high-temperature synthesis (SHS). The principle is that after the raw materials are ignited, the reaction diffuses to the unreacted region until the reaction is complete. Its characteristic is that no external heat source is needed, and the heat required is provided by its own combustion. This method for preparing powder has the advantages of high efficiency, energy saving, and environmental protection, but during powder preparation, because the reaction speed is too fast, the reaction process is difficult to control.
Carbothermal reduction method. Using SiO2 as the silicon source and carbon as the reducing agent, silicon nitride is synthesized through carbon reduction and nitridation reaction above 1400°C under N2 atmosphere. The raw materials used in this method have low cost, the prepared powder product has small particle size, the reaction speed is fast, the α-Si3N4 content is high, and it is suitable for large-scale production. However, the prepared silicon nitride powder often contains residual carbon or silicon carbide, so the purity of the prepared powder is not high, which affects the quality and application of the product.
Silicon imide pyrolysis method. A method for preparing Si3N4 powder with a relatively high α-phase content through interfacial reaction and thermal decomposition. This method is difficult to prepare, has high technical barriers, and has high requirements for raw material purity. Its difficulty lies in the fact that it is not easy to obtain stable solid silicon imide (Si(NH)2). This method can produce high-purity, high-quality silicon nitride powder.
Process flow diagram of the silicon imide pyrolysis method
02 Diverse Applications: Three Major Products Seize the High-End Materials Track
Silicon nitride ceramic products mainly include three major categories: ceramic substrates, ceramic balls, and structural components. At present, the fields of ceramic substrates and high-end hot isostatic pressing (HIP) ceramic balls are still dominated by Japanese and American enterprises such as Kyocera, Toshiba Materials, Maruwa, and CoorsTek, while domestic enterprises mainly include Sinoma Advanced Materials, Guoci Jinsheng, Hengyang Kaixin, and others.
Ceramic substrates
Silicon nitride is considered the ceramic substrate material with the best comprehensive performance, with thermal conductivity reaching 90–120 W/(m·k), a thermal expansion coefficient of 3.2×10-6/°C, and excellent mechanical strength, good chemical stability, and thermal shock resistance. With the rapid development of third-generation semiconductor devices and high-power-density applications, market demand for high-performance ceramic substrates continues to grow, and silicon nitride substrates are gradually becoming one of the important technical routes for high-end power modules.
Silicon nitride ceramic sheet, image source: Ferrotec
At present, the method commonly used in industry to prepare silicon nitride ceramic substrates is organic tape casting. First, silicon nitride powder, dispersant, sintering aid, and organic solvent are mixed by primary ball milling, and at the same time the powder is ball-milled to the required particle size; then binder and plasticizer are added to the ball-milled slurry for secondary ball milling to make it uniformly mixed; after defoaming, the required slurry is obtained. The slurry is added to a tape casting machine for tape casting to obtain a green body, and then the green body is cut to the required size, subjected to binder removal and sintering, and finally a sintered silicon nitride ceramic substrate is obtained. The sintering methods mainly include reaction sintering (RS), hot pressing sintering (HPS), pressureless sintering (PLS), and gas pressure sintering (GPS).
Before use in packaging, ceramic substrates need to be metallized, that is, a conductive layer is formed on the ceramic surface by copper plating or copper coating, so as to connect electronic components and realize the conductive function of the circuit. Common ceramic metallization processes include DPC, DBC, and AMB.
China's silicon nitride ceramic industry started relatively late. High-end silicon nitride ceramic products still have gaps compared with foreign countries in some key performance indicators, and some rely on imports, but low- and mid-end products have basically achieved domestic production. The gaps are mainly reflected in technological improvement, dependence on imported equipment, unstable product quality, irrational industrial structure, and insufficient market development.
Ceramic balls
Compared with steel balls, silicon nitride ceramic balls have the advantages of high limit speed, strong load-bearing capacity, high-temperature resistance, and acid and alkali corrosion resistance, and also exhibit certain self-lubricating properties. With absolute advantages as rolling elements, they are widely used in high-speed bearings and have become key components of high-speed, high-precision bearings.
The preparation process of silicon nitride ceramic balls is to first obtain a dense, high-strength ceramic ball sintered body blank, and then precisely machine the blank to achieve the required size, roundness, and roughness. The preparation process generally includes four stages: raw material preparation, green body forming, high-temperature sintering, and grinding. Dry forming has the advantages of high forming efficiency, short forming cycle, and low binder content, and is the preferred forming method for the industrialization of Si3N4 ceramic balls. Dry forming includes dry pressing and isostatic pressing, both of which have been commonly used in the green body forming of Si3N4 ceramic balls.
The sintering methods suitable for mass production of silicon nitride ceramic balls are gas pressure sintering (GPS) and hot isostatic pressing sintering (HIP). Among them, hot isostatic pressing technology uses gas as the pressure medium and applies equal pressure in all directions of the body during sintering, achieving densification of ceramics under high temperature and high pressure. It is the best process for manufacturing high-performance silicon nitride ceramic balls.
Comparison table of properties of GPS silicon nitride and HIP silicon nitride
After sintering, silicon nitride ceramic balls also need surface finishing to meet the requirements of high precision and low roughness, including grinding, polishing, and superfinishing processes.
Ceramic structural components
Precision ceramic structural components are key components of semiconductor equipment, and their research, development, and production directly affect the development of the semiconductor industry. Silicon nitride has a low thermal expansion coefficient, high thermal conductivity, chemical corrosion resistance, and excellent thermal shock resistance. Hot-pressed sintered Si3N4 has extremely high hardness and is extremely high-temperature resistant. Its strength remains at 1200°C without decreasing, and it will not melt into a liquid after heating. It is called the ceramic material with the best comprehensive performance and is used in semiconductor equipment to manufacture wafer carriers, heaters, etching machine components, and others.
Silicon nitride ceramic structural components, image source: CeramTec
03 Summary
The future incremental market for silicon nitride is expanding from traditional structural ceramics to more high-value applications. Power semiconductors need high-thermal-conductivity, high-reliability ceramic substrates; new energy vehicles need high-wear-resistance, high-reliability ceramic balls and related components; semiconductor equipment needs high-purity, low-contamination, corrosion-resistant precision ceramic structural components. With continuous technological breakthroughs, there is broad space for the high-end industrialization and domestic substitution of silicon nitride ceramics, and huge future development potential.