Silicon nitride is recognized at home and abroad as the best ceramic substrate material with high thermal conductivity, high reliability and other comprehensive properties. The AMB ceramic substrate made of silicon nitride ceramic substrate has a thermal expansion coefficient closer to that of the third-generation semiconductor substrate SiC crystal material, and the match is more stable. This makes Si3N4 a highly thermally conductive substrate material for the third-generation SiC semiconductor power device. first choice. Domestic and foreign companies are also gradually increasing investment in silicon nitride substrates. However, for silicon nitride substrates, sustained and stable mass production is still an industry problem. The following will briefly analyze and discuss its molding and sintering processes. 

Forming of Silicon Nitride Ceramic Substrate In order to prepare anisotropic ceramic materials, the arrangement and growth of grains can be controlled through molding methods to ensure that the grains can produce better orientation effects in one or two dimensions. At present, the molding methods of high thermal conductivity silicon nitride substrates include tape casting, film rolling, pouring and injection molding, among which tape casting is recognized as the most suitable engineering preparation technology.

 ① Tape casting 

The tape casting process includes casting and drying processes. The degassing tank is connected to the feed pipe of the casting machine, and the slurry is pressurized by nitrogen to enter the casting machine, spread into a uniform film on the film belt, and transported to the drying tunnel via the conveyor belt. The hot air generated by the electric heating The slurry is dried at a heating temperature of about 150~180°C. During the drying process, some organic matter with a lower boiling point is volatilized and removed from the slurry to obtain a dry substrate.

▲The process route of tape casting 

In silicon nitride ceramics prepared by tape casting, the Si3N4 grain arrangement has a certain orientation. Thermal conductivity is also different in different directions, and has higher thermal conductivity in specific directions. 

② Casting molding 

The grouting molding method is also called pouring molding. The prepared ceramic slurry is injected into the plaster model. Since the plaster model has breathable and water-absorbing properties, after the slurry contacts the model, the moisture in the slurry will gradually be sucked into the model wall. The fine particles in the material will be evenly arranged into a thick mud layer according to the shape of the model. When the thick mud layer reaches the expected thickness, the excess slurry in the model can be poured out. After the moisture in the thick mud layer continues to be absorbed by the model and independent molding is achieved, the green body can be taken out and dried for repair. 

③ Injection molding 

Injection molding is also called hot die casting. This technology adds a certain amount of polymer and additive components and slightly heats it to give metal powder, ceramic powder and polymer similar fluidity, and fills the slurry into the metal mold under pressure. , remove the blank after cooling to obtain the blank. It is listed as an important "national key technology" by developed countries such as the United States.

▲Ceramic injection molding process route

The injection molding method has high raw material utilization rate and can be quickly and automatically mass-produced; it can prepare special-shaped parts with small size, complex shape and high dimensional accuracy; due to the flow die, the density of the green body is uniform and the performance of the sintered product is superior; the production cost is low.

Sintering process of high thermal conductivity silicon nitride ceramics

The main sintering methods for preparing high thermal conductivity Si3N4 ceramic materials include hot isostatic pressing sintering (HIP), hot press sintering (HPS), reaction sintering-resintering sintering (SRBSN) and gas pressure sintering (GPS). Early research mostly used hot isostatic pressing sintering methods, but hot isostatic pressing sintering has problems such as expensive equipment, complex operations, and high preparation costs. At present, engineering preparation mainly adopts two methods: air pressure sintering and reaction sintering-resintering sintering.

① Reactive sintering-resintered sintering process (SRBSN)

Oxygen impurity content is the most important factor affecting the thermal conductivity of Si3N4 ceramics. Even the most pure commercial Si3N4 powder contains oxygen impurities with a mass fraction of more than 1%. Thanks to the progress of the modern semiconductor industry, the oxygen content of high-purity Si powder The content of impurities and metal impurities is significantly lower than that of Si3N4 powder.

② Gas pressure sintering process (GPS)

Improving the GPS process is also an effective way to improve the thermal conductivity of Si3N4 ceramics. The research group of Xi'an Jiaotong University developed a new two-step air pressure sintering method for preparing high-strength and high thermal conductivity Si3N4 ceramics by studying the effect of pre-sintering temperature on particle rearrangement and α→β phase transformation during liquid phase sintering. When the pre-sintering temperature is 1525°C, thanks to the optimized particle rearrangement and appropriate α→β phase transition rate, the Si3N4 ceramic is almost completely dense after the second step of high-temperature sintering at 1850°C, forming a prominent double-peak shape. microstructure, while achieving optimal comprehensive performance, with a thermal conductivity of 79.42W·m-1·K-1 and a bending strength of 801MPa.

① Nitride sintering aid

Nitrides that can be used as Si3N4 sintering aids include: VN, YN, Mg3N2, AlN, Ca3N2 and MgSiN2, etc. Among them, MgSiN2 can not only effectively lower the melting point of silicate glass, but also does not introduce excess oxygen impurities. More importantly, it is now possible to produce high-purity MgSiN2 powder in large quantities through the combustion synthesis process to reduce costs. Therefore, the nitride sintering aid MgSiN2 shows great promise in preparing high thermal conductivity Si3N4 ceramics.

② Reducing sintering aids

Add reducing additives such as metal hydride, a small amount of silicon powder or carbon powder, and reduce the oxygen content with the help of metal hydride reduction reaction, silicon thermal reduction reaction or carbothermal reduction reaction, and increase the N/O ratio of the intergranular second phase. It can promote the abnormal growth of β-Si3N4 grains and reduce the lattice oxygen content, thereby effectively improving the thermal conductivity of Si3N4 ceramics.

③ Ternary composite sintering aid

Using non-oxides instead of oxides as sintering aids can reduce the oxygen content in the liquid phase, thereby reducing the lattice oxygen content of Si3N4. However, the increase in the N/O atomic ratio in the liquid phase will inhibit densification to a certain extent, which often requires Solving the problem by increasing the sintering temperature or extending the holding time will undoubtedly increase the preparation cost, and abnormal grain growth will also harm the mechanical properties. Therefore, researchers are trying to prepare Si3N4 ceramics with both thermal and mechanical properties at low cost through low-temperature and low-pressure or low-temperature pressure-free sintering, and the use of ternary sintering aids has become the focus of attention.