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Sintering process of silicon nitride ceramics

The current methods to improve the thermal conductivity of Si₃N₄ ceramics mainly include: (1) using high-purity Si3N4 powder or Si powder with lower oxygen content as raw material; (2) selecting effective non-oxide sintering aids; (3) optimizing the sintering process Or perform vacuum annealing heat treatment on the sample at high temperature. However, no matter which method is used, obtaining Si3N4 ceramics with thermal conductivity >100 W·m^-1·K^-1 often requires long-term sintering at high temperatures (such as 1900°C), resulting in abnormally grown β-Si3N4 crystals. Although particles can increase thermal conductivity, the sacrificed mechanical properties will cause Si3N4 ceramics to lose its advantages as a substrate material.

Effective sintering aid for silicon nitride ceramics

Due to the strong covalent bonding characteristics of the Si-N bond in Si₃N₄, sintering Si₃N₄ceramics requires the addition of a certain amount of sintering aids, which form a liquid phase with the SiO₂ film on the surface of the Si₃N₄ particles and a small amount of Si₃N₄ at high temperatures, which is achieved with the help of liquid phase sintering Densification.

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 reduce costs and produce high-purity MgSiN2 powder in large quantities through the combustion synthesis process. Therefore, the nitride sintering aid MgSiN2 shows great promise in preparing high thermal conductivity Si₃N₄ ceramics.

Reducing sintering aid

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. YH2, GdH2, YbH2 and ZrH2 were selected to form composite sintering aids with MgO respectively, and high thermal conductivity Si₃N₄ ceramics were prepared through two-step air pressure sintering. Under the action of ZrH2, SiO2 on the surface of Si3N4 powder is eliminated through the path of SiO2→ZrO2→SiO(g). Ultimately, it benefits from less glass phase content and more sufficient contact between grains. Thermal The conductivity is up to 116.4 W·m^-1·K^-1. In addition, by adding a small amount of silicon powder, Si3N4 ceramics with an obvious bimodal microstructure were prepared using two-step air pressure sintering and a new silicothermal reduction reaction. Compared with without adding silicon powder, the thermal conductivity increased from 90.03W·m^-1 ·K^-1 increased to 104.5 W·m^-1·K^-1, and the fracture toughness increased from 8.56 MPa·m^1/2 to 9.91 MPa·m^1/2.

Sintering process of silicon nitride ceramics

Reactive sintering-resintered sintering process (SRBSN)

The 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. Inspired by this, we used high-purity Si powder as the starting material and Y2O3-MgO as the sintering aid. By developing and improving the SRBSN process (M-SRBSN), we obtained a thermal conductivity as high as 182 W·m^-1·K^-1 Si3N4 ceramics, no one has surpassed it so far. The team used high-purity Si powder as raw material to prepare Si3N4 ceramics through reaction sintering-resintering. Compared with Si3N4 ceramics prepared through traditional air pressure sintering using α-Si3N4 powder as raw material, the former showed greater advantages in high thermal conductivity. .

Gas pressure sintering process (GPS)

Improving the GPS process is also an effective way to improve the thermal conductivity of Si₃N₄ ceramics. By studying the effect of pre-sintering temperature on particle rearrangement and α→β phase transformation during liquid phase sintering, a new two-step air pressure sintering method for preparing high-strength and high thermal conductivity Si3N4 ceramics was developed. 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 79.42 W·m-1·K-1 and a bending strength of 801 MPa.

In order to meet the performance requirements of third-generation semiconductor chip packaging for ceramic substrates, the preparation of high-strength and high-thermal conductivity Si3N4 ceramics that take into account both mechanical and thermal properties will be the focus of research for a long time to come. Although it has been successfully achieved to simultaneously achieve high thermal conductivity (>150 W·m^-1·K^-1) and high toughness (>10 MPa·m^1/2), the opposite trends in thermal conductivity and flexural strength remain unresolved. The next step is to increase the thermal conductivity of Si₃N₄ ceramics while sacrificing as little as possible the bending strength.