Silicon nitride ceramics have excellent properties such as high strength, high toughness, corrosion resistance, high temperature resistance, oxidation resistance, low specific gravity and thermal shock resistance, and have good development prospects. In addition, silicon nitride ceramics have relatively high theoretical thermal conductivity, which makes them considered as a potential heat dissipation and packaging material for high-speed circuits and high-power devices.
Preparation of high thermal conductivity silicon nitride ceramics
Selection of raw powder
Silicon nitride has two crystal types: α-Si3N4 and β-Si3N4. The α phase is in an unstable state at high temperature and is easily transformed into a high temperature stable β phase. The study found that as the content of β phase in silicon nitride ceramics gradually increases in the range of 40%-100%, the thermal conductivity of silicon nitride ceramics increases linearly, so high purity β phase is the key to obtaining high thermal conductivity silicon nitride ceramics factor. Both α-Si3N4 and β-Si3N4 powder can be used as raw materials for preparing β-Si3N4 ceramics.
Using α-Si3N4 powder as the raw material, during the sintering process, the α→β phase transition is promoted through the dissolution and precipitation mechanism. The sintering driving force is relatively high, and high β phase silicon nitride ceramics can be obtained. However, pure β-phase silicon nitride ceramics can be obtained by using β-phase as the raw material, but there is no phase change during the sintering process, the driving force is small, and sintering is relatively difficult, and because Si3N4 is prone to decompose above 1800°C, in order to ensure compact sintering , Air pressure sintering is often used to increase the driving force of sintering and its decomposition temperature, so the production cost is increased more.
Selection of sintering aids
Silicon nitride ceramics are compounds with strong covalent bonds, which are difficult to sinter and compact by solid phase diffusion. Sintering aids such as MgO, Al2O3, CaO and rare earth oxides must be added. During the sintering process, the added sintering aids can be combined with The primary oxide on the surface of the silicon nitride powder reacts to form a low-melting eutectic melt, and the liquid phase sintering mechanism is used to achieve densification.
However, the grain boundary phase formed by the sintering aid has a low thermal conductivity, which has an adverse effect on the thermal conductivity of silicon nitride ceramics. For example, the commonly used Al2O3 sintering aid for silicon nitride ceramics will interact with nitride at high temperatures. Silicon and its surface oxide form a SiAlON solid solution, which causes distortion of the crystal lattice near the grain boundary and hinders the heat transfer of phonons, thereby greatly reducing the thermal conductivity of silicon nitride ceramics. Therefore, selecting a suitable sintering aid and formulating a reasonable formula system are the key ways to improve the thermal conductivity of silicon nitride.
Oxide sintering aid is a commonly used sintering aid system for silicon nitride ceramics. The most common is the combination of metal oxides and rare earth oxides. The sintering aid of Y2O3-MgO system is a high thermal conductivity silicon nitride material. It is widely used In addition, Yb2O3 is also a common rare earth oxide sintering aid system. In addition to the commonly used oxide sintering aids, in recent years, a research hotspot in the preparation of silicon nitride ceramics, especially high thermal conductivity silicon nitride ceramics, is the study of non-oxide sintering aids.
The advantage of non-oxide sintering aids is that they can reduce the extra oxygen introduced, which is of great significance for purifying the silicon nitride crystal lattice, reducing the glass phase of the grain boundary, and improving the thermal conductivity and high temperature performance. In addition to the research on rare earth oxides being replaced by rare earth non-oxides as sintering aids, there are also some studies using Mg non-oxides instead of MgO as sintering aids in order to reduce the oxygen content of the crystal lattice and increase the thermal conductivity. . However, non-oxide sintering aids also have problems such as rare raw materials, high cost, high sintering difficulty, and high conditions. Therefore, non-oxide sintering aids have not been widely used in the mass preparation of high thermal conductivity silicon nitride materials.
Selection of sintering method
At present, the main sintering methods used for sintering silicon nitride ceramics include hot pressing sintering, air pressure sintering, and spark plasma sintering. These sintering methods have their own advantages in the sintering of silicon nitride ceramics. The spark plasma sintering method is very fast. It only takes about 1 hour to cool from sintering. It is very suitable for rapid sintering and is helpful for studying the sintering characteristics of ceramics. The advantage of gas pressure sintering is that the sintering cost is low and it can prepare products with more complex shapes. Enable mass production. For the hot-pressing sintering method, the driving force of sintering is greatly improved due to the external mechanical pressure. It is a very effective densification for covalent compound ceramics that are difficult to sinter. Sintering technology.
Application of silicon nitride ceramics in electronic packaging substrates
Modern microelectronics technology is developing very rapidly, and electronic systems and equipment are developing in the direction of large-scale integration, miniaturization, high efficiency, and high reliability. The increase in electronic system integration will lead to increased power density and increased heat generated by the overall operation of electronic components and systems. Therefore, effective electronic packaging must solve the heat dissipation problem of the electronic system, and the thermal conductivity of the substrate material in the electronic packaging will affect The key to heat dissipation of the entire electronic system. Silicon nitride ceramics is a structural ceramic material with the best comprehensive performance. The theoretical thermal conductivity of single crystal silicon nitride can reach 400W/(m·k), which has the potential to become a high thermal conductivity substrate. In addition, the thermal expansion coefficient of Si3N4 is about 3.0×10-6/℃, which is well matched with materials such as Si, SiC and GaAs, which makes Si3N4 an attractive substrate material with high strength and high thermal conductivity for electronic devices.
Comparison of the performance of three ceramic substrate materials
Compared with other ceramic materials, silicon nitride ceramics have many excellent characteristics, such as higher theoretical thermal conductivity, good chemical stability, non-toxicity, higher bending strength and fracture toughness. In the current research reports on high thermal conductivity silicon nitride ceramics, the maximum thermal conductivity can reach 177W·m-1·K-1, and the mechanical properties are also excellent (bending strength reached 460MPa, fracture toughness reached 11.2MPa· m1/2), these characteristics make it considered as a potential heat dissipation packaging material for high-speed circuits and high-power devices.
The broad market prospects of Si3N4 ceramic substrates have attracted the attention of international ceramic companies. At present, the main international suppliers of high thermal conductivity silicon nitride ceramic substrates include Rogers Corporation of the United States and Toshiba Corporation of Japan, which produce high thermal conductivity silicon nitride ceramics. The thermal conductivity can reach 90W·m-1·K-1, and the flexural strength and fracture toughness can reach 650MPa and 6.5MPa·m1/2, respectively.