現在、SiCやGaNに代表される第3世代半導体の登場により、パワーモジュールは小型化、高電圧、高電流、高電力密度の方向に発展しており、使用中に発生する熱量が増加し、デバイスの放熱パッケージに対する要求が厳しくなっています。新世代の高出力モジュールでは、セラミック基板が 主にチップサポート、電気絶縁、熱伝導チャネルの役割を果たしており、窒化ケイ素セラミックは、高熱伝導率と高機械特性の利点により、大きな応用可能性を秘めた放熱基板材料となっています。
機械特性と熱伝導性の両方を備えた高性能のシリコン窒化物基板をいかに得る かは、今日の業界で最も懸念される課題の 1 つです。シリコン窒化物セラミック基板内の Si 原子と N 原子の拡散係数が低いため、相転移、粒子発達、緻密化は液相焼結によって達成する必要があります。したがって、適切な焼結添加剤を使用して焼結プロセスで液相と微細形態を調整することは、シリコン窒化物セラミックの機械特性と熱特性を改善する効果的な方法です。
非酸化物焼結助剤の種類
フッ化物
ケイ酸塩溶液では、フッ素原子がケイ酸塩ネットワークの構造を破壊し、分解させ、液相形成の温度と粘度を下げ、焼結の緻密化を促進します。例えば、焼結剤としてMgOの代わりにMgF2を使用すると、より低い温度で液相を形成でき、セラミックスの緻密化を加速し、MgF2の添加量が増えるにつれて粒径が大きくなり、窒化ケイ素セラミックスの熱伝導率を効果的に向上させることができます。また、LiF、希土類フッ化物、二元フッ化物などがあります。
Si3N4セラミックスの変位温度曲線は、焼結助剤としてMgF2とMgOを使用して作成された。
フッ化物焼結添加剤は、システムへの追加の酸素原子の導入を避け、液相中のSiO2の活性を低下させ、溶解沈殿プロセス中の格子酸素の形成を妨げます。同時に、フッ素原子エネルギーは液相の粘度を低下させ、低温で液相を形成するのを助け、大型β-Si3N4粒子の発達を促進し、調製された窒化ケイ素セラミックは格子酸素と熱伝導率の低い結晶間相含有量が低く、セラミックの熱伝導率が高くなります。ただし、SiF4の過度の揮発はセラミックの多孔性を高め、機械的特性と熱伝導率を低下させるため、適切な添加量を制御する必要があります。
窒化物および窒素含有化合物
Metal nitride has good compatibility with silicon nitride ceramics, and is often used as a sintering assistant to prepare silicon nitride ceramic materials, which can enhance the thermal conductivity of silicon nitride ceramics and strengthen its mechanical properties. MgSiN2, as a potential high thermal conductivity ceramic, has attracted much attention as a sintering aid for silicon nitride ceramics in recent years. At high temperature, SiO2 on the surface of MgSiN2 and Si3N4 powder will form Mg-Si-O-N liquid phase and promote densification. In addition, a portion of Si and N atoms are separated out in the form of Si3N4 to optimize grain boundaries. In recent years, by using MgSiN2 instead of MgO, researchers have prepared silicon nitride ceramics with high thermal conductivity and excellent mechanical properties at lower sintering temperature and shorter holding time. Li Jiangtao's team from the Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences prepared MgSiN2 in batches through self-propagating sintering, laying a material foundation for the large-scale application of MgSiN2. It is expected to be used as an efficient sintering aid for silicon nitride ceramics with high thermal conductivity.
The silicon nitride ceramics with low lattice oxygen content and high thermal conductivity were prepared by using MgSiN2 as sintering assistant
In addition, some researchers used solid phase reaction to synthesize a new non-oxide sintering agent Y2Si4N6C to replace Y2O3 as a sintering agent, which not only has low lattice oxygen content, but also weakened the scattering of phonons by intergranular phase and grain boundary film, which can greatly improve the thermal conductivity of silicon nitride ceramic substrate. However, the preparation process of Y2Si4N6C is complex, and it cannot be synthesized in large quantities for the time being, which limits its application.
Borides
The densified Si3N4 ceramics with LaB6 as the sintering agent do not introduce additional oxygen into the system, and can remove lattice oxygen through the dissolution precipitation process, improve the thermal conductivity of β-Si3N4 grains, and at the same time, the content of low thermal conductivity intercrystalline phases is less, the grain size is larger, and the phonon scattering between grains is weakened. This is because during the sintering process, B atoms enter the glass network, and the formed [BO3]- structural unit will replace the [SiO4]- structural unit in the original network, destroying the integrity of the glass network, reducing the liquid phase viscosity, and thus promoting low temperature sintering.
Silicides
It has been found that iron silicide (FeSix) has a certain regulatory effect on the phase transition and grain growth of silicon nitride ceramics. For example, FeSi2 can generate β-Si3N4 phase before the α-Si3N4 phase transition, providing nucleation and growth points for the later α-Si3N4 phase transition. It is helpful to regulate the phase transition and grain growth process in the sintering of silicon nitride ceramics. ZrSi2 can react with SiO2 on the surface of silicon nitride powder to produce ZrO2 and β-Si3N4 crystal seeds. In situ ZrO2 and MgO assistant form a low temperature eutectic liquid phase, which promotes ceramic densification and β-Si3N4 grain development through dissolution precipitation mechanism. Due to the consumption of SiO2 by ZrSi2, the oxygen content in the liquid phase is reduced, thereby impeding the generation of lattice oxygen dynamically and reducing lattice defects. In addition, Zr elements in the sintered body are precipitated in the form of ZrN (or ZrO2) phase, and there is no obvious amorphous grain boundary film between Si3N4 grains, which reduces the scattering of phonons at the grain boundaries. Thanks to the above three factors, the thermal conductivity of Si3N4 has been greatly improved. However, the difficulty of both thermal conductivity and mechanical properties limits the application of ZrSi2 as a sintering aid for high-conductivity silicon nitride ceramics.
Schematic diagram of densification mechanism of silicon nitride ceramics containing ZrSi2-MgO additive
Hydrides and Metal Particles
Metal hydride is a commonly used oxygen consumption agent in powder metallurgy industry, which is decomposed into metal elements and H2 at high temperatures, H2 can remove the oxide layer on the surface of metal particles, and the highly active metal elements generated by decomposition play the role of absorbing impurity oxygen in the metal matrix, which can effectively improve the performance of metal products. Rare earth hydrides such as YH2 can reduce the activity of SiO2 in the liquid phase and facilitate the removal of lattice oxygen during dissolution precipitation. In addition, the "nitrogen-rich" liquid phase formed by the addition of YH2 is also conducive to the nucleation and development of β-Si3N4, and the grain size is significantly larger than that of Y2O3 additive system. However, excessive hydride makes the liquid viscosity too high, inhibits the densification process, and the fully developed β-Si3N4 grains cross to form a porous skeleton, which can not prepare high density silicon nitride ceramics. Therefore, it is still necessary to determine the optimal amount of rare earth hydride according to the oxygen content of α-Si3N4 raw material powder.
Ternary Layered Compound
The fracture toughness can be improved by introducing layered compounds into silicon nitride ceramic matrix through crack deflection, bridge and other mechanisms. In recent years, researchers have investigated the effect of layered compounds on the thermal conductivity of silicon nitride ceramics, and found that layered compounds can effectively improve the mechanical thermal properties of ceramics. YB2C2 can react with SiO2 on the surface of silicon nitride powder to reduce the liquid phase oxygen content and promote densification. The remaining YB2C2 layer improves the bending strength and fracture toughness of the ceramics through crack deflection mechanism.
Carbon, Silicon Sintering Additives
Carbon is widely used to remove oxygen impurities from ores because of its strong reducibility. In the study of silicon nitride, a small amount of carbon can promote α→β phase transition in silicon nitride sintering. Carbon thermal reduction deoxygenation can adjust the composition and properties of the liquid phase, and then regulate the relative rate of phase transition and densification, so that the silicon nitride ceramics with good morphology can be obtained without adding β-Si3N4 seed.
(a, c) Sample microstructure after nitriding without addition and (b, d) sample microstructure
after nitriding with buried powder containing C (a, b) and microstructure of silicon nitride after air
It is worth noting that the introduction of C needs to be controlled precisely in the sintering system of silicon nitride ceramics, the addition amount is too small, and the control effect of liquid phase is not good. Excessive addition will lead to residual SiC in the sample, which will adversely affect the density and electrical properties of silicon nitride ceramics. The results show that Si can also remove the surface oxide layer by silicothermal reduction reaction with SiO2. Unlike toner, which requires precise control of the amount of addition, excess Si is nitrided to Si3N4 in a nitrogen atmosphere, without the formation of harmful by-products.
The research of optimizing raw material powder of silicon nitride by carbothermal reduction and silicothermal reduction, and improving the performance of silicon nitride ceramics by means of liquid phase regulation provides a solution for the preparation of high thermal conductivity silicon nitride ceramics by using low-cost silicon nitride powders with high oxygen content.
格子酸素含有量の低い窒化ケイ素粉末がまだブレークスルーを達成していないという背景の下で、対応する酸化物焼結添加剤の代わりに非酸化物を使用し、液相組成を調整することにより、窒化ケイ素セラミックスの熱伝導率を向上させることが経済的で効果的な方法です。窒化ケイ素原料粉末の継続的な最適化、新しい多機能焼結添加剤の継続的な開発、成形および焼結プロセスの継続的な改善により、高強度で高熱のSi3N4基板の大規模生産が現実のものとなり、パワー半導体デバイスの開発に強力なサポートを提供します。
