Electronic equipment is evolving quickly in the direction of miniaturization, thinning, integration, high power, and high frequency with the growth of modern industry. Heat dissipation performance has also developed into a key indicator for assessing product service life and reliability. Due to the semiconductor power module's high level of integration and significantly increased heat generation, clear requirements for the equipment's heat dissipation capacity are presented. The main factor affecting the device's ability to dissipate heat is its thermal conductivity because it is a ceramic insulating substrate packed with semiconductor components.
Three Commonly-Used Ceramic Substrates
Alumina Ceramic Substrate
Low dielectric loss, outstanding chemical stability, and high mechanical strength are advantages. But it is u nable to meet the demands of the growing electronic components in integrated circuits due to the low thermal conductivity.
Aluminum Nitride Ceramic Substrate
Benefits include excellent insulation and a high thermal conductivity that is 8 times greater than alumina ceramics. In addition to having poor mechanical properties and being a high-strength, hard, and brittle material that is easy to damage in complex service environments, this material also has some challenges with powder transportation and high production costs.
Silicon Nitride Ceramic Substrate
High strength, high hardness, high electrical resistivity, good thermal shock resistance, low dielectric loss, and low expansion coefficient are all advantages of this material. But it is challenging to meet the demands of thermal conductivity and mechanical properties at the same time because the actual thermal conductivity of silicon nitride ceramics is much lower than the theoretical thermal conductivity. Some silicon nitride ceramics with high thermal conductivity (>150 W/(m.K)) are still in the laboratory stage.
In terms of overall performance, silicon nitride outperforms other ceramic materials. It is also a substrate material that is ideal for heat dissipation. Alumina and aluminum nitride ceramic substrates have currently reached industrialization in the domestic market, but silicon nitride ceramics with high thermal conductivity continue to pose a significant challenge as substrate materials for commercial electronic devices.
Why Lattice Oxygen Affects Silicon Nitride Ceramics
Since silicon nitride is a covalent compound with bound electrons, heat conduction can only occur through lattice vibration. Phonon transport predominates in the lattice vibration. Because lattice vibration is nonlinear, there is a certain coupling effect between lattices, and phonon collisions reduce the mean free path of phonons. Additionally, different flaws, impurities, and grain boundaries in Si3N4 crystals will result in phonon scattering, which will further cause a decrease in the mean free path of phonons and lower thermal conductivity.
Lattice oxygen is one of the most essential lattice defects affecting the thermal conductivity of silicon nitride ceramics. The oxygen atoms in the silicon nitride powder undergo a solid solution reaction in the form of silicon dioxide during the sintering process. As a result of the reaction, silicon vacancies are created. Atomic substitution will lead to crystal distortions that will scatter phonons, decreasing the thermal conductivity of Si3N4 crystals.
The hot gas extraction method was used to measure the oxygen content of the crystal lattice. The results showed that grain growth can successfully lower the crystal lattice's oxygen content, significantly enhancing the thermal conductivity. Large-sized grains are therefore necessary for the production of silicon nitride ceramics with high thermal conductivity.
Methods to Reduce Lattice Oxygen Content of Silicon Nitride Ceramics
Raw Material Powder Selection
Currently, there are two methods for reducing lattice oxygen from raw material powder: one is to use silicon nitride powder (α-Si3N4 powder orβ-Si3N4 powder) directly for sintering, and the other is to use high-purity silicon powder, in order to make silicon nitride ceramics with a high density and high thermal conductivity that using the two-step process, which involves nitriding silicon powder and re-sinter.
Among them, α-Si3N4 is made by directly nitriding silicon powder, has a relatively high oxygen content, and produces silicon nitride ceramic with a low thermal conductivity. Although silicon nitride ceramics can be made from high-purity, low-oxygen α-Si3N4 powder created by the liquid-phase reaction of SiCl4 and NH3, using high-purity raw materials will increase costs, which is not ideal for mass production.
The oxygen content of β-Si3N4 is low, and the prepared silicon nitride ceramics have a high thermal conductivity. However, t he rod-shaped grains of β-Si3N4 will prevent grain rearrangement, making it difficult to densify and resulting in poor sintering activity. By utilizing special sintering techniques and lengthening the holding time, the sintering activity can be improved, but the technical difficulty and cost will rise as well.
The production of high-purity silicon powder has very mature technology thanks to the development of the modern semiconductor industry. The sample made of Si powder as the raw material can achieve high thermal conductivity, but it is vulnerable to oxidation during the grinding process, and the experimental process is laborious and time-consuming, which is unsuitable for industrial production.
Sintering Aids Selection
Si3N4 is a covalent compound with a low coefficient of self-diffusion. During the sintering process, self-diffusion struggles to form a dense crystal structure. As a result, high thermal conductivity can be achieved by adding the right sintering aids and optimizing their ratio. The various silicon nitride sintering agents available today can be roughly categorized into oxides, non-oxides, and carbon reduction.
MgO and Al2O3 are both sintering aids that can enhance sintering performance, but Al2O3's absorption into the Si3N4 grains will impede grain growth, lowering thermal conductivity. Therefore, silicon nitride's thermal conductivity can be increased by selecting MgO with a larger particle size.
Non-oxide sintering aids are frequently used to create silicon nitride ceramics with higher thermal conductivity and better performance by further lowering the lattice oxygen content and raising the nitrogen-oxygen ratio. Non-oxide sintering aids are superior to oxide sintering aids in that they can add more nitrogen atoms, raise the nitrogen-to-oxygen ratio, encourage crystallization, and reduce SiO2 to lower the oxygen content of the lattice and the grain boundary phases.
Additionally, carbon is frequently used as a sintering additive, and its primary purpose is to remove oxide impurities from the surface of ceramic powders that don't contain oxide.By significantly lowering the amount of lattice oxygen, the addition of carbon increases the thermal conductivity of silicon nitride ceramics. Furthermore, the preparation cost is low, and there are few requirements placed on the oxygen content of the powder and sintering aids. This approach is anticipated to be extensively used in actual industrial production as technology advances.
Conclusion
Si3N4 ceramic substrates have entered a new era of rapid growth with the widespread adoption of third-generation SiC-based semiconductor chips across the new energy vehicles, telecommunications (5G), and the new energy sectors. However, silicon nitride ceramics' actual thermal conductivity is less than anticipated due to the presence of lattice oxygen. As a result, in order to prepare silicon nitride substrates with high thermal conductivity, the lattice oxygen content of silicon nitride ceramics must be reduced.