Aluminum nitride is a kind of ceramic material with excellent comprehensive properties. It was synthesized artificially for the first time in 1877, but it has no practical application in the following 100 years, and it is only used as a nitrogen fixer for fertilizers.
Aluminum nitride is a covalent compound with a small self-diffusion coefficient and a high melting point, so it is difficult to sinter.
With the deepening of research, the production process of aluminum nitride is becoming mature, and its application scope is also expanding.
Since the beginning of the 21st century, with the rapid development of microelectronics technology, electronic complete machines and electronic components are developing in the direction of miniaturization, light weight, integration, high reliability and high power output, and more and more complex devices, higher requirements are placed on the heat dissipation of substrates and packaging materials, which further promotes the development of the aluminum nitride industry.
Aluminum Nitride Properties
Aluminum nitride (AlN) is a covalent bond compound with a hexagonal wurtzite structure with lattice parameters a=3.114 and c=4.986.
Pure aluminum nitride is blue-white, usually gray or off-white, and is a typical third-generation wide-bandgap semiconductor material.
Aluminum nitride (AlN) has the characteristics of high strength, high volume resistivity, high insulation withstand voltage, thermal expansion coefficient, and good matching with silicon.
In the field of ceramic electronic substrates and packaging materials, its performance far better than that of alumina.
Thermal conductivity W/(m·K): 320 (theoretical value)
Thermal expansion coefficient 10^-5／℃: 4.5
Insulation properties: Room temperature resistance 10^14 Ω·cm / Breakdown field strength 11.7 x 10^6 V/cm
Dielectric Constant: 8.8
Band gap: 6.2eV
Mechanical properties at room temperature: Hardness 12GPa, elastic modulus 314GPa / Flexural strength 300~400 MPa
High temperature mechanical properties: 20% lower at 1300℃ (compared to properties at room temperature )
Others: Non-toxic, high temperature corrosion resistance, normal pressure decomposition temperature 2000~2450℃
Compared with several other ceramic materials, aluminum nitride ceramics have excellent comprehensive properties, are very suitable for semiconductor substrates and structural packaging materials, and have great application potential in the electronics industry.
Thermal Conduction Mechanisms
The most notable property of aluminum nitride is its high thermal conductivity.
The main thermal conduction mechanism of aluminum nitride: Heat transfer through lattice or lattice vibration, that is, by means of lattice waves or heat waves.
Theoretically, the thermal conductivity of AlN can reach 320 W/(m·K), but the thermal conductivity of the actual product is less than 200 W/(m·K) due to impurities and defects in AlN. This is because the structural elements in the crystal cannot be completely uniformly distributed, and there are always sparse and dense different regions, so the carrier phonons will always be disturbed and scattered during the propagation process.
Applications of Aluminum Nitride
High Power Electronics
Aluminum nitride has high resistivity, high thermal conductivity (8-10 times that of Al2O3), and a low expansion coefficient similar to that of silicon. It is an ideal material for high temperature and high power electronic devices.
Electronic Packaging Substrate
Commonly used ceramic substrate materials include beryllium oxide, aluminum oxide, aluminum nitride, etc. Among them, the thermal conductivity of aluminum oxide ceramic substrate is low, and the thermal expansion coefficient does not match that of silicon; Although beryllium oxide has excellent properties, its powder is highly toxic.
Among the existing ceramic materials that can be used as substrate materials, silicon nitride ceramics have the highest flexural strength and good wear resistance, and are the ceramic materials with the best comprehensive mechanical properties, while their thermal expansion coefficient is the smallest. Aluminum nitride ceramics have high thermal conductivity, good thermal shock resistance, and still have good mechanical properties at high temperatures.
From a performance point of view, aluminum nitride and silicon nitride are currently the most suitable materials for electronic packaging substrates, but their disadvantage is the high cost.
The maximum width of the direct band gap of aluminum nitride is 6.2 eV, which has higher photoelectric conversion efficiency than indirect band gap semiconductors. As an important blue and ultraviolet light-emitting material, AlN is used in ultraviolet/deep ultraviolet light-emitting diodes, ultraviolet laser diodes, and ultraviolet detectors.
In addition, AlN can form a continuous solid solution with group III nitrides such as GaN and InN, and its ternary or quaternary alloys can achieve continuous tunability of its band gap from the visible band to the deep ultraviolet band, making it an important high-performance luminescent material.
AlN crystals are ideal substrates for GaN, AlGaN and AlN epitaxial materials.
Compared with sapphire or SiC substrates, AlN has better thermal matching and chemical compatibility with GaN, and less stress between the substrate and the epitaxial layer.
Therefore, when AlN crystal is used as a GaN epitaxial substrate, the defect density in the device can be greatly reduced, the performance of the device can be improved, and it has a good application prospect in the production of high-temperature, high-frequency, and high-power electronic devices.
In addition, using AlN crystal as the AlGaN epitaxial material substrate with high alumina (Al) composition can also effectively reduce the defect density in the nitride epitaxial layer, and greatly improve the performance and service life of the nitride semiconductor device.
Ceramics and Refractories
Aluminum nitride can be used as a structural ceramic.
Aluminum nitride ceramics have good mechanical properties, higher flexural strength than Al2O3 and BeO ceramics, high hardness, and high temperature corrosion resistance. Therefore, AlN ceramics can be used to make high temperature corrosion resistant parts such as crucibles and Al evaporating dishes.
Pure AlN ceramics are colorless and transparent crystals with excellent optical properties, which can be used to manufacture high-temperature infrared windows for electronic optical device equipment and heat-resistant coatings for fairings.
The epoxy resin/AlN composite material is used as a packaging material and needs good thermal conductivity and heat dissipation. As a polymer material with good chemical properties and mechanical stability, epoxy resin is easy to cure and has low shrinkage, but its thermal conductivity is not high. Adding AlN nanoparticles with excellent thermal conductivity into epoxy resin can effectively improve the thermal conductivity and strength of composites.