Aluminium Nitride: Key Properties and Applications

Aluminium nitride (AlN) is a non-oxide technical ceramic. It is well known for its combination of very high thermal conductivity, resistance to high temperatures and as an electrical insulator. For this reason, it is often used as a heat sink or as a substrate for electronics where a high thermal conductivity is required, for example for LEDs and semiconductors.

The most important properties and applications of aluminium nitride are outlined below.

Properties of aluminium nitride

Aluminium nitride’s functionality comes from its combination of thermal, electrical and mechanical properties.

Structural properties

Aluminium nitride has the chemical formula AlN. It is a covalently bonded inorganic compound with a hexagonal wurtzite crystal structure. It has a density of 3.3 g/cm³ and a molar mass of 40.99 g/mol.

Thermal properties

In contrast to most ceramics, aluminium nitride has a very high thermal conductivity. In fact, AlN has one of the highest thermal conductivities of any ceramic, surpassed only by beryllium oxide. For monocrystalline AlN, this value can reach as high as 285 W/(m·K). For polycrystalline material, however, a value in the range of 70–210 W/(m·K) is more common.

The high thermal conductivity of aluminium nitride is a result of its low molar mass (40.99 g/mol compared to 101.96 g/mol for alumina, Al2O3), strong bonding and relatively simple crystal structure. More of the properties of aluminium nitride are compared to other similar technical ceramics below.

Aluminium nitride exhibits a coefficient of thermal expansion of 4.8✕10^-6 1/K at 20 °C. This is similar enough to that of silicon (3.5✕10^-6 1/K at 20 °C) that AlN is often used as a substrate material for silicon processing.

Additional properties of aluminium nitride relevant for use at elevated temperatures are high thermal shock resistance and resistance to corrosion from molten metals, chemicals and plasmas at high temperatures.

Aluminium nitride has a melting temperature of 2200 °C and a boiling point of 2517 °C.

Electrical properties

Similar to other ceramics, AlN has a very high electrical resistivity, in the range of 10-16 Ω⋅m. This makes it an electrical insulator.

AlN also has a relatively high dielectric constant of 8.8-8.9 (for pure AlN), which is a similar value to that of Al₂O₃, but much lower than SiC. 

The breakdown electric field of AlN is 1.2–1.8 x 10⁶ V cm^-1. 

AlN also displays piezoelectricity, which is useful in thin film applications. 

Mechanical properties

AlN has a flexural strength of 350 MPa at 20 °C, which is the same as Al₂O₃ and just lower than that of SiC. 

It has a high Young’s modulus of 343 GPa, compared to in the 200 GPa range of steel. This value is just lower than that of Al₂O₃. 

AlN also has a high Vickers hardness of around 1000 at 20 °C. 

Aluminium nitride compared to other common ceramics

Below is a comparison table of some of the relevant properties of some popular technical ceramics, including aluminium nitride.

Property	AlN	SiC	BeO	Al2O3
Thermal conductivity W/(m·K) at 20 °C	170–210	150	250–300	20–30
Coefficient of thermal expansion at 20 °C	4.3–4.8	3.7	7.5–8.0	6.7–7.3
Dielectric constant	8.8–8.9	40–42	6.5–6.7	8.5–8.9
Electrical resistivity (Ωcm)	>1014	>1014	>1014	>1014
Density (g/cm3)	3.26	3.21	3.01	3.97
Flexural strength (MPa)	350	370	240	350
Young’s modulus (GPa)	343	460	378	378

Applications of aluminium nitride

The high temperatures present in semiconductor manufacturing processes combined with the sensitivity of the devices under fabrication make aluminium nitride particularly useful as a substrate material. Its high thermal conductivity allows it to act as an excellent heat sink while remaining electrically insulating and not fracturing under high temperatures. In addition, it has a very similar coefficient of thermal expansion to silicon. 

AlN is a common alternative to beryllium oxide in similar applications, as it is not a health hazard when machined. 

As a result of its high resistance to corrosion at high temperatures, AlN is also used as a crucible material for molten metals. 

In the form of thin films, AlN has a number of applications. As a result of its piezoelectrical properties, it is used for surface acoustic wave (SAW) sensors. It is also used for thin film bar acoustic resonators (FBAR), a microelectromechanical system (MEMS) device used for RF filters in mobile phones. Research is also underway to develop gallium-aluminium nitride-based light-emitting diodes. 

Some other common applications of AlN include:

• Heat spreaders
• Packages for microwave devices
• Molten metal fixtures
• Components for laser heat management
• Electrical insulation components for lasers
• Substrates for electronic packages
• Substrates for microelectronic & optoelectronic devices
• Insulators for microelectronic & optoelectronic devices
• Chip carriers for sensors and detectors
• Chiplets
• Collets
• Silicon wafer handling and processing
• Electrical insulators
• Chucks and clamp rings for semiconductor processing equipment
• Steel manufacturing equipment

^{Aluminium nitride parts. Credit: Innovacera}

Processing of aluminium nitride

Aluminium nitride is a non-naturally occurring material and so must be chemically synthesized. It is formed via one of two processes. This first is the carbothermal reduction of aluminium oxide, Al₂O₃, in the presence of nitrogen gas or ammonia at a temperature of 1400–1700 °C. The reaction involving nitrogen is as follows:

1. Al₂O₃ + C → Al₂O₂ + CO
2. Al₂O₂ + 2C + N₂ → 2AlN + 2CO

The second synthesis process is the direct nitridation of aluminium.

AlN parts can be formed by a number of routes, including:

• Ceramic injection moulding
• Low-pressure injection moulding
• Cold isotactic press
• Dry press
• Tape casting
• Precision machining

Following synthesis, and the formation of parts, the material is usually sintered. Sintering aids such as calcium oxide, CaO or yttrium oxide, Y₂O₃ are often added to enhance is sintering densification process.

Sourcing aluminium nitride

For the processing of advanced products such as semiconductors, high-quality materials are a necessity. Material selection is a fundamental step, and finding the right material is no easy task. Luckily, material suppliers, such as Xiamen Innovacera Advanced Materials, provide a broad spectrum of high-quality technical ceramic materials, including aluminium nitride. They have listed their materials here on Matmatch to ensure that engineers, researchers, and product developers can easily find their desired materials and reach out to them directly.

Visit the Xiamen Innovacera Advanced Materials supplier page for more information on their materials and to get in direct contact with them.