Jul 26 2001
AlN was first synthesised in 1877 but it was not until the mid 1980s that its potential for application in microelectronics spurred development of high quality commercially viable material.
AlN is synthesised by carbothermal reduction of alumina or by direct nitridation of aluminium. It has a density of 3.26 g.cm-3 and although it does not melt, it dissociates above 2500 °C at atmospheric pressure. The material is covalently bonded and is resistant to sintering without the assistance of liquid forming additives. Typically oxides such as Y2O3 or CaO allow sintering to be achieved at temperatures between 1600 - 1900 °C.
Key Properties
- AlN is resistant to attack by most molten metals, most notably aluminium, lithium and copper
- It is resistant to attack from most molten salts including chlorides and cryolite
- High thermal conductivity for a ceramic material (second only to beryllia)
- High volume resistivity
- High dielectric strength
- It is attacked by acids and alkalis
- In the powder form it is susceptible to hydrolysis by water or humidity
Table 1. Typical Physical and Mechanical Properties of Aluminium Nitride
| Property |
Value |
| Density (g.cm-3) |
3.32 |
| Modulus of rupture (MPa) |
300 - 350 |
| Modulus of elasticity (GPa) |
310 |
| Fracture toughness (MPa.m-1/2) |
3.35 |
| Coeff of thermal expansion RT-1000 °C (x10-6 K-1) |
5.6 |
| Thermal conductivity (W/m.K) |
140 - 177 |
| Specific Heat (J.kg.K-1) |
780 |
| Volume resistivity (ohm.cm) |
1010 |
| Dielectric Strength (kV.mm-1) |
>20 |
| Dielectric constant |
8.6 |
| Loss tangent at 1 MHz |
5x10-4 |
Applications
Microelectronics
The most remarkable property exhibited by AlN is its high thermal conductivity - in ceramic materials second only to beryllia. At moderate temperatures (~200 °C) its thermal conductivity exceeds that of copper. This high conductivity coupled with high volume resistivity and dielectric strength leads to its application as substrates and packaging for high power or high-density assemblies of microelectronic components. One of the controlling factors which limits the density of packing of electronic components is the need to dissipate heat arising from ohmic losses and maintain the components within their operating temperature range. Substrates made from AlN provide more efficient cooling than conventional and other ceramic substrates, hence their use as chip carriers and heat sinks (see figure 1).
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Figure 1. AlN substrates for microelectronic applications (Photo Courtesy of Ceram Research Ltd)
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Other Applications
Because of the cost of AlN its applications have been developed mainly for military aeronautics and transport fields.
Other applications of AlN lie in refractory composites for handling of aggressive molten metals, and high efficiency heat exchange systems.