Overview

Have you considered that gold-colored drill bit in your tool box? A titanium nitride coating gives it wear resistance. How about that smoky gray coating on your kitchen faucet? That color comes from chromium nitride. Consider also that the lightweight and compact – yet higher power – electrical charger you have relies on gallium nitride films. Nitrides are a class of compounds with a variety of useful properties. Deposition in thin film form allows coatings to be formed on almost any surface, imparting new functionalities.

Let’s look briefly at some examples, to give a better understanding of these fascinating materials.

• Titanium Nitride (TiN) has a high hardness for wear-resistant coatings, good electrical conductivity (it is used in semiconductors as a conductive barrier layer), and a strong gold color that recommends it for decorative coatings.

• Chromium Nitride (CrN) has high abrasion and corrosion resistance and is used in a variety of wear-intensive applications. Half of the hardest known materials are nitrides.

• Aluminum Nitride (AlN) has a unique combination of very high electrical insulation while also offering high thermal conduction. It is used as a heat sink material in high-power electronics and in high-temperature applications. Nitrides rank high for thermal stability and oxidation resistance.

• Gallium Nitride (GaN) is the most widely used III-V semiconductor. It is used in power electronics and is also the basis for the blue light LED.

The range of properties from this class of materials is impressive. However, sputter deposition from compound targets can be challenging. The sputtering yields and hence growth rates are low. Targets are more expensive. Some nitrides require RF sputtering, which further slows the process. Stoichiometric transfer is also an issue, i.e. the film does not have the same stoichiometry as the target. For these reasons, reactive sputtering may be practiced to make coatings of these materials.

In reactive sputtering, the target is made of the metal only. Thus, it sputters well. Nitrogen is injected to the system, and the nitride is formed reactively on the substrate as it grows. Although there are a wide variety of techniques to realize a good nitride coating, we summarize briefly some key points. To maintain a metallic target surface, a low percentage of nitrogen is often used in the sputtering gas. For the same reason, is preferable to introduce nitrogen at the substrate rather than at the sputtering source. Thus, the gas distribution must accommodate different delivery points. To promote the reaction, RF bias can be applied to the substrate. This increases the ionization fraction of the N2, making it more reactive. The RF bias also adds energy to the growing film, promoting the proper crystalline orientation. Finally, substrate heating can be applied to anneal the structure and promote the nitriding reaction [2].