Oxides and Nitrides of Aluminum
Applications & Deposition Processes
This article will compare the properties, applications and deposition processes of films of the Oxygen and Nitrogen compounds of Aluminum; specifically Aluminum Nitride, AlN, and Aluminum Oxide, Al2O3 or Alumina. Although they are neighbors on the periodic table of elements, oxide and nitride compounds of various metals exhibit different physical properties and therefore are used to satisfy different applications. While Alumina is found naturally as the material Corundum, AlN does not occur naturally but must be artificially grown.
Comparison of Properties
The common oxide compounds of Aluminum, Tantala, Silica, Titania and Alumina are transparent between near-UV and Mid-IR wavelengths, with Alumina transmission reaching ~250 nm. For that reason, these compounds are often used in thin-film optical coatings. Alumina optical films provide some degree of protection against abrasive wear and chemical reaction. However, they are never as hard as crystalline sapphire, because the density and morphology of the films are not of the same form.
The common nitride compounds of Aluminum Nitride (AlN) and Silicon Nitride (SiN) are transparent at wavelengths longer than mid-visible wavelengths and produce hard coating layers. Compounds of transition metals to form Titanium Nitride (TiN), Chromium Nitride (CrN) and others are not transparent, but find application as tribological and wear resistant coatings such as on tools, decorative surfaces, and high-temperature applications.
Applications
Aluminum Oxide films are used in optical coatings several ways: as an intermediate index material, as a protective overcoat for aluminum and silver mirror films, and in thick layers as a barrier to salt, steam, and other corrosive agents. Thick Aluminum Nitride films are used as piezoelectric transducers. Depending on the crystal growth axis, the piezoelectric (PZ) properties differ with orientation [1]. Thick layers of both Zinc Oxide (ZnO) and AlN have desirable PZ properties; each suitable for a specialized application. Orientation of the film layer along the c-axis produces the largest PZ effect, therefore crystalline growth morphology and axis orientation must be appropriately controlled. The ability to sputter thick (>20 µm) layers of oriented AlN with superior PZ properties provides an alternative to ZnO. Martin [1] mentions two advantages: AlN processing is compatible with silicon semiconductor technology processes, and AlN tends to possess and maintain higher conductivity and thus higher efficiency over a range of driver frequencies.
Piezoelectric devices are used to produce small mechanical motions, either linear or angular, as applied to micro-mechanical motion (MEMS) actuation using electrical stimulation. PZ devices can be driven at high frequencies as transducers. Conversely, small currents can be generated by mechanical stressing a PZ material. Figure 1 illustrates the configuration that can be used for either application.
Figure 1. General configuration of a PZ actuator/transducer. Electrical stimulation causes mechanical motion; mechanical stress causes electrical current generation. The electrodes are metal (Al/AlN/Al) or they can be transparent conductors such as ITO or AZO.