Thinner is Better – Advances in Thin Film Coatings

The most remarkable thin film properties are: optical, mechanical and chemical properties. Monolayer thin films often provide these properties, although multi-layer thin films are sometimes required. Of course, in many applications, it is exactly the combination of properties (often the transparency combined with chemical or mechanical properties) that exploits the full range of this technology.

Thin films could be designed to have chemical properties like water repellence, anti-fogging, chemical barriers and chemical inertness, oxygen or moisture barriers over polymers or antimicrobial surfaces. The function of polar/apolar surfaces is crucial for sensors’ applications and together with hydrophilic/hydrophobic balance is essential in hard coatings. By increasing the inorganic content, thin film stability could also be improved. The appropriate chemical composition (e.g. hybrid coatings) also provides good etch barrier characteristics (e.g. for plastics on automotive bodies) or has also a great impact on electrical properties and especially on insulation properties (relevant for semiconductor circuit structures).

Optical properties include  light emission, trapping, transmission, opaqueness, fluorescence, waveguides, anti reflection, etc. Some thin films have the ability to emit light without the need for a backlight; these are applied in displays. Thin films can be designed to be transparent or opaque (or both depending on the applied voltage or incident light) and applied,  in windows, displays or solar cells. In some cases multi-layered thin films are required for achieving the desired properties (e.g. OLEDs use small molecules in multi-layers for up to full colour displays). Thin films of high refractive materials could be designed to be planar waveguides for photonics’ applications. Dielectric thin films could also be used to generate surface plasmons resonance that’s exploitable in optical modulators or chemical sensors. These properties have particularly benefited photovoltaic systems.

Wear/ abrasion resistance, hardness, scratch resistance, dry lubrication, reduced Strain-to-failure, etc are some of the mechanical properties that thin films can achieve.

Thin film coatings are applied to improve the thermal properties of windows or more sophisticated applications like aircraft engines (e.g. Thermal Barrier Coatings based on Zirconia).  Application of multi-layered thin films allows, blocking the travel of atomic vibrations that produces heat flow while still letting the electrons flow as a current (application in thermoelectric devices).

Progress in each of these areas depends upon the ability to selectively and controllably deposit thin films – thickness ranging from tens of ångströms to micrometers – with specified physical properties. This, in turn, requires control – often at the atomic level – of film microstructure and microchemistry. There are a vast number of deposition methods available and in use today. However, all methods have their specific limitations and involve compromises with respect to process specifics, substrate material limitations, expected film properties, and cost. This makes it difficult to select the best technique for any specific application.

The process of deposition depends on whether chemical means are deployed, or mechanical. Chemical means involve a so-called `fluid precursor’ that leaves behind a thin coating on the solid surface after initiating a chemical reaction. The fluid precursor may be a liquid reagent, often with a dissolved salt of the metal to be deposited on the required surface. This method is commonly used in electroplating of metals.

The fluid could also be a gas, as in the metal-organic chemical vapour deposition (MOVCD) which is used to facilitate the epitaxial growth of layers of semiconducting materials on a substrate. Usually, a gas which is the hydride or halide of the element to be deposited is used for the reaction. Precursor gases are used at very low pressures.

Mechanical processes involve the deposition of certain materials onto substrates by physical means like thermal evaporation and electron beam evaporation. The first method is suitable for the deposition of metals with high vapour pressures while the second is suitable for the deposition of metals with low vapour pressures. To eliminate the problem of uneven evaporation of the metal to be deposited, a new method called sputtering is used. This involves the use of a noble gas like argon in order to dislodge atoms one by one.

Other important methods are reactive sputtering, in which oxygen or nitrogen may be used in order to deposit an oxide or nitride of the target material and molecular beam epitaxy (MBE), in which a stream of the metal to be deposited is directed to the substrate.

With recent advances in engineering, thin film technology is playing an increasingly important role in spearheading technological advancements for future society. Aside from traditional applications, thin film technology is also closely tied to nanotechnology, which is one of the key technologies of the near future. Nanocomposite, nanophase or nanostructured bulk materials and coatings will become tomorrow’s workhorse in new-generation manufacturing and precision engineering industries.