Thin-film coatings are extremely thin layers of material placed on the surface of substrate. They can vary in thickness from several micrometers, or millionths of a meter, to less than 1 nanometer, or a billionth of a meter. Thin-film coatings can be made from various substances, including silicon, ceramics and gallium arsenide. They are commonly used in optical devices such as photovoltaic cells used for solar power generation, for components for electronics and in tools and other equipment that requires resistance to surface damage.
The composition of a thin-film coating depends on its purpose. Thin-film coatings used in photovoltaic cells to generate solar power are made of semiconducting materials such as silicon and gallium arsenide. These materials can convert energy from light into electrical current when incoming photons of light excite the semiconducting material's electrons to higher energy levels. These coatings are also used in electronics, such as integrated circuits.
Semiconducting thin films are often applied in a series of layers with varying chemical compositions to affect the properties of the film. These layers can be incredibly thin, and some thin-film coatings have layers that are only a single atom thick. Thus, a thin-film coating can contain a complex internal structure of varying layers in an extremely small space.
Thin-film ceramics are used in applications that require materials resistant to wear or corrosion. Cutting tools, for instance, can have greatly extended working lifetimes if they are made with a thin ceramic outer layer on the cutting edge to protect the rest of the blade from wear. Devices that use reactive chemicals, such as fuel cells, might use thin-film ceramic to contain them because of the ceramic film's relative chemical inertness.
Other uses of thin-film coatings include rechargeable lithium batteries, which can be made with a cathode composed of a thin film of lithium and metal oxides to produce batteries that are much less bulky and more flexible than conventional batteries. Mirrors contain a thin metal film, usually made of aluminum in modern mirrors, behind glass to create a reflective surface. Thin aluminum film coatings are also used in the packaging of many modern consumer goods, especially foods, to increase the shelf life of perishable items and make the package shinier and more noticeable. Aluminum films are also used as insulation, especially for applications that require flexibility and portability, and they are incorporated in such things as space suits, firefighters' protective gear and emergency insulating blankets for shock victims.
Thin-film coatings are applied to a surface through one of several techniques collectively referred to as thin film deposition. Most of these methods are are categorized as either chemical deposition, in which the thin film is applied through a chemical reaction between the surface and a fluid called a precursor, or physical deposition, in which the film is applied through mechanical, thermal or electrical means. Methods of chemical deposition include plating, which uses a liquid precursor solution containing dissolved metal ions, and chemical vapor deposition, in which the surface reacts with gaseous compounds.
Physical methods of deposition are carried out in a vacuum chamber containing the substrate and the substance. The particles of the latter are knocked off through various means, such as heating, laser pulses or electrical arcs, creating a vapor that forms a film on the substrate. There are also methods that use both physical and chemical processes and do not fit neatly into either category, such as reactive sputtering and molecular beam epitaxy.