Thin film
[1] The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many applications.
A familiar example is the household mirror, which typically has a thin metal coating on the back of a sheet of glass to form a reflective interface.
In addition to their applied interest, thin films play an important role in the development and study of materials with new and unique properties.
Desorption reverses adsorption where a previously adsorbed molecule overcomes the bounding energy and leaves the substrate surface.
Physisorption describes the van der Waals bonding between a stretched or bent molecule and the surface characterized by adsorption energy
Chemisorption describes the strong electron transfer (ionic or covalent bond) of molecule with substrate atoms characterized by adsorption energy
Cluster coalescence through processes, such as Ostwald ripening and sintering, occur in response to reduce the total surface energy of the system.
It is useful in the manufacture of optics (for reflective, anti-reflective coatings or self-cleaning glass, for instance), electronics (layers of insulators, semiconductors, and conductors form integrated circuits), packaging (i.e., aluminium-coated PET film), and in contemporary art (see the work of Larry Bell).
This allows creating thin films of various molecules such as nanoparticles, polymers and lipids with controlled particle packing density and layer thickness.
Unlike the soot example above, this method relies on electromagnetic means (electric current, microwave excitation), rather than a chemical-reaction, to produce a plasma.
Facing this source is a cooler surface which draws energy from these particles as they arrive, allowing them to form a solid layer.
Since particles tend to follow a straight path, films deposited by physical means are commonly directional, rather than conformal.
Examples of physical deposition include: A thermal evaporator that uses an electric resistance heater to melt the material and raise its vapor pressure to a useful range.
In molecular beam epitaxy, slow streams of an element can be directed at the substrate, so that material deposits one atomic layer at a time.
If a reactive gas is introduced during the evaporation process, dissociation, ionization and excitation can occur during interaction with the ion flux and a compound film will be deposited.
Through the influence of electric field, the liquid coming out of the nozzle takes a conical shape (Taylor cone) and at the apex of the cone a thin jet emanates which disintegrates into very fine and small positively charged droplets under the influence of Rayleigh charge limit.
The second stage commences as these individual islands coalesce and begin to impinge on each other, resulting in an increase in the overall tensile stress in the film.
The overall shape of the stress-thickness vs. thickness curve depends on various processing conditions (such as temperature, growth rate, and material).
There is also a possibility of developing a mixed Zone T/Zone II type structure, where the grains are mostly wide and columnar, but do experience slight growth as their thickness approaches the surface of the film.
Although Koch focuses mostly on temperature to suggest a potential zone mode, factors such as deposition rate can also influence the final film microstructure.
This technology is used, for instance, to grow a film which is more pure than the substrate, has a lower density of defects, and to fabricate layers having different doping levels.
The assumptions made regarding the Stoney formula assume that the film and substrate are smaller than the lateral size of the wafer and that the stress is uniform across the surface.
Since layers are thin with respect to some relevant length scale, interface effects are much more important than in bulk materials, giving rise to novel physical properties.
It may also be understood as any form of painting, although this kind of work is generally considered as an arts craft rather than an engineering or scientific discipline.
Today, thin-film materials of variable thickness and high refractive index like titanium dioxide are often applied for decorative coatings on glass for instance, causing a rainbow-color appearance like oil on water.
A well-known example for the progress in optical systems by thin-film technology is represented by the only a few mm wide lens in smart phone cameras.
For instance, plastic lemonade bottles are frequently coated by anti-diffusion layers to avoid the out-diffusion of CO2, into which carbonic acid decomposes that was introduced into the beverage under high pressure.
Another example is represented by thin TiN films in microelectronic chips separating electrically conducting aluminum lines from the embedding insulator SiO2 in order to suppress the formation of Al2O3.
Quantum effects in such thin films can significantly enhance electron mobility as compared to that of a bulk crystal, which is employed in high-electron-mobility transistors.
[55][56] Thin-film printing technology is being used to apply solid-state lithium polymers to a variety of substrates to create unique batteries for specialized applications.
