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Thin films are layers of material deposited on a bulk substrate in order to impart properties that cannot be easily attained (or not attained at all) by the base material. Thin film deposition refers to the action of applying a thin film of any substance on a surface, whether a substrate or already deposited layers.
What Does ‘Thin’ Mean?
‘Thin’ is, of course, a relative layer, but in most cases, the techniques of thin film deposition are designed to produce layers which are only a few tens of nanometers thick. Other newer methods such as molecular beam epitaxy, Langmuir-Blodgett method, and atomic layer deposition allow thin films to be deposited just one layer of molecules or atoms at a time.
In today’s world, thin films are commonly thought of in connection with semiconductors. However, thin films are important in a multiplicity of other areas where coatings only a few microns thick are needed.
The thin film is important because it changes the surface interactions of the newly formed platform from the bulk substrate properties.
For instance, chromium films are used to create hard metallic coatings on automobile parts and to protect them against ultraviolet rays, without having to use a large amount of the metal, saving both on weight and on cost.
TiN coatings are extremely efficient at creating an edge on cutting tools, imparting hardness and low friction coefficients, and also offering a chemical barrier to alloying so that the tool can be safely used with other workpieces.
Thin film absorptive coatings are also used to make objects less visible, by bending radiation in the visual, infrared, audio and radio frequency regions of the electromagnetic spectrum.
Are All Thin Films the Same?
Thin films have a range of properties, based on which different types are used in a plethora of applications. These include the following, among many others:
- Optical thin films – to create reflective or anti-reflective coatings, solar cells, displays, waveguides, photodetector arrays, and memory discs
- Electrical or electronic thin films – to manufacture insulators or conductors, semiconductor devices, integrated circuits, solar cells, and photodetector arrays, and piezoelectric drives
- Magnetic thin films – in memory discs
- Chemical thin films – to resist alloying, diffusion, oxidation or corrosion, and in sensors for gas and liquids
- Mechanical thin films – tribological coatings to protect against wear, to impart hardness and micro-adhesion, and to take advantage of micromechanical properties
- Thermal thin films – to create barrier layers and heat sinks
Thickness is the fundamental property of these thin films and is closely linked to other thin film properties. Thus it is often essential to measure film thickness and to understand how it can be controlled, such as when microlenses require to be optically coated.
How Thin is Thin?
Thin films are not defined by their thickness alone, however, as they are of various thicknesses depending on their properties. For instance, a thin film at an atomic level is created by the deposition of atoms or molecules, as by evaporation. On the other hand, particle deposition would create a thick film, as when paint particles are deposited.
Overall, however, a thin film would probably be thinner than one-micron, or at most a few microns. This may perhaps be better visualized in terms of a single strand of spider silk, hundreds of which are contained in a single thread of a spider web.
A silicon chip covered with a coating of silicon oxide would not show the oxide coating at all to the naked eye. Thus a particle is basically indistinguishable to the human eye when it is sized below one-micron since light waves have a wavelength of about this size.
However, a chromium film is visible because the one micron thickness makes up only one dimension of the film, namely, its thickness. Similarly, a film of oil on water is well below a micron in thickness, but it is visible as a multicolored layer, largely because of refraction at the interface, related directly to the film thickness.
On the other hand, electron microscopy techniques can be used to ‘see’ thin films. Various refinements of these methods have been used such as scanning electron microscopy (SEM). Thus the wavelength of light is not a hard and fast rule either when it comes to defining thin films.
In most situations, a thin film may be as thick as one layer of atoms, while a thick film is defined by the nature of the film, the thickness required for its function, or the thickness that is ineffective use.
While traditional thin films are defined a lot of the time by the equation dz < d0 with d0 = 5 µm, for instance, it is more accurate to say that a thin film is considered ‘thin’ if the thickness of the film is measurable in the same or a lesser order of magnitude compared to the scale of length that is intrinsic to the measured system.
In fact, thin film properties differ significantly from the bulk substrate only when these characteristics are an expression of the internal length scale.
To complicate matters further, it should be understood that while films of various metal oxides such as TiO2, SiO2, and Ta2O5 at thicknesses of about 100 nm are thin films, this is not the case with the same thickness of the aluminum film. The latter behaves like bulk material, making it not useful for thin film properties.
Thus both the properties and the length scale should be considered when attempting to classify a material as a thin or thick film.
Sources and Further Reading
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