Layers of material called thin films are applied to a bulk substrate to impart qualities that the base material cannot readily or at all acquire. The process of putting a thin coating of any material to a surface, be it a substrate or layers that have already been deposited, is known as thin film deposition.
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What Is Meant by “Thin”?
Naturally, “thin” refers to a relative layer, but most thin film deposition methods are intended to create layers that are only a few tens of nanometers thick. One layer of molecules or atoms can be deposited in a thin film at a time using other, more recent techniques including atomic layer deposition, Langmuir-Blodgett method, and molecular beam epitaxy.
In the modern world, thin films are frequently associated with semiconductors. Nonetheless, thin films play a significant role in several other fields that need coatings that are just a few microns thick.
Because it modifies the surface interactions of the newly created platform from the bulk substrate characteristics, the thin film is significant.
For example, chromium films are used to cover automotive parts with a strong metallic finish and shield them from UV light without using a lot of metal, therefore reducing weight and expense.
TiN coatings are incredibly effective in giving cutting tools an edge, hardness, and low coefficients of friction. They also provide a chemical barrier against alloying, allowing the tool to be used safely with different workpieces.
By absorbing radiation in the visible, infrared, radio frequency, and audio spectrums, thin film absorbent coatings can also be employed to reduce an object’s visibility.
Are Thin Films All Identical?
Because of their many qualities, thin films are employed in a wide variety of applications. These comprise, among many other things, the following:
Optical thin films are used to make memory discs, solar cells, waveguides, displays, photodetector arrays, and reflective or anti-reflective coatings.
Electrical or electronic thin films are used in the production of semiconductors, integrated circuits, photodetector arrays, solar cells, piezoelectric motors, and insulators or conductors.
The thin magnetic sheets found in memory disks
Chemical thin films are used in gas and liquid sensors as well as to withstand alloying, diffusion, oxidation, and corrosion.
Tribological coatings used in mechanical thin films to impart hardness and micro-adhesion, prevent wear, and use micromechanical characteristics
Thermal thin films: for heat sinks and barrier layers
These thin films’ primary characteristic, thickness, is intimately related to many other characteristics. As a result, measuring film thickness and comprehending its controllability are typically crucial, particularly in situations where optical coating of microlenses is necessary.
How thin is the thinness?
However, thin films can in a variety of thicknesses based on their unique features, thus thickness is not the only characteristic that defines them. For example, atoms or molecules can deposit, as in the case of evaporation, to form a thin film at the atomic level. Particle deposition, on the other hand, would result in the formation of a thick coating, similar to that of paint particles.
However, a thin film would likely be thinner overall—at best, a few microns—than a micron. This could be best understood as a single spider silk strand, hundreds of which are included in a single spider web thread.
A silicon oxide layer on a silicon chip would be completely invisible to the unaided eye. Since light rays have a wavelength of around one micron, a particle below this size is almost undetectable to the human eye.
Nonetheless, a chromium film is visible because its thickness—which is its only dimension—is just one micron. Similar to this, an oil coating over water has a thickness far below a micron, yet it is visible as a colorful layer mostly due to refraction at the interface, which is closely correlated with the film thickness.
On the other hand, thin films may be “seen” using electron microscopy techniques. Numerous improvements on these techniques, including scanning electron microscopy (SEM), have been employed. Thus, when it comes to defining thin films, light wavelengths are also not set in stone.
A thick film is determined by the nature of the film, the thickness necessary for its function, or the thickness that is inefficient for usage. In most cases, a thin film can be as thick as one layer of atoms.
It is more accurate to say that a thin film is ‘thin’ if its thickness is measurable in the same or a lesser order of magnitude compared to the scale of length that is intrinsic to the measured system. Traditional thin films are often defined by the equation dz < d0 with d0 = 5 µm. Actually, only when the thin film properties are an expression of the internal length scale do they deviate greatly from the bulk substrate. To add to the complexity, it should be noted that whereas films of different metal oxides, such as SiO2, TiO2, and Ta2O5, are thin films when they have a thickness of around 100 nm, the aluminum film does not meet this requirement. The latter is useless for thin film qualities as it acts like bulk material. In order to categorize a material as a thin or thick film, it is therefore necessary to take into account both the characteristics and the length scale.