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Thin films

What are thin films?

Thin films read "Hakumaku" in Japanese. In some cases, it refers to the film used in filters or a thin gold leaf. However, it may also refer to a thin and flat film on a glass or silicon substrate. Such thin films cannot exist by themselves and are always placed on the substrate.
Therefore, the thin film usually refers to the film made by vacuum deposition technology. Depending on the materials used, there are metal thin films, semiconductor thin films, insulator thin films, etc. In addition to thin optical films used for lens coatings, it is increasingly being applied to a wider range, including electronic components such as diodes, transistors, and integrated circuits, magnetic thin films used for memory elements of computers, and superconducting thin films used for elements such as cryotrons. A memory element stores information in some form that can be read later, such as in a computer circuit.
A thin film is literally a very thin film, but when its thickness reaches and exceeds 1 micron, it can generally not be called a thin film. In other words, thin films are nano-level films. A thick film is called a plating film, and when it is thickened and removed from the substrate, it may become an electroforming coating.


As previously mentioned, thin films are usually films made by vacuum deposition technology, so what are their applications? Thin films are mainly used in electronic components. For example, the hard disk recording surface is a magnetic thin film. The screen of the liquid crystal display is also made by the lamination of various thin films. The fine circuit of a semiconductor is made by repeatedly processing thin films into patterns. 
In addition to electronic components, thin films can be used in various ways, such as strengthening or protecting the surface of an object. The surfaces of molds and tools used in the manufacturing industry are often protected with a layer of tough thin film.
Thin films can also be used to control light. Furthermore, the surface of metal products may be colored with a thin film. Adding colors by using thin films is more stable than painting because the quality is stable. In addition, thin films are also applied around the light source of automobile headlights for light reflection purposes.
The following divides the applications of thin films into optical, electromagnetic, mechanical, and scientific aspects and gives separate explanations.

(1)Optical applications
①Anti-reflective film
This film makes use of light interference phenomenon to prevent the reflection of glasses, cameras and other lenses, and headlights of automobiles to increase light transmittance. Raw materials generally used include magnesium fluoride, silica, zirconia, alumina, and tantalum pentoxide.

②Filter membrane
It is a type of optical thin films that only transmits or reflects light of a certain wavelength. For example, only green light can pass, or visible light is allowed to pass while infrared rays are reflected.

③Transparent conductive film
This film is used as a transparent electrode in solar cells, liquid crystal display panels, etc. Tin oxide and indium oxide mixed with tin are used.

(2)Electromagnetic applications
①Integrated circuit

Integrated circuits (ICs) confine complex electrical circuits to such a small space that they can only be viewed with a microscope. It is made by applying thin film processing technique, which uses a vacuum.

②Display devices
Today, the flat panel display described in another section has become the mainstream display device. As for the manufacturing of flat panel displays, thin film preparation technology is generally considered a key technology. In addition to the transparent conductive film mentioned earlier, a variety of thin films are used.

③Optical disks
Reflective films of aluminum are formed on various compact discs and digital discs.

④Solar cell films
Amorphous silicon films obtained by the plasma-enhanced chemical vapor deposition of substances called silane gas are used in solar cells, which contributes to the cost reduction of solar cells.

(3)Mechanical applications
①Wear-resistance film

Hard films are formed on the surface of cutting tools, such as drills or various molds, to prolong their service life.

②Solid lubricant film
Lubricant and grease are usually used to reduce friction on the sliding surfaces of bearings and others. However, such lubricating oil cannot be used in some environments, and a typical example is space. Therefore, solid lubricant films are used for bearings and gears of machines in space, such as satellites.

(4)Chemical applications
①Corrosion resistant films
In general, painting and plating techniques are used to prevent metal from rusting. In addition, thin films made by physical vapor deposition and chemical vapor deposition methods are also used. In either case, ceramic and metal films will be produced, so it is necessary to select the type of film and film formation method that is compatible with an environment where the metal product is used.

②Barrier films
Plastic film is used for food packaging. In order to preserve food, it is necessary to block the ingress of oxygen and reduce UV rays. For this purpose, plastic films are formed into thin films using the physical vapor deposition technique.

How to make a thin film

Thin films can be made in several ways. One typical way is vacuum deposition, in which the film material is deposited onto the surface of the substrate in a vacuum.
There are several different vacuum deposition methods. One is called vacuum evaporation, that is, to evaporate the material by heat and cool it to a solid state on the substrate. The other one is called sputtering, in which the material is bombarded with ionized atoms and sputtered towards the substrate.
In addition to vacuum deposition, there are also other ways to form thin films,  including electrolytic plating, electroless plating, and wet coating.
Furthermore, there are also thermal spraying, painting, spray, etc. methods, in which metal powder softened by heat is sputtered and solidified, but many of these usually have a film thickness exceeding the micron level, so it is more appropriate to call them coatings or membrane instead of films.

About sputtering thin films

Sputtering is one of the representative thin film forming techniques. Many of the films used in semiconductor wafer processing are sputtered films. Moreover, transparent conductive films used in LCDs and LEDs are also made by sputtering.
The sputtering process is characterized by a wide selection of materials, high adhesion, and ease of control. It is used for coating tools and forming functional films on semiconductors, liquid crystals, optical elements, etc. Furthermore, it is also used to decorate the surface of plastics.
Sputtering is a phenomenon in which molecules are knocked out by causing ions to collide with materials at high speed. This phenomenon is used to coat the objects or change the quality of the surface.
To cite a familiar example, when using a fluorescent lamp, the end of a glass tube gradually becomes darker because the filament of the fluorescent lamp is gradually dispelled by the discharge and becomes adhered to the glass tube. This is also a sputtering phenomenon.
However, like other coating methods, films made by sputtering can be greatly affected by the surface condition of the object. In case the surface is fragile, or if any foreign matter is attached or it is dirty, it is impossible to apply a high-quality film. Therefore, before sputtering, the surface of the object needs to be strong and clean. Moreover, if the object releases gas in the vacuum, the gas may mix in, the quality of the film may change.
Sputtering can be applied to heat-sensitive materials, but in some cases, it may be necessary to heat the object itself to form a high-quality film. It is also necessary to consider the possibility of the surface being damaged by exposure to plasmas.

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