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Extremely thin films of material are used to make everything from potato chip bags to solar cells, and vapor deposition processes are the common techniques used to make thin layers.
Vapor deposition encompasses a variety of production techniques involving the vaporization of a solid in a high-vacuum environment and the resulting vapor being deposited onto a target substrate. Capable of applying a coating at the single-atom or single-molecule level, vapor deposition techniques can create very pure, high-performance films of material.
The two main categories of vapor deposition are physical vapor deposition (PVD) and chemical vapor deposition (CVD).
Physical Vapor Deposition
Various PVD methods make use of the same essential steps but vary in some of the processes used to produce and lay down coating material.
The two most common PVD operations are thermal evaporation and sputtering. In both, the resulting vapor phase is put onto the target substrate via condensation.
One of the most basic PVD methods, thermal evaporation involves heating a material in a vacuum chamber until the atoms on its exterior have enough energy to be released, a process known as vaporization. After being vaporized, the atoms are channeled through the vacuum chamber to coat a target substrate situated above the source material.
Sputtering is a plasma-aided process that produces vapor from the source material by bombarding it with high-speed plasma ions. The evaporated source material atoms, bunches of atoms or molecules travel in a straight line. If a "substrate" like a silicon wafer is put in the way of these streaming particles, it will be covered by a thin film of source material.
Source material atoms atomically bond to the substrate to develop a thin film. There are quite a number of kinds of sputtering methods, including a diode, ion beam and magnetron sputtering.
To generate a consistent thin film just a few atoms or molecules thick, a target item can be rotated on various axes or put onto a conveyor belt that travels through the plasma stream. Single or multiple coatings can be administered with the same deposition process.
Furthermore, reactive gasses like oxygen or acetylene can be placed into the deposition chamber to generate an extremely strong bond between the coating and substrate. Even though the thin films produced by these processes are just microns thick, they are extremely strong, making PVD an ideal option for many applications.
Chemical Vapor Deposition
Chemical vapor deposition is a highly versatile, popular process that can be modified to a multitude of different applications.
The major difference between PVD and CVD is the use of one or more chemical precursors that break down the source material and carry it to the substrate, where it is deposited.
In the standard CVD sequence, the substrate material is put into a vacuum chamber and a source material is put either inside the same chamber or in a neighboring chamber. Next, the source material is heated or the atmospheric pressure is decreased until the source material vaporizes. Then, one or more precursors are introduced to react with the source material, allowing for it to be deposited on the substrate.
The vaporized material then reacts with the substrate to create a uniform thin film. Modifying the temperature and duration of the sequence helps to manage the thickness of the film.
The development of high-temperature CVD processes in recent years has allowed for many more commercial uses. For instance, researchers have been using high-temperature processes to fabricate sheets of graphene and massive arrays of carbon nanotubes, both of which hold untold potential for the creation of new electronics and other products.
One major benefit of CVD is that it can develop coatings of consistent thickness even over intricate shapes. For instance, CVD can be used to apply a consistent coating on carbon nanotubes in order to tweak their mechanical qualities, such as to make them chemically react in a certain way.
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