Image Credit: Laremenko Sergii/Shutterstock.com
There are many ways to deposit a nanosized film, in what is termed ‘bottom-up’ deposition methods. Nanosized films can also be produced by top-down approaches—such as lithography or etching—where larger materials are broken down to create nanosized structures. However, there are limits to how thin top-down approaches can go, so bottom-up methods are required to realize extremely thin films.
There are a number of bottom-up deposition methods that can be used to grow a nanofilm. In all cases, these nanofilms are deposited on to a substrate—because the films are built up and fabricated atom by atom—but can be removed in many cases after the film has been created. The most common bottom-up deposition methods for fabricating nanofilms are chemical vapor deposition (CVD), physical vapor deposition (PVD) and atomic layer deposition (ALD), and these are covered in more detail below. In addition to the methods mentioned here, there are also various nucleation-growth methods that rely on wet chemical methods, but these approaches are often unpredictable and can suffer from grain boundary defects where nano-islands/nano-clusters join together during the film formation process. So, for that reason, this article looks at different high-quality techniques rather than wet chemical methods.
Chemical Vapor Deposition (CVD)
Chemical vapor deposition (CVD) is a class of techniques. In recent years, they have become best known for the method which can produce single-layer graphene on different substrates, but it is also a method that can be used with a wide range of materials. It has become a well-established method that is known for creating uniform nanofilms that are of high quality. In many cases, it is a process that is associated with harder materials.
Even though there are different methods, a lot of them work along with the same basic working principles, with how the atoms themselves are deposited being the main differentiator. For example, atoms can be deposited as a plasma, in low pressures, through laser irradiation and photochemical reactions, to name a few. However, all the processes are performed in a vacuum environment, and regardless of the deposition mechanism itself, all the methods use a volatile precursor material. These precursors are vaporized under high temperatures within a reaction chamber, and the vaporized atoms are then deposited, which as mentioned, can take numerous forms depending on the specific sub-method in question. The vaporized atoms then decompose/react on the surface of a substrate, and this enables a thin nanosized film to built up as the atoms react with each to create a chemically bonded film.
Physical Vapor Deposition (PVD)
Much like CVD, physical vapor deposition (PVD) is another general class of methods that can be used to deposit atoms on to a surface to create a nanosized thin film. PVD is much like CVD in many respects and is also performed in a vacuum environment. Where CVD differs from method to method in the deposition method, PVD does also. However, PVD also varies in the methods used to generate the vaporized atoms that enter the reaction chamber.
PVD relies on taking solid materials and vaporizing them into a gas. Given that many materials can be used, this vaporization can take many forms. Regardless, of the vaporization method, the vaporized atoms then travel through a reaction chamber where they are condensed on to a substrate. The deposition stage can take the form of thermal deposition using evaporation, or sputtering, which uses an accelerated plasma, to name the most common methods. Because there a few different methods, PVD methods can be used to generate nanosized thin film coatings on a wide variety of materials with a wide variety of film compositions.
Atomic Layer Deposition (ALD)
Atomic layer deposition (ALD) is actually one of the methods that fall within the class of CVD methods, but in recent years, it has become a method that has stood on its own merits. So, even though it can be bundled in with various CVD methods, it is a method that has earned its own distinction and mention. It is a growing method for producing highly uniform and conformal nano-thin films that can be used on a wide range of complex geometries and curved surfaces (if it is deposited as a nanofilm coating directly), as well as substrate materials. It is a method that can even be used to deposit a nano-thin film directly on to a nanoparticle, and this is a variation known as particle ALD.
ALD uses two precursor materials to build a nanofilm. Where ALD differs from other CVD methods is that the different precursors are never present within the reaction chamber at the same time—they are deposited sequentially. ALD is also a type of layer-by-layer (LbL) method and is often used to create films with multiple atomic layers, whereas CVD is commonly used for single-layer films. Once one of the precursors has been deposited across the whole surface, the next precursor material is layered on top, where it reacts with the first deposited material, creating a chemically bonded multi-layer film. This process repeats until the desired thickness has been reached. Another key difference from CVD is that the reaction is performed in a controlled temperature range, whereas CVD uses high temperatures to vaporize the atoms. While it is often a more controlled approach than other deposition methods, it is also a method that requires a lot more monitoring and expertise to perform.
Sources and Further Reading