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Atomic layer deposition (ALD) is a bottom-up nanofabrication deposition method that technically falls within the remit of chemical vapor deposition (CVD) methods; but it has become well-regarded as a stand-alone class of deposition methods. While there is a standard way of utilizing ALD, there are many different variations—which mostly depend on the material being deposited or how the reactions between the surface and film precursors are initiated—that will be discussed in this article.
ALD is a method that has gained a lot of interest as a way of producing nanosized thin films and nanosized coatings on various substrates. The thin films/coatings produced by ALD are highly conformal and can be utilized on a wide range of geometrically complex surfaces, and even spherical particles. Applications wise, it is a method that has seen use in a wide range of scientific fields, including battery electrodes, semiconductor technologies (including solar cells), transistors and other electronic components, microelectromechanical systems (MEMS), drug delivery, and tissue engineering applications; but this is by no means an exhaustive list.
Basic Principles of ALD
ALD is a sequential deposition method, and this is where it varies from most bottom-up deposition methods. In ALD, two different precursor materials are used to build up the film/coating, and these precursors are never present within the reaction chamber at the same time. The sequential deposition method means that one of these precursor materials gets deposited, followed by the other precursor material, so in essence, it is also a type of layer-by-layer (LbL) deposition method.
In the reaction chamber, it is the occurrence of surface reactions which are responsible for the formation of the film/coating. The precursor materials which are used are vaporized in a controlled temperature range before they are deposited. The first precursor material enters the reaction chamber, where it is deposited uniformly across the whole surface. The molecules bind together chemically to form a completely bonded coating layer. Once this layer has been deposited, the second precursor material is deposited on top of the first coating layer, and the second layer not only binds to itself, but also to the first coating layer. The process is repeated sequentially until the film/coating reaches the desired thickness.
Thermal ALD
While a controlled approach to atomic vaporization is commonplace in most ALD methods, higher temperatures are sometimes required. This is often the case for certain types of molecules, and in particular, aluminum containing molecules. The temperature ranges utilized in thermal ALD are generally between 150-350 °C. The most common example is the creation of Al2O3 which is formed through the reaction of water and trimethylaluminum on the surface of a substrate.
Metal ALD
A wide range of metals (aside from aluminum) can be deposited on to a surface by utilizing elimination reactions between a halogen-functionalized metallic molecule (commonly a metal fluoride) and a silicon-based molecule. Most of the reactions used in metal ALD processes utilize exothermic fluorosilane elimination reactions, which deposits the metal on the surface of the substrate. Like thermal ALD, metal ALD can use higher temperatures of around 175-325 °C and can be used to deposit a wide range of metals on a surface. Even though both thermal and metal ALD use higher temperatures than other methods, the temperatures used are significantly less than other CVD methods.
Particle ALD
Particle ALD is much like conventional ALD. However, where most ALD method focus on a flat, or slightly curved surface, particle ALD is concerned with coating the entire surface of a particle (including the surface of nanoparticles). Many materials can be coated on the surface of a particle, with a high uniformity and conformability, without any areas of the particle being missed. This method essentially takes the principles of ALD and applies it to the complex coating geometry of spherical particles—but it is unique, as there are not many methods which can do this.
Other ALD Methods
Other ALD methods include plasma ALD and photo-assisted ALD, both of which use low temperatures. Plasma ALD uses a plasma to reduce the temperature of molecular vaporization and can be used with a wide range of precursor materials. Photo-assisted ALD, on the other hand, uses ultra-violet (UV) light to initiate and accelerate the surface reactions on the substrate, so it does not require a high temperature to initiate reactions. This method is easy to control because the wavelength, intensity and illumination time of the UV light can be used to tune the surface reactions.
Sources
Forge Nano: https://forgenano.com/
“New development of atomic layer deposition: processes, methods and applications”- Jen T-C. et al, Science and Technology of Advanced Materials, 2019, DOI: 10.1080/14686996.2019.1599694
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