Thin film deposition is crucial in fabricating many optoelectronics, solid-state, and medical devices, including consumer electronics, lasers, LED displays, optical filters, micro-analysis sample slides, and medical implants.1
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Atomic layer deposition (ALD) and chemical vapor deposition (CVD) are widely used deposition techniques. This article explores these two methods, comparing their uses, advantages, and disadvantages.
ALD vs. CVD: Process Mechanisms
The term "ALD" was first used around 2000. This technique achieves atomic layer control and conformal deposition through successive, self-limiting surface reactions. It involves introducing chemical reagents to the substrate surface sequentially, forming sub-monolayers of film.2
ALD is mainly applied to thin films below 100 nm but can also be used for thicker films up to a few micrometers. In this process, two or more precursors sequentially react in a chamber with a substrate at a specific temperature and pressure to deposit materials on the substrate’s surface layer by layer.2
An ALD cycle generally consists of four stages: the introduction of reactants, the purging of excess reactants with an inert gas, the insertion of counter-reactants, and the purging of unused reactants and by-products.3
CVD, on the other hand, is a vacuum-based method for creating high-quality, high-performance solid materials. It is widely used in material processing, utilizing gas-phase precursors in a chemical reaction to form thin layers on a heated substrate.
The substrate reacts with one or more volatile precursors, which decompose on the substrate material surface, resulting in the deposition of a thin film.4 In simple terms, a gas mixture interacts with the substrate surface, causing the chemical breakdown of some gas components and forming a solid coating on the material's surface.
While ALD involves sequential pulsing of precursors, CVD involves a continuous chemical reaction by the simultaneous introduction of reactants. ALD's sequential process ensures self-limiting precursor adsorption, distinguishing it from the simultaneous reactions in CVD.
Applications and Benefits of ALD and CVD
ALD is ideal for ultra-thin films with high conformity. It is used in nano-patterning for microelectronics, energy storage systems, desalination, catalysis, and medical fields. Industrial applications include gyroscopes, accelerometers, nano-machines, GPS navigation, thin-film magnetic heads (TFMH), and passive electrical devices.
ALD is also used to coat porous materials, nanomaterials, and nanoparticles, such as lithium battery cathode materials and membranes for water desalination.5 Additionally, ALD helps prevent environmental pollution and is used in water purification and gas separation sectors.
CVD is suited for thicker films and is primarily used in the semiconductor industry to create materials for solar panels, LEDs, perovskite cells, and integrated circuits for devices like phones and televisions. It is also used to prepare single-crystal metal oxides, such as sapphire and ferrites, and to produce specific shapes like tubes by removing the substrate.6
Previously, coating lenses and other optical devices with thin films was challenging due to resilience issues and difficulties with uneven surfaces. Researchers have made a significant breakthrough by using a new ALD method to apply an ultra-black coating to an uneven lens surface on a space exploration microscope.7
This upgraded coating is more resilient and effective than previous versions. Designed for aerospace-grade magnesium alloys, this durable ultra-black thin-film coating is crucial for space exploration, where weight and durability are essential.
Current coating techniques use fragile materials that cannot withstand harsh environments. This novel coating is effective for space exploration and optics and can also be applied to solar cells and energy storage, significantly increasing efficiency.
Major Challenges
Despite their extensive application, several challenges hinder the applicability of these techniques.
ALD can create high-quality films with excellent conformity, but its high precision often produces significant precursor gas and energy usage. Approximately 60 % of the precursor dosage is wasted during the ALD process, raising concerns about its economic viability due to low material utilization efficiency, around 50.4 %.
CVD, while versatile, requires precise control of temperature, pressure, and chemical reactions, making it more challenging to implement compared to ALD. The equipment needed for CVD can be costly, particularly for large-scale production, affecting overall production efficiency. Additionally, coating large and bulky structures with CVD is limited due to the size constraints of the reaction chamber.8
Efforts are being made to make deposition techniques eco-friendly. Developing CVD chemistry that utilizes precursors reacting with lower energy input and producing fewer harmful by-products is a promising step toward sustainable practices.
Additionally, designing CVD reactors with minimal vacuum volume and thermal budget and incorporating recirculation of process gases can reduce energy consumption and increase atom efficiency.9
Future Perspective
With the recent shift towards miniaturization, ALD and CVD processes are being used even more extensively. As devices become smaller and more complex, the precise and controlled nature of these deposition processes becomes indispensable.
The quest for more efficient energy storage solutions has propelled ALD into the spotlight, significantly enhancing the performance and longevity of energy storage devices.10
For instance, atmospheric-pressure ALD is an emerging technology set to revolutionize the commercial production of large-area substrates and unlock new applications in high-porosity and 3D materials. This technology promises high substrate throughput and reduced reaction time, improving manufacturing efficiency across various industries.
As researchers continue to prioritize ALD and CVD technologies for renewable energy and efficient energy storage, significant progress will be made to make these methods eco-friendly and cost-effective.
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References and Further Reading
[1] Dento Vaccum. (2023). What is Thin Film Deposition? [Online]. Dento Vaccum. Available at: https://www.dentonvacuum.com/resources/what-is-thin-film-deposition/#:~:text=Thin%20film%20deposition%20is%20an,precision%20optics%2C%20microscopy%20%26%20microanalysis%20sample (Accessed on May 19, 2024)
[2] Kunene, T., et al. (2022). Review of atomic layer deposition process, application and modeling tools. Materials today: proceedings.doi.org/10.1016/j.matpr.2022.02.094
[3] Ukoba, O., et al. (2019). Review of atomic layer deposition of nanostructured solar cells 4. In Journal of Physics: Conference Series. doi.org/10.1088/1742-6596/1378/4/042060
[4] Martin, PM. (2009). Handbook of deposition technologies for films and coatings: science, applications, and technology. [Online] William Andrew Publications. Available at: https://ebin.pub/handbook-of-deposition-technologies-for-films-and-coatings-science-applications-and-technology-3rd-ed-9780815520313-081552031x.html
[5] Ritala, M., et al. (2009). Industrial applications of atomic layer deposition. ECS transactions. doi.org/10.1149/1.3207651
[6] Provac. (2022). Chemical Vapor Deposition And Its Application. [Online] Provac. Available at: https://www.provac.com/blogs/news/chemical-vapor-deposition-and-its-application (Accessed on May 20, 2024)
[7] Hamilton, D. (2024). Atomic Layer Deposition (ALD) Used to Create Ultrablack Coating Perfect for Viewing the Cosmos. [Online] Available at: https://www.securities.io/atomic-layer-deposition-ald-used-to-create-ultrablack-coating-perfect-for-viewing-the-cosmos/ (Accessed on May 21, 2024)
[8] Kintek Solutions. (2024). What Are The Advantages Disadvantages Of Chemical Vapor Deposition Method? [Online] Kintek Solutions. Available at: https://kindle-tech.com/faqs/what-are-the-advantages-disadvantages-of-chemical-vapor-deposition-method (Accessed on May 22, 2024)
[9] Pedersen, H., et al. (2021). Green CVD—Toward a sustainable philosophy for thin film deposition by chemical vapor deposition. Journal of Vacuum Science & Technology A. doi.org/10.1116/6.0001125
[10] Verified Market Reports. (2024). Advancing Innovation: Unveiling Trends in the Atomic Layer Deposition (ALD) Market. [Online] Verified Market Reports. Available at: https://www.verifiedmarketreports.com/blog/top-7-trends-in-atomic-layer-deposition-ald/
(Accessed on May 23, 2024)
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