Atomic layer deposition (ALD) originated from atomic layer epitaxy, which was introduced in 1970 and initially used in electroluminescent displays. It rapidly revolutionized semiconducting applications, pushing the limits of Moore’s law. Today, ALD is a well-established ultrathin film deposition method, allowing control over the film’s composition, conformity, self-saturation, thickness, and uniformity.1
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Since Intel’s first industrial introduction of ALD in 2007, it has found applications in sensors, energy storage, catalysis, memory devices, and several others.1 ALD facilitates atomic and near-atomic scale production of various materials, structures, components, and systems.2
Principles and Mechanisms of ALD
ALD is a bottom-up method typically involving four steps. First, a substrate is exposed to a precursor that undergoes physisorption and chemisorption.2 The second step involves purging to eliminate by-products and the unreacted precursor.
The precursor and a co-reactant undergo a self-limiting surface reaction on the substrate. Finally, another purge removes by-products and unreacted elements, leaving a deposited layer.1 The process is repeated multiple times to obtain a layer-by-layer deposition with the desired thickness.
ALD operates within a specific temperature window to ensure the self-saturation of the resulting film.1 Beyond this range, issues like inadequate condensation or thermal decomposition of the precursor can occur.2
Material Deposition Techniques
Various ALD techniques have been developed for material deposition, including thermal (TALD), plasma-enhanced (PEALD), flash-enhanced (FEALD), and photo-assisted (PAALD), with TALD and PEALD being the most common. 1
TALD operates between 150 to 350 °C, offering precise thickness and geometry control. However, its application is limited by the high-temperature range, which is overcome using PEALD.1
PEALD uses strongly reacting plasma species, allowing a low-temperature deposition without impacting film quality. This technique uses several precursors to produce pure films on heat-sensitive substrates at a faster growth rate. The highly reactive catalysts produced by PEALD boost the deposition process.1
PAALD uses ultraviolet (UV) light as the energy source for deposition reactions, which allows better control of film properties by varying its intensity, wavelength, and exposure time. Alternatively, in FEALD, the substrate is exposed to high-intensity xenon flash lamps (visible and near-infrared light).
While these ALD techniques produce high-quality films, they have limited application due to the need for special reactors.1
Applications in Thin Film Fabrication
ALD exhibits great potential as a thin-film fabrication method in various applications. It supports the continuous downscaling of integrated circuits (ICs), which form the foundation of information technology.
Layer-by-layer ALD enhances the efficacy of barrier/seed layers and metal contact/interconnects, reducing interface-related defects in ICs. Additionally, low-temperature deposition using PEALD prevents functional losses in thin films.2
In photovoltaics, spatial ALD (SALD) is preferred for passivation in solar cells due to its low cost, high throughput, and easy-to-scale deposition of metal oxides, significantly increasing the efficiency of solar cells.2
Organic light-emitting diodes (OLEDs) and quantum light-emitting diodes (QLEDs), commonly used in displays, suffer erosion by water and oxygen during processing, resulting in defects and short service life. Thin-film encapsulation by ALD enhances the water-oxygen barrier without affecting the flexibility and luminescence of OLEDs. ALD-deposited conformal films of atomic-scale thickness can also stabilize quantum dot QLEDs.2
In energy applications, ALD is used to coat nanoparticles with ultrathin films, improving their stability without negatively impacting their functionality. ALD-coated energetic nanoparticles are utilized in batteries and high-energy propellants.2
Advantages Over Other Deposition Methods
Chemical vapor deposition (CVD) and plasma-enhanced CVD (PECVD) are generally comparable to ALD. However, CVD and PECVD require higher vacuum levels, have a lower working temperature range, a feature size limitation of 60 nm, conformity only up to 70 %, and particle-contamination risks. In contrast, ALD offers sub-nm feature size, up to 100 % conformity, and minimal particle-contamination risks without high vacuum requirements.3
Sputtering is widely used in industries for large-area thin film deposition. However, sputtered films suffer from varying surface roughness, possible plasma damage, inefficient doping, poor step coverage, lack of sharp interfaces, and pinholes.3 The self-limiting property of ALD results in soft deposition; thus, it can overcome these issues.2
ALD can also address the challenges of conventional top-down photolithography using area-selective methods that allow self-aligned nanopatterning with atomic-scale accuracy.2 Additionally, advances like SALD can help achieve large-area thin films at lower costs than conventional methods.1
Challenges and Limitations
Although ALD is incredibly promising for fabricating highly functional thin films, its commercial application is limited by various challenges, including low growth rate, substantial material waste, disproportionate energy usage, nanoparticle emission risks, and intense processing.3,4
ALD techniques for several metals, halides, sulfides, nitrates, organometallic reactants, and mixed metal oxides are inefficient due to difficult control over the reactivity of the precursors.4 Additionally, the commonly used precursors are expensive and environmentally harmful.3,4
The inherently self-limiting nature of ALD makes it slower than conventional film growth methods like sol-gel, sputtering, pulsed-laser deposition, and CVD, raising questions about its economic feasibility.4 Current reactors are also complex, requiring a compromise between throughput and accuracy.2
Recent Advances and Research Case Studies
Significant efforts are being made to overcome the challenges inhibiting the applications of ALD.
For instance, a recent study in the Journal of Vacuum Science & Technology A explored the use of deep neural networks to optimize ALD processes. Machine learning was employed to predict optimal saturation times in an ALD reactor based on thickness values from a single trial growth, reducing the number of experiments needed to develop novel ALD methods in existing reactors.5
Another recent study in Dalton Transactions highlighted the upcoming atmospheric-pressure ALD techniques with potentially lower reactor costs and novel applications in high-porosity/three-dimensional materials. The researchers focused on atmospheric temporal ALD (t-ALD) for high-porosity particle coatings, functionalization of capillary columns for gas chromatography, and membrane modification in water treatment and gas purification.6
Industrial Applications and Commercialization
The application of ALD in industrial processes offers multiple advantages. For example, precise ALD-based fabrication of advanced catalysts with enhanced stability, activity, and efficiency accelerates and guides complex chemical reactions.2
ALD can deposit functional layers such as thermal barrier coatings for gas turbines and anti-corrosion and UV-resistant coatings for aircraft bodies, overcoming typical bottlenecks in aerospace coatings like fall-off, corrosion, and wear.2
ALD technology has also demonstrated significant potential in biomedical device manufacturing, including micro/nanorobots for diagnosis and drug delivery, sensor coatings, and surface modification of implants.2
Optimized ALD processes can produce complex nanostructures and improve device performance for future intelligent manufacturing.2
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References and Further Reading
1. Oke, JA., Jen, T. (2023). Atomic layer deposition thin film techniques and its bibliometric perspective. The International Journal of Advanced Manufacturing Technology. hdoi.org/10.1007/s00170-023-11478-y
2. Zhang, J., Li, Y., Cao, K., Chen, R. (2022). Advances in Atomic Layer Deposition. Nanomanufacturing and Metrology. doi.org/10.1007/s41871-022-00136-8
3. Oke, JA., Jen, TC. (2022). Atomic layer deposition and other thin film deposition techniques: From principles to film properties. Journal of Materials Research and Technology. doi.org/10.1016/j.jmrt.2022.10.064
4. Ansari, MZ., Hussain, I., Mohapatra, D., Ansari, SA., Rahighi, R., Nandi, DK., Song, W., Kim, S. (2023). Atomic Layer Deposition–A Versatile Toolbox for Designing/Engineering Electrodes for Advanced Supercapacitors. Advanced Science. doi.org/10.1002/advs.202303055
5. Yanguas-Gil, A., Elam, JW. (2022). Machine learning and atomic layer deposition: Predicting saturation times from reactor growth profiles using artificial neural networks. Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. doi.org/10.1116/6.0001973
6. Chen, M., Nijboer, MP., Kovalgin, AY., Nijmeijer, A., Roozeboom, F., Luiten-Olieman, MWJ. (2023). Atmospheric-pressure atomic layer deposition: recent applications and new emerging applications in high-porosity/3D materials. Dalton Transactions. doi.org/10.1039/d3dt01204b
