From the presented overview of the research and development on ALD processes, it is clear that ALD is a technique of high interest for anyone who wants a high-quality thin film. The unique depositing mechanism ensures the uniform and pinhole-free thin films. Because of the layer-by-layer deposition nature, the crystalline property is much improved which results in better chemical, thermal, electrical, and mechanical performance. ALD builds up thin film not from gas-phase reactions but through surface reactions, and its reaction speed is controlled mostly by the gas diffusion speed. Therefore, even on very high-aspect-ratio structure surfaces such as nanoparticles, nanowires, and 3D structures, ALD is able to produce uniform and conformal coating. Plasma-enhanced ALD uses plasma to activate precursor molecules to achieve higher reactivity on the surface, which reduces the temperature to form thin film. It enables deposition of ALD thin films on thermally sensitive substrates such as biodegradable polymer and biological template. ALD is used to produce various inorganic materials ranging from metal oxide, nitride, sulfide to fluoride. Metal thin film is also obtained by ALD with reducing agent used during the process.
The development of ALD reactors is another focus of ALD research. The introduction of the flow gas to the ALD reactor marks the first productivity leap of. The integration of the plasma power sources to ALD reactors versatile the usage of ALD and greatly shortens the gap the real application of ALD system. The first application of ALD system in industry occurs in producing high-quality insulating layer for semiconductor and solar cell sections. Because of the high equipment cost, the relative low throughput hindered the expansion of the ALD to other industry field. This driving force pushed the emergence of the recently high-throughput reactor designs such as batch ALD reactor and spatial ALD reactor. Other derivative like roll-to-roll ALD reactor is also invented to meet the needs of mass production.
Increasing the versatility and productivity is the main theme for future development of ALD systems. To increase the versatility, researchers in ALD field focus on growing different functional thin films to different substrates for novel applications. To increase the productivity, more exploration is needed to coat faster and on bigger substrates. The research and development of batch ALD and spatial ALD system are going to be paid more attention too.
New energy exciting methods are equipped to ALD to promote the surface reaction. Since the conventional low-pressure plasmas cannot be used for the spatial ALD, atmospheric pressure plasma is a logical choice to be used to the spatial ALD in the future. The exciting methods used now include thermal and plasma energy. Photons, laser, and microwave can be the next exciting method too for ALD application. The research on the precursor for ALD is also crucial as the surface chemistry determines the ultimate speed of ALD process. More user-friendly precursors are needed for ALD process in the future for new application.
ALD is capable to produce superior property thin film with precise control, and it provides an alternative solution other than traditional CVD and PVD for special coating needs. The industry applications of ALD are going to be expanded from existing area like semiconductor and solar cell to other fields like oil and gas, marine, precision engineering, medical device, food processing, optical device, and others. With the continuous demands in top-quality coating for products from every aspect of the life, ALD will become more and more popular.

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