As stated before, the application of ALD in industry right now is limited within the semiconductor and solar cell areas. Because of the superb quality of ALD thin film, its applications on other areas are very promising. Here in this section, the areas of potential usage of ALD are introduced.

Semiconductor

Applications in semiconductor industry have been the main driving forces for the recent development of ALD technologies. The International Technology Roadmap for Semiconductors (ITRS) has included ALD for high-dielectricconstant gate oxides in the MOSFET structure and for copper diffusion barriers in back end interconnects. Ultrathin high-к dielectric films with precise control of the thickness are a major bottleneck for the semiconductor industry. Traditional CVD and PVD processes are not capable to realize such task. The unique capability of ALD to engineer composite films helped to solve this problem (Sneh et al. 2002).
Most semiconductor ALD applications aim at the thickness range from 20 to 200 Å. With the help of batch and spatial ALD, industry standard throughput of 10–20 wafers/h/module is not very difficult to achieve. Temperature uniformity is another key requirement because the deposition rate depends on temperature. Right now the temperature control across the wafer by the batch-type ALD is good enough to have the max thickness inhomogeneity in wafer and in batch within 1 %.

MEMS

The ongoing reduction in the dimensions of MEMS devices and components dramatically increases the reliability requirement of every part within. The existing techniques such as CVD and PVD are not able to meet the requirement for some MEMS devices. ALD is a very promising replacement process to accomplish such goal since the coatings made by ALD are usually denser and smoother and with less defects. Researchers have used ALD process to build multiple parts of MEMS such as protective layers for biocompatible coating, high-dielectric-constant layers in RF MEMS, low-temperature insulating layers, lubrication layer of moving parts, or controlled gap filling (de Groot et al. 2009). Since ALD process can be easily integrated into the standard CMOS processing, there are not much obstacles for commercial MEMS fabrication lines to use it in the future.
For example, the sliding elements of MEMS devices are prone to have wear failures as they were rubbed constantly. A protective interface to MEMS structures helps to reduce such failures. It has been shown that with a 10-nm-thick Al2O3 coating on polysilicon structure via ALD (Mayer et al. 2003), less wear particle is generated than a native oxide layer in a MEMS microengine.
TiN can be used as diffusion barrier layers for through-silicon vias (TSV) in the MEMS industry, but the high deposition temperature (> 300o C) in conventional CVD or PVD processes limits its application in this field. PEALD can produce high-quality TiN film at low temperature (< 200o C) which is compatible with MEMS fabrication processes. The surface morphology, stress, electrical, and wet etching properties of the thin film are well satisfied the requirement of the barrier layer of as a ruthenium (Ru) preseed liner (Samal et al. 2013).

Corrosion and Wear Resistant

The ability of ALD thin film to cover high-aspect-ratio surface without pinhole enables its excellent corrosion-resistant capability. For the other coating methods, such as CVD, PVD, and sol-gel, the anticorrosion coating cannot uniformly cover the entire surface, and the defects are almost inevitable. Since every part of the surface can be covered with conformal and almost defect-free film, there is no weak point against corrosive species in an ALD process part. For example, the conformal, adherent, and defect-free TiO2/Al2O3 coatings by ALD strongly improve the corrosion resistance of stainless steels in chloride-containing media. At least two orders of magnitude the passive current density decreases, and breakdown potentials shift toward more positive values (Marin et al. 2011).
The metal oxide layer made by ALD is also used for solid lubrication purpose since, compared to bulk oxide ceramics, the nanocrystalline oxide ceramics have better mechanical and tribological properties. It has been shown that oxide nanolaminates (ZnO/Al2O3/ZrO2) made by ALD on porous carbon-carbon composites (CCCs) and graphite foams have better thermal stability and sliding wear resistance (Mohseni and Scharf 2012). A significant reduction in the sliding wear factor (2.3 x 10-5 to 4.8 x 10-6 mm3/Nm) and friction coefficient (0.22–0.15) were observed with the ALD nanolaminate in comparison to uncoated CCCs.

Optical Applications

Metal fluoride thin films have excellent light transmission in a broad wavelength region and have been used for many optical applications. Mainly physical vapor deposition (PVD) method has been used to deposit these metal fluoride films before. However, the main problems of evaporated fluoride films are their porous nature and low packing densities. The thickness control of the multilayer stacks is also very challenging. The intrinsic advantages of ALD can solve these issues and produce high-quality films.
The metal precursors to produce ALD metal fluorides are metal-thd (2,2,6,6-tetramethyl-3,5-heptanedione) compounds. HF, by thermally decomposing NH4F, was used as the fluorine precursor as in ALD for depositing CaF2, ZnF2, and SrF2 films. However, HF is not an ideal choice for ALD because in addition to its unsafe chemical nature, it etches glass and many oxides. TiF4 is used then to improve the quality and productivity in ALD (Pilvi et al. 2007).

Organic light-emitting diode (OLED)

Most organic devices are extremely sensitive to moisture, and its moisture-driven degradation can be only prevented by proper encapsulation. OLED encapsulation has rigorous requirement of the water vapor transfer rate (< 10-5 g/m2/day). However, flexible polymer substrates are highly permeable to all atmospheric gases. The water and gas diffusion barrier quality is directly responsible for the quality and lifetime of products with polymer materials as substrates. The encapsulation of OLEDs today remains heavily dominated by relatively simple cover glass technologies using epoxies to seal the edges. But OLED device market is changing to larger and flexible format and has longer lifetime requirements. The existing encapsulation methods are not suitable for future development. Thus, the industry is shifting toward simpler, better, and less expensive barrier technologies in order to unlock the huge market potentials.
For example, ALD Al2O3 film as thin as 10 nm on polyethylene naphthalate (PEN) and Kapton reduces the water vapor transmission rate (WVTR) over three orders of magnitude to 10-3–10-4 g/m2/day (Carcia et al. 2006). Even lower WVTRs were achieved by bilayer or multilayer barriers using ALD Al2O3 and SiN by plasma-enhanced CVD (Groner et al. 2006), ALD Al2O3 and rapid ALD SiO2 (Carcia et al. 2009), and ALD Al2O3 and ALD ZrO2 (Dameron et al. 2008). ALD offers the potential for deposition of pristine, pinhole-free thin layers of encapsulation material with excellent barrier performance which could provide the solution to the requirement of the OLED industry.

Lithium Ion Battery

The capacity fading and potential safety risks are the major challenges for lithium ion batteries (LIBs). The origins of these challenges lie in the unwanted side reactions during charge-discharge cycles, which cause electrolyte decomposition, solid electrolyte interphase formation, and active material dissolution. Surface modification of LIB electrodes is a viable strategy to address these issues.
The solution-based methods are unable to modify prefabricated electrode surface evenly. ALD was demonstrated to be highly effective in enhancing the electrochemical performance of LIB electrodes, for both the cathode and the anode. It is a cost-and time-saving coating technique for LIBs. A number of coating materials (Al2O3, TiO2, ZnO, and TiN) were investigated (Meng et al. 2012). The ultrathin ALD films down to subnanometers on the electrodes are uniform and conformal with very facile operation. They can be successfully deposited at low temperatures (typically less than 200o C with possibility of going down to RT). Specifically, ALD thin films serves to fasten micro- and nano-sized particles to electrodes, protect metal surfaces against diffusion and corrosion, tailor the composition of porous material surface passivation, and passivate electrode surface in general.

Solar Cell

ALD has unique features which can be used to face processing challenges in different types of solar cells. The uniformity and the pinhole-free nature make the ALD an attractive approach to deposit CdS-free buffer layers in CulnS2 (CIS) and Cu(ln,Ga)S2 (CIGS) thin-film solar cells. For a-Si:H solar cells, ALD is also used to make ALD ZnO thin film as front contact. For c-Si solar cell, ALD produces excellent surface passivation layer made of Al2O3 films on both low resistivity n-type and p-type Si (Hoex et al. 2006b).
ALD is also used to make ZnO nanotubes and nanowires for dye-synthesized solar cells. The ALD Al2O3 passivation film also helps to improve the efficiency of the dye-synthesized solar cells. Since only ultrathin films are needed and the emerging of the batch ALD and spatial ALD reactors, the solar cell manufacturing is adopting this method to make the passivation layer.

Packaging Material

Biopolymers (modified natural polymers and biodegradable synthetic polymers of bio-based monomers) have been considered to be the environmentally friendly solution for packaging materials in the future to replace the nonbiodegradable polymers like polystyrene, polypropylene, and polyethylene terephthalate. However, similar to OLED, poor barrier properties, especially of natural polymers, and sensitivity to moisture remain to be the block to wider use of these materials. One way to improve the barrier properties of biopolymers is to coat them with a thin inorganic layer. Suitable depositing methods need to deposit this thin film at very low temperature but with low thickness and high quality. Low-temperature ALD is able to achieve this goal. The developed coatings with ALD on biodegradable polymers have excellent gas and water vapor permeation resistance (Hirvikorpi et al. 2010), which reached the level required in commercial packaging applications for dry food and pharmaceuticals.