ALD (atomic layer deposition), originally known as ALE (atomic layer epitaxy), is a special kind of chemical vapor deposition process. It was firstly developed by T. Suntola in Finland as early as 1974. It was intended to fabricate polycrystalline luminescent ZnS:Mn and amorphous Al2O3 insulator films for electroluminescent flat panel displays. Because of the lack of understanding on the surface chemistry, there was not much improvement in this area till 1985. This technique then received a huge push in the mid-1990s when semiconductor industry was forced to decrease device dimensions and increase aspect ratios. In order to achieve faster computational speed and smaller compacter size, the thickness of these films has to be controlled precisely to the order of a few nanometers and avoid pinholes at the same time. ALD process became the logical choice because of its layer-by-layer deposition which produces high-quality film of uniform thickness and excellent conformality.

ALD Theory and Chemistry

ALD was developed from chemical vapor deposition (CVD) and uses similar precursors. For CVD, all the precursors are introduced onto the substrate surface simultaneously. But each of precursors in ALD was alternatively exposed on the surface. Generally, there are four basic steps for every ALD process as shown in Fig. 1. First, one precursor (A) gas or vapor was introduced onto the substrate surface, whereas chemisorption or surface reactions take place. The entire surface is saturated with A in a certain amount of time. Secondly, the ALD reactor is purged with inert gas like N2 to remove the unreacted precursor from the surface and ensure that only single layer of the precursor A is attached onto the surface. Thirdly, the other precursor (B) is released into the ALD reactor, where it reacts with the previous precursor (A) on the substrate surface. At last a purging step takes place to remove excessive B at the surface to form an atomic layer of targeted thin film (AB). This four-step cycle is repeated multiple times to produce thicker film on the surface.

The surface reactions in ALD are all self-limiting, which is the unique advantage of ALD (Sneh et al. 2002). Because of the self-limiting characteristics, the coating thickness can be precisely controlled. Furthermore, for the same reason, every part of the substrate surface takes the same amount of the precursor each cycle which results in a pinhole-free and conformal thin film.
One of the self-limiting mechanisms in ALD is sequential surface chemical reactions. The film deposition in this process is as a result of the chemical reactions between the reactive molecular precursors and the substrate. The first half reaction of process sequence is given in Fig. 2.

Taking the Al2O3 ALD process as example, the top figure shows that at the beginning, the substrate surface is activated by OH groups. This surface is then exposed to the first metal precursor Al(CH3)3 (TMA). TMA molecules react with the surface OH-reactive species to form OAl(CH3)2 groups. The reaction is given in Eq. 1:

Al(CH3)3 + -OH → OAl(CH3)2 + CH4 (1)

where CH4 is the by-product, and this reaction self-stopped when all the OH groups on the surface are converted to OAl(CH3)2 groups. Then the surface is fully passivated by TMA. Some of the Al(CH3)3 molecule reacts with two OH groups on the surface and produces two CH4 molecules. A purging process is then followed to remove the first precursor molecules physisorbed on the substrates and on the reactor wall. It also removes the precursor molecules which are flowing inside the chamber. Once the first precursor (TMA) is cleared, the second precursor, H2O, is released onto the surface fully covered with OAl(CH3)2. The reaction is given in Eq. 2:

2OAl(CH3)2 +3H2O →  O3Al2(OH)2 + 2CH4 (2)

As shown in Fig. 3, this reaction self-saturates until all OAl(CH3)2 are converted to O3Al2(OH)2. The H2O molecules completely passivate the CH3-terminated surface to stop further reactions after the saturation. The surface is then converted similar to the initial surface, terminated with OH groups. A purging step is followed to remove excessive H2O on the surfaces (substrate and reactor wall) and inside the chamber to reduce the chance to gas-phase reaction. Until then, the first atomic layer of Al2O3 is formed uniformly on the surface. These four steps are called one cycle and repeated to grown films layer by layer. Each deposition cycle includes two half reactions (Eqs. 1 and 2), and during each half reaction, the surface functional groups change from one surface species to another alternatively (George and Ott 1996).

ALD process typically runs in temperature range from 200o C to 400o C. If the deposition temperature is too high, chemical bonding cannot survive on the surface or the density of the chemically reactive site is reduced; thus, the deposition rates reduced. If the deposition temperature is too low, thermally activated chemisorption is suppressed and film-forming reaction rate decreases. As the deposition temperature increases, the deposition rate increases to a peak and then starts to drop. The temperature operation window for maximum deposition rate is relatively wide compared to CVD processes, which is much more temperature sensitive. Figure 4 illustrates the allowable temperature window for ALD. It shows the stability and reactivity of the precursors determined the working temperature of ALD process. Furthermore, as a general rule, the higher the process temperature within the allowable temperature window, the higher the quality of the thin film as it can anneal better at higher temperature. However, the process temperature needed for the plasma-enhanced ALD is much lower. The plasma-enhanced ALD integrates plasma source to the precursor feed and creates very active precursor species such as the radicals which react much faster to the surface species. The detailed introduction will be discussed in section “Plasma ALD.”

The pulsing time, the duration for a precursor gas to stay in the chamber before being purged, of the precursor into the reactor is important because it has to meet the minimum saturation time to create a fully covered monolayer. As shown in Fig. 5, the reaction rate reaches to maximum after certain length of the pulsing time. The reactivity of the precursors to the substrates and to each other plays a major role in determining the pulsing time. As the reactivity of precursors increases with the temperature increase, shorter pulsing time can be used. The other critical factor is the surface geometry of the substrate. The higher the surface area or surface aspect ratio, the longer the pulsing time is needed to let the precursor gas reach to every location of the substrate surface.
Based on its working mechanism, there are several critical elements to have good ALD performance: the volatility and stability of the precursor materials, their interaction with the substrate surface and each other, and their pulsing time. The volatility of the precursor material determines the working temperature for this ALD process. The stability of the precursor material affects the film thickness and uniformity. The pulsing time of precursor decides the total process time. The interaction of precursor with substrate surface and each other impacts the film quality. This part will be discussed in detail in section “Coating Material and Precursor.”

Advantages of ALD

Because of its self-limiting growth process, ALD has atomic level control of film composition. It enables ALD to produce sharp interface and superlattice. It alsomakes the interface modification possible. Since the film thickness depends only on the number of the deposition cycles, ALD is capable of precise, reproducible, and simple thickness control. For the reasons that ALD is only driven by interface reactions and has no requirement to the consistency of the reactant fluxes, large area and large batch deposition becomes practical. At the same time, ALD can achieve great conformality over any shape of substrate, and the final coating is not affected by the vaporization rate of the precursor as long as the surface is saturated for every step.
Also because of layer-by-layer growth mechanism of ALD at the surface of the substrate, the thin film with great qualities, such as high hardness, high density, and low defects, can be produced at lower temperature. The other advantage of ALD is that the scaling up in mass production is very straightforward. There are high-throughput ALD reactors developed aiming to increase the production yield such as the batch ALD reactor and spatial ALD reactor. Both of the reactors will be discussed in the section “High Throughput ALD Systems.” ALD is also powerful to create special structure materials. Due to the wide process temperature window, creating thin film with unique structure, such as nanolaminate, doped thin film, graded layer, and ternary thin film, can be done in a continuous process.
Other advantages include low deposition temperature to produce thin film on the thermally sensible substrates and wide range of selection of deposition material from oxide, nitride, and fluoride to elemental metal.
Compared to other methods to produce thin film, as shown in Fig. 6, ALD has very distinct advantages on the conformal coverage especially on high-aspect-ratio surface. As for liquid-phase coating method, the sol-gel process produces low-cost thin film, but the film uniformity is poor. Electroplating and electroless plating process is a surface-initiated process similar to ALD and produces relatively good coverage for 3D material, but its limitations are obvious. Its deposition speed depends on the ion diffusion in liquid phase, which is much slower than that of reactant diffusion in gas phase. It is usually for depositing much thicker film and for low-aspect-ratio structure.

As for vapor-phase deposition methods, CVD (chemical vapor deposition) provides better coverage than PVD (physical vapor deposition), but both of them have trouble to achieve uniform coating on the surface with very high aspect ratio feature. For the most time, the protruding areas are coated with thicker film. PVD operates by vaporizing the desired film material and condensates them onto various substrate surfaces. Those heavy “vapors” cannot diffuse to the back of substrate to form film. Even with the rotation of the substrates, because of the shadowing effect, the coverage on the high-aspect-ratio structure is still poor. CVD uses similar precursors as ALD to create gas-phase reaction, but the distinct difference is that reactions between the precursors in CVD occur frequently outside the substrate surface and ALD only allows those reactions to take place on the surface. CVD can deposit thin film on the shadowed area, but it is much thinner than the area fully exposed to the precursors. For both CVD and PVD depositions, pinholes are inevitable and thickness control cannot be done as accurately. Those intrinsic drawbacks limit their application on the thin-film deposition for the 3D substrates. As shown in Fig. 7 (Ritala et al. 1999), ALD is capable to produce ultra-uniform thin films even on extremely high-aspect-ratio surface, which are impossible to achieve by other methods.

Although there are distinct advantages over other thin-film coating methods, the limitation of ALD is also obvious. Its deposition speed is slow compared to other methods. Thus, the low productivity of ALD limits its application. But there are new types of ALD such as batch-type ALD and special ALD designed to improve the productivity. The details of such systems are going to be discussed in section “ALD Process and Equipment.”

Coating Material and Precursor

Precursor chemistry plays an essential role in ALD (George and Ott 1996). Precursors must be volatile and thermally stable. They are preferably liquids and gases, but solids are also used, provided they have no sintering-related problems under high temperature. They need to be efficiently transported within the reaction chamber, so that the ALD process will not be limited by precursor flux. The approximate vapor pressure of precursors should be about 0.1 Torr. The reactivities of precursors to the surface and between each other are very important. Precursors must chemisorb on the surface very quickly or react rapidly with surface groups. The aggressive reactions help to attain saturation over the entire surface within short cycle times, complete reactions with high film purity, and avoid gas-phase reactions. On the other hand, precursors should not etch and/or dissolute into the substrate, as this would prevent the self-limiting film growth. Precursors need to be very stable within the allowable temperature window and should not undergo self-decomposition. Self-decomposition destroys self-limiting film growth, which results in deterioration of thickness uniformity, accuracy, and film contamination level.
Purging is crucial to separate the gaseous contact between the precursor molecules and remove excessive precursors on the surface and reactor wall. The substrate temperature, reactor-wall temperature, and purging time should be fixed so that the monolayer remains on the surface and does not desorb during the purging period and maximum cleaning effect can be attained (Suntola and Simpson 1990).
Reported materials deposited by ALD (selected) are given in Table 1, ranging from oxides, nitrides to fluorides and others.
Table 2 shows the corresponding precursor combinations for selected coating materials reported in the literature, and its application is listed for reference.
The process details of typical thin-film materials for each category will be discussed in the following sections.

Inorganic Thin Film

Metal oxide, metal nitride, and metal chalcogenides are discussed here in detail. Other inorganic materials like elemental film will be discussed in section “Plasma ALD.” The metal fluoride film will be briefly introduced in section “Optical Applications.” More information can be further traced in the review paper of Puurunen (Miikkulainen et al. 2013).

Metal Oxide

High-k oxides for both transistors and DRAM capacitors are the major driving forces for ALD metal oxide research. The research and development on transparent conducting oxides, such as ZnO:An, represent the increasing need of organic and flexible electronics and displays. The properties of the oxide films strongly depend on the crystal structure.
The property of the thin film is affected by not only the difference between amorphous and crystalline phases but also the difference between the different crystalline phases of the oxides. Here two kinds of oxide film are discussed to explain the ALD process to produce metal oxide thin film and their properties.

Al2O3

The working mechanism of Al2O3 deposition was discussed in section “ALD Theory and Chemistry,” and it has been developed as model ALD system (Puurunen 2005). As shown in Table 2, the metal precursors for Al2O3 include AlCl3, AlBr3, and Al(CH3)3 (trimethylaluminum [TMA]). Because of the highly reactive nature of TMA, it is now the most used precursor for ALD to synthesize Al2O3 thin films. Besides water, ozone is also used with TMA as the other precursor. The use of O3 instead of H2O is because in some cases O3 has higher activity in ligand elimination. Since O3 does not absorb onto the reactor walls as easily as H2O, the purging process is more thorough. The focus here will be on ALD Al2O3 using TMA and H2O.
The growth of Al2O3 per ALD cycle is much less than one Al2O3 “monolayer.” The growth rate of Al2O3 per AB cycle is 1.1–1.2 Å which is estimated using the density of 3.0 g/cm3 for Al2O3 ALD films grown at 177° C. Quartz crystal microbalance (QCM) is usually employed to record the mass increase during each ALD process to estimate the growth rate (Groner et al. 2004). The monolayer thickness calculated from bulk Al2O3 is 3.8 Å, which is much thicker than the growth per AB cycle. The big difference is due to the surface chemistry of Al2O3 ALD. For every AB cycle, the growth of every layer is not always matched with stoichiometry of “monolayer.” For instance, when the ALD process temperature increases from 177° C to 300° C, the number of surface “OH” or “CH3” species on a given area decreases. This in turn decreases the growth rate for every AB cycle (Ott et al. 1997).
Al2O3 films grown below 600° C are amorphous regardless of the types of the substrates. Post-deposition annealing of ALD Al2O3 thin film changes the crystallinity of the film to increase the dielectric constant and hardness. ALD of alumina films that are deposited above 600° C are crystalline when AlCl3 is employed as the precursor. The stability of AlCl3  at such high temperature is the major concern.
Because of the continuous and pinhole-free nature of Al2O3, it presents better electrical properties than the film made from other techniques. The ALD Al2O3 films have a dielectric constant of ~7 and display very low electron leakage (Groner et al. 2002), which are distinct signs of the absence of any defects or pinholes in the ALD Al2O3 films. Based on these excellent properties, the ALD Al2O3 film was used as gate oxides (Huang et al. 2005) and to passivate semiconductor (Ye et al. 2003).

ZnO

ZnEt2 and H2O are commonly used to produce ZnO ALD film, and two half reactions are proposed to occur as (Ferguson et al. 2005)

- OH + Zn (CH2CH3)2 → - OZn (CH2CH3) + C2H6 (3)

OZn(CH2CH3) + H2O → - OZnOH + C2H6 (4)

where –CH2CH3 and –OH are surface species after each step. The two half reactions are repeated to deposit thicker ZnO films. The growth rate is 2.2–2.5 Å per cycle at 100–160° C.
ZnO is a versatile material because of its beneficial physical, electrical, and chemical properties. ZnO is a semiconductor with a wide bandgap of 3.37 eV and large exciton binding energy, 60 meV, which makes it a promising optoelectronic material. Zinc oxide thin films made by ALD are typically crystalline even when deposited at low temperature. Only on certain materials thin amorphous zinc oxide films can be produced by ALD. Crystalline ZnO films produced by ALD are all of hexagonal phase. The crystal orientation determines the behavior of hexagonal ALD ZnO. The orientation appears to be a function not only of substrate and deposition temperature but also of other deposition conditions, such as precursor purge time.

Metal Nitride

Metal nitrides are hard, chemically resistant, sometimes catalytically active, and often electrically conductive. Since the increasing interest in applications as diffusion barriers and electrodes in microelectronics, nitride deposition by ALD has developed rapidly. Metal chloride precursors or other halides and ammonia are mostly used for growing ALD metal nitride thin films. These processes give nitrides at relatively high temperatures (typically 350–500° C) (Satta et al. 2002). The residues of the respective halogens are one of the concerns for this precursor. Metal-organic alkylamide-based compounds have been targeted for some transition metals (TiN, ZrNx, MoNx, HfNx, TaNx, WNx) as the metal precursor in order to solve this problem. With the alkylamides, the process temperatures are much lower (typically ca. 150–250° C) than with the halides (Elam et al. 2003). But the biggest limitation is that the thermal decomposition temperature of amide compounds is very low (even during storage).

TiN

TiN is one of the most investigated ALD processes for metal nitride (Miikkulainen et al. 2013). TiCl4–NH3 process is the most commonly applied process to deposit TiN. Since the oxidation state of Ti in the reactants is +4, whereas it is +3 in the desired product nitride, TiN, a reducing agent is needed. In many cases, the nitrogen source (e.g., NH3) also serves for this purpose, while in other processes, separate reducing agents have been added. Growth-per-cycle values between ca. 0.2 and 0. 4 Å  have been reported (Cheng and Lee 2006). Cl and H are the surface species for each step. As seen in Eq. 5, the surface –OH group reacts with TiCl4 to form –OTiCl3 species and generate HCl as by-product. The surface –OTiCl3 species then encounter NH3 to form –TiNH2 (Eq. 6) with NH3 also reducing Ti from oxidation state of +4 to +3. This half reaction terminates at –TiNH2. For the next cycle (Eq. 7), the surface species are –NH2, and they react with TiCl4 to form another layer of TiN:

In addition to inorganic halides, metal-organic precursors have been used for TiN deposition. They have been used for special cases, where the substrates, such Cu, are sensitive to the halide residue or where the ALD reactor material, such as stainless steel, is not compatible with halide. Growth-per-cycle values are usually very high (2 Å up to several nanometers per cycle) when alkylamide precursors are used. This growth rate equals to several TiN monolayers per cycle which is due to the decomposition of the alkylamide precursor.
The amorphous/crystalline nature of TiN depends on several ALD process parameters. When the metal halide reactants are used, the TiN films are mainly crystalline, while TiN films deposited from the alkylamide reactants are mainly amorphous – both with some exceptions. Temperature and impurities also affect crystalline form of the film.

Metal Chalcogenides

ZnS

From metal chalcogenides, most researchers paid their attention to ZnS because of its applications in the thin-film electroluminescence (EL) panel (Suntola and Antson 1977). ZnCl2 and H2S were first employed as precursors in 1977, but the halide residue is detrimental to the performance of the EL panel. Since then, Zn acetates, Zn (CH3)2, Zn(thd)2 (thd = 2,2,6,6-tetramethyl-3,5-heptanedione), and Zn(C2H5)2, have been studied as zinc precursors. For ZnCl2 and Zn(C2H5)2, similar growth rates of about 1 Å/cycle can be achieved at 500° C and 150° C, respectively. Process temperature for alkyl compounds is room temperature which for halides, it needs to be over 500° C. Cl (or CH2CH3) and H are the surface species for each step.
Because it is known that crystalline order is beneficial for light emission, there has been no interest in amorphous ZnS films. The phase depends on the temperature and at atmospheric pressure. The low-temperature cubic phase transforms to hexagonal form at 1,020° C.

Other Special Material

Polymer

Other than inorganic materials as metal oxide and nitride, ALD can be used to grow organic polymers employing similar self-limiting surface reactions. This is described as molecular layer deposition (MLD) or alternating vapor deposition polymerization (AVDP) because a molecular fragment is deposited during each ALD cycle. Figure 8 illustrates the MLD process (Du and George 2007). Acyl chlorides and amines were used to produce various polyamides. For example, poly (p-phenylene terephthalamide) (PPTA) MLD layer is made by precursor of terephthaloyl chloride (ClCOC6H4COCl) and p-phenylenediamine (NH2C6H4NH2) (PD). Every precursor is a bifunctional molecule. The surface reactions for PPTA MLD are proposed as follows:

where –NH2 and –Cl are the surface species after each reaction. The PPTA MLD growth is linear but growth rate between 0.5 and 4.0 Å per AB cycle was achieved for individual experiments. This variation was attributed to varying numbers of “double” reactions between the bifunctional precursor and the surface species (Adamcyzk et al. 2008).
Similar processes have been developed to produce other organic polymers using MLD techniques. For example, 1,4-phenylene diisocyanate and ethylenediamine were used to make MLD polyurea (Kim et al. 2005). Area-selective patterning and orientation control (Yoshimura et al. 2006) of polyazomethine MLD have been also demonstrated.

Special Substrates

Because of the advantages of ALD process, researchers have been using it to produce uniform films on various substrates from ceramic, metal, to fiber for various applications. Among those substrates, three kinds of special substrates for ALD process are discussed here to illustrate its versatility.

Nanoparticles and Other Nano-objects

ALD has been used to change the surface chemical properties of particles while retaining the bulk properties of the original particles. ALD thin layer can serve for purposes as the oxidation protection, electrical conduction, optical enhancer, and mechanical supporter. It is very natural to produce core/shell structures by ALD.
Proper agitation in particle ALD is necessary. During agitation, the upward force caused by vibration or upward gas pulse to particles bed equals the downward force of gravity. The equal forces result in the constant movement of the particles. Without agitation, a static particle bed takes long for gaseous ALD precursor to infiltrate, similar to the porous substrates. There is also a possibility of “glue effect” after the ALD film is formed when the particles are not moving. The so-called glue effect is referring to the newly formed ALD thin film that connects the neighboring particles and “glues” them together. With agitation, the moving particles are fully exposed to the precursors in a much shorter time, and chance for particles to glue together is very low. Although particle aggregates still form during agitation, the aggregation processes are dynamic and the constant exchange of particles between the aggregates prevents the particles from being “glued” during ALD. The specially designed fluidized and rotary ALD reactors are going to be discussed in the ALD reactor section.
Ultra-uniform coatings of ALD Al2O3  were observed on BN particles with a platelet shape as in Fig. 9 (Wank et al. 2004). Producing ALD thin film on other large quantity of nano-objects such as nanotubes and nanowires also faces the same challenges as to nanoparticles. The fluidized or rotary ALD reactor is needed to agitate these nano-objects to obtain efficient surface reactions (Lee et al. 2003).

Polymer

Because of the thermal stability of the polymer material, low-temperature deposition is needed to coat thin film. ALD has produced thin film on polymers to alter its surface functional groups, to create unique inorganic/organic polymer composites, and to deposit gas diffusion barriers.
As low as 33° C as the deposition temperature for ALD Al2O3 has been developed. The resulting thin film has very low surface roughness values, low leakage currents, high dielectric constants, and growth rates in excess of 1 Å/ cycle. These properties are comparable with the properties of ALD Al2O3 films grown at higher temperatures of 177° C. However, decreasing densities and increasing hydrogen concentrations were observed at lower temperatures.
Since most polymers do not contain the necessary surface chemical species to initiate ALD, it has a different growth mechanism compared to ALD on other substrates. For ALD Al2O3 on polyethylene using TMA and H2O as precursors, the TMA precursor initiates the reaction by firstly diffusing into the polyethylene and then reacting with the subsequent exposure H2O to form –AlOH species. These –AlOH species then serve as the bed to grow more –AlOH cluster in the nearsurface region of the polyethylene. With more –AlOH clusters forming on the surface, they start to coalesce and cover the whole surface to stop the diffusion process. A normal Al2O3 ALD process is then followed after that. This mechanism is illustrated in Fig. 10 (Wilson et al. 2005).

ALD thin film on polymer is mostly used to serve as the water barrier and gas barrier. Detailed discussion is in the application on Organic light-emitting diode (OLED). ALD Al2O3 has also been effective as a capping layer and a gate dielectric for polymer-based transistors. Surface modification of natural fiber and woven fabric materials has utilized ALD Al2O3.

Biological Template

Low-temperature ALD also enables uniform coating on biological templates. One of the first demonstrations of ALD on biological templates was ALD TiO2 and ALD Al2O3 on tobacco mosaic virus (TMV) and ferritin (Knez et al. 2006). The outer sheath of the virus is replicated by a thin film made of TiO2. The duplication of the shape of butterfly wing by ALD Al2O3 was also accomplished (Huang et al. 2006).

Selective-Area ALD

To realize the selective-area ALD, a mask has to tightly cover the surface and block the diffusion of the precursor gases to the protected areas. For example, the patterned self-assembled monolayer (SAM) is a very good mask layers because of the strong chemical bond between the monolayer and the surface. The SAM passivate the surface against the ALD growth, and ALD film is deposited only on areas without an SAM. Alkyl silane SAM on hydrogen-terminated silicon (Farm et al. 2008) and alkyl thiol SAM on metals (Farm et al. 2012) helped to form patterned ALD layer. Unreactive and thermally stable polymer films have also been used as mask layers for selective-area ALD (Gay 2010). A lift-off process is carried out to selectively remove the unwanted areas.