Conformality in atomic layer deposition: Current status overview of analysis and modelling

Atomic layer deposition (ALD) is a gas phase thin film deposition technique which has been discovered and developed independently in the 1960s in the Soviet Union and in 1974 in Finland.^1–3 This technique is characterized by exposing the substrate to an alternating sequence of vapor phase reactants. Due to the self-saturating nature of the surface reactions, the film thickness can be controlled at the atomic scale.^4–8 A typical ALD process consists of several ALD cycles with each ALD cycle comprising four characteristic steps, which are shown in Fig. 1 for the prototypic^4–6 trimethylaluminum (TMA)/H₂O process:

1. Step 1: The first reactant A (TMA) reacts in a self-terminating way with the available functional groups on the OH-terminated surface.

2. Step 2: The excess of reactant A (TMA) and the gaseous by-products (CH₄) are purged or pumped away.

3. Step 3: The second reactant B (H₂O) reacts in a self-terminating way with the adsorbed species (A) on the surface.

4. Step 4: The excess of reactant B (H₂O) and the gaseous by-products (CH₄) are purged or pumped away.

In this review, we will call steps 1 and 3 the first and second reactions of the ALD cycle, respectively. After each ALD cycle, a certain amount of material (coating) is deposited on the surface, which is called the growth per cycle (GPC). The GPC can be expressed in various units such as those of the thickness (e.g., nm), mass gain (e.g., g), and an increase in the areal density (e.g., atoms/nm²).

As opposed to “line-of-sight” deposition techniques such as physical vapor deposition (PVD),^9,10 ALD has the capability to grow uniform and conformal films in 3D structures with complex shapes and with a large depth to width ratio or in more general terms a large aspect ratio.^11–15 For deposition techniques that are flux controlled [such as chemical vapor deposition (CVD)¹⁶ and PVD], film growth depends on the local gas flux. Because of the inherent kinetics of gas transport within narrow trenches, the flux of reactant molecules can be several orders of magnitude larger near the entrance as compared to the bottom of the structure. Therefore, the entrance region to narrow trenches tends to get clogged at the beginning of the deposition, making it difficult for reactant molecules to diffuse deeper into the structure. Various approaches exist to improve the conformality of CVD processes, e.g., inhibiting the growth,¹⁷ using pulsed CVD,^18,19 and using surfactants to catalyze the growth.^20,21 Also during ALD deposition, the entrance region will receive a higher flux of reactant molecules. However, the self-saturating nature of the surface reactions during ALD results in surface-controlled deposition. Higher flux near the entrance region will result in faster coverage at this location, but once the surface is saturated, no further reaction can take place at this site, avoiding clogging of the trench entrance and allowing the reactant molecules to diffuse deeper into the trench and coat the entire structure.

The miniaturization of semiconductor devices leads to the introduction of more complex 3D geometries with an increasing aspect ratio, often termed high aspect ratio (HAR) structures. Due to the self-limited surface reactions, ALD is one of the most suitable deposition techniques to deposit thin coatings with an excellent control of the layer thickness onto such structures. ALD is, for example, used for the fabrication of trench capacitors in dynamic random-access memory (DRAM),^22,23 in FinFETs,^24,25 and 3D NAND structures.^26,27 Also, outside the field of semiconductor applications,²⁸ ALD is being considered to deposit films onto 3D substrates, e.g., for surface functionalization and protection of microelectromechanical systems (MEMSs),^29,30 and coatings for fuel cells,³¹ solar cells,^32–34 batteries,^35–39 catalytic surfaces,^40–46 membranes,^47,48 textiles,⁴⁹ and pharmaceuticals.⁵⁰ 

In this review article, we explore the status of current understanding of the conformality of ALD processes, providing an overview of published experimental and theoretical studies focusing on this topic. First, conformality-related concepts of ALD processes will be introduced in Sec. II. In Sec. III, we discuss methods and dedicated test structures that have been used to quantify the conformality of ALD processes. An overview of experimental reports on the conformality of thermal, plasma-enhanced, and ozone-based ALD processes is presented in Sec. IV. In Sec. V, different models are discussed that have been used to simulate ALD in narrow structures. These models can be used to optimize process parameters towards improved conformality. Fitting of model parameters to experimental data can also provide insights into the ALD surface reactions, e.g., in sticking probabilities and the impact of non-ideal side-reactions during ALD. In Sec. VI, we simulate the influence of different ALD parameters on the conformality with a Monte Carlo model⁵¹ and discuss how one can obtain relevant information on the growth mechanism from the analysis of such thickness profiles.

Due to the self-limited nature of the chemisorption and subsequent surface reactions, it is possible to grow with ALD uniform and conformal films in structures with a large depth to width ratio. Uniform films have an equal thickness and composition (and other properties) at each position along a planar substrate, e.g., along a 300 mm wafer. Conformal films have the same thickness (and properties) also inside 3D features (“around-the-corner”). Figure 2 shows a schematic representation of the coating of a 3D structured surface with a non-conformal line-of-sight technique (a) and with the typical conformality of an ALD coating (b). In the absence of the 3D feature, both films could be considered uniform.

During reactant transport, the reactant molecules penetrate into narrow structures in a certain flow regime. To distinguish the different flow regimes, Knudsen⁵² introduced a dimensionless parameter, the Knudsen number Kn, which is defined as the ratio of the mean free path λ (m) and the pore diameter d_p (m)

If the mean free path of the reactant molecules is much larger than the dimensions of the structures (Kn ≫ 1), gas transport takes place in the molecular flow regime.⁵² In this regime, the transport is dominated by particle-surface interactions and inter-particle interactions can in most cases be neglected.

The mean free path λ (m) of molecules is given by⁵³ 

with k_B (m² kg/(s² K)) being the Boltzmann constant, T (K) being the temperature, P (Pa) being the pressure, and d (m) being the diameter of the molecules. The mean free path depends strongly on pressure, while the molecule size and temperature affect the mean free path to a minor extent, as illustrated in Fig. 3. At sufficiently low pressure, a molecular flow regime can be obtained in structures with macroscopic (i.e., mm to cm) dimensions, while for near-atmosphere pressures, molecular flow will only be achieved in nanometer scale features.

If the mean free path of the reactant molecules is much smaller than the dimensions of the features (Kn ≪ 1), gas transport is governed by viscous flow,⁵² also known as continuum flow.⁵⁴ In viscous flow, there are frequent particle-particle interactions in the gas phase. The transition flow between molecular and viscous flow takes place when Kn ≈ 1 and is often termed Knudsen flow although the latter term is also used as a synonym for molecular flow.¹⁶ 

When the gas phase consists of different types of molecules, e.g., when a mixture of reactant molecules (species A) and carrier gas molecules (species B) is used in ALD, one has to adapt the above formula [Eq. (2)] of the mean free path. Chapman et al.⁵⁵ derived a specific scattering length λ_0,A(m) for a particle of species A taking into account the interaction with particles of species B

with k_B (m² kg/(s² K)) being the Boltzmann constant, T (K) being the deposition temperature, P_i (Pa) being the partial pressure, and m_i (kg) being the mass of molecules of type i ∈ {A, B}. The cross-section between the molecules i and j with radii r_i (m) and r_j (m) is represented by⁵⁵ 

One of the distinguishing factors among the different types of ALD reactors is whether they operate in the temporal or spatial ALD mode, with the first mode being the most conventional one. During temporal ALD, the sample is stationary and the different reactants are sequentially injected and removed from the sample cell. In spatial ALD, there is a continuous supply of the reactants in isolated injection regions which are separated by an inert gas curtain, while the substrate moves between the different zones.^56–58 Suntola developed in 1974 his first ALD reactor where he applied spatial ALD.⁵⁹ The reactor comprised a carousel that rotated at several rounds per second and worked at a base pressure of 10⁻⁴Pa.³ In another reactor design, Suntola et al.⁶⁰ used a carrier gas to separate the different surface reaction steps from each other in a temporal deposition mode.

As discussed above, the reactant (partial) pressure is a determining factor for the gas transport regime in high aspect ratio structures, influencing in turn the process of conformal coating during ALD. Therefore, we classify the different designs of ALD reactors according to the typical pressure ranges that are applied. Following the classification by George,⁴ we define pump-type and flow-type (temporal) ALD reactors. In pump-type reactors, reactants are added into the reactor chamber without the use of a carrier gas and the typical reactant pressures are in the range of 10⁻³–10⁰ Pa. After the reactant exposure, the chamber is evacuated by pumping down to a base pressure in the range of 10⁻⁴–10⁻³Pa. Because of the low pressure and the corresponding molecular flow regime, the reactor design is not restricted to specific geometric constraints and can easily be adapted to accommodate plasma sources and in-situ characterization techniques. The main disadvantage of pump-type reactors concerns the long cycle times in the range of 10¹–10² s, due to the slow evacuation of the reaction chamber without the use of a purge gas. In the classical flow-type reactor design,^3,60 the reactants are entrained in an inert carrier gas which flows through the reactor in a viscous flow regime. Flow-type reactors are typically operated at a pressure near 100 Pa, and cycle times are on the order of 10⁰ s.⁴ Next to pump-type and flow-type reactors, atmospheric pressure (AP-type) reactors in which ALD processes take place at (or near) atmospheric pressure (∼10⁵ Pa) form the third class of ALD reactors. Over the past few years, there has been increasing interest in spatial ALD approaches applying atmospheric pressures for high throughput ALD for a number of applications including photovoltaics.^56–58 In this way, high deposition rates can be achieved for certain processes, e.g., ∼1 nm/s for ZnO.⁶¹ 

As shown in Fig. 3, different flow regimes occur during ALD within high aspect ratio structures, depending on the characteristic size of the 3D features that are present on the substrate and the absolute pressure range, linked to the type of ALD reactor used. For instance, in millimeter-sized features, reactant transport occurs in the molecular flow regime in pump-type reactors, but in flow-type and AP-type ALD reactors, it occurs in the viscous flow regime.

Pressure plays a crucial role in the ALD process, as it determines the impinging flux of reactant molecules on the substrate. At a given pressure, a certain minimum amount of time is needed before the sample surface is fully covered with adsorbed reactant molecules and saturation is reached. A useful measure for the reactant exposure is therefore given by the product of the reactant partial pressure and the pulse time. In this way, the exposure is expressed in Langmuir (1 L = 10⁶ Torr s = 7500 Pa s). As an example, Gordon et al.⁶² estimated that the exposure required to saturate a flat surface with Hf(NMe₂)₄ molecules during ALD of HfO₂ at 200 °C is in the range of 3–43 L. Much larger exposures are commonly required during ALD on high aspect ratio structures to compensate for diffusional limitations. To deposit a conformal coating of HfO₂ into holes with a pore diameter of 0.17 μm and a depth of 7.3 μm, an exposure of 9000 L was required.⁶² Applying sufficiently large exposures is one of the essential conditions to obtain a conformal coating by ALD as will be exemplified later in this review article.

The surface reaction kinetics of ALD processes can be complex^5,6,63 and should not be considered as trivial as depicted in Fig. 1. For many processes, even the reaction stoichiometry during an ALD cycle is not necessarily known.⁶⁴ To cope with this complexity when modelling the conformality of ALD processes, the reaction chemistry is often simplified by using irreversible Langmuir surface kinetics.^65–67 The sticking probability s is introduced as the probability that a reactant molecule reacts upon collision with the surface and contributes to film growth. It is often assumed that the sticking probability has a first order dependence with the available surface sites¹⁶ 

with s₀ being the initial sticking coefficient (i.e., reaction probability with a bare surface) and θ being the fraction of covered sites. This expression implies that the surface reactivity gradually decreases with the increase in the coverage and eventually becomes zero. This simple model thus reflects the self-limiting nature of the surface reactions during ALD, while the reaction kinetics (fast/slow) can be implemented via the initial sticking coefficient s₀ (high/low values).

Taking a step back and considering in the first instance reversible Langmuir adsorption, the first reaction of an ALD cycle (Reaction A) can be represented by

with A_g being the gaseous reactant A, ^* a vacant surface site, and A^* the chemisorbed reactant A. A^* can be interpreted as a “lumped reaction product” (and not the result of an elementary reaction), containing multiple types of surface species simultaneously.

The adsorption rate r_ads is equal to the product of the adsorption rate constant k_ads, the partial pressure of reactant A, P_A, and the fraction of uncovered surface sites, giving the overall second-order⁶⁸ surface reaction rate equation

The desorption rate r_des is equal to the fraction of covered sites times the desorption rate constant k_des

The rate of change in the surface coverage θ is obtained by subtracting the desorption rate from the adsorption rate

At equilibrium, $dθdt$ is zero. In practice, gaseous byproducts are often continuously pumped out, making reverse reactions unlikely and justifying the assumption of irreversibility. When irreversible adsorption is assumed, the second term, r_des, in Eq. (9) will be ignored.

The second reaction of the ALD cycle (reaction B) can be described, in the lumped way, assuming an “average” reaction product AB^*

In practice, in many models, reaction B is not modelled separately, and it is only considered that the surface left behind by exposure to reactant A, saturated with θ ≈ 1, is rendered reactive again in reaction B.

Furthermore, it is often assumed that the total number of adsorption sites for the Langmuir adsorption model is obtained from the number of metal atoms deposited per cycle and thus growth per cycle (GPC). The current authors understand this to be a gross oversimplification, as, e.g., the number of OH groups on alumina, which act as (reactive) adsorption sites in the trimethylaluminum reaction, is known to be 7–9 per nm² at typical ALD Al₂O₃ conditions, while the number of aluminium atoms deposited per cycle is around 4.5 per nm².⁵ Despite the fact that this assumption is clearly oversimplified, it is being repeatedly used as it can give modelling results with a satisfactory fit.

For radical-assisted ALD processes (e.g., plasma-enhanced or PE-ALD) or ozone-based ALD processes, reactant molecules that collide with the surface can undergo recombination processes. For example, an O radical can recombine with an adsorbed O atom and form molecular O₂ that leaves the surface. The probability that a species recombines during a collision with the surface is usually defined as the recombination probability r.^66,69,70 Although it depends on the process, the recombined species can often no longer contribute to film growth upon subsequent collisions with the surface, e.g., when the surface is only reactive towards O radicals and not towards molecular O₂. Hence, recombination processes are usually considered loss processes.

Elam et al.⁶⁵ introduced the concepts of a diffusion versus a reaction limited growth type for ALD growth in narrow features in the molecular flow regime.⁷¹ In the diffusion limited growth type, it is the geometry of the feature that causes the main difficulty in coating.⁷² During deposition, the most accessible sites will be covered first, resulting in a clear front between the accessible (covered) sites and the less accessible (uncovered) sites. During the deposition, this front will gradually penetrate deeper into the feature. In the reaction limited growth type, the difficulty of coating is mainly related to a low sticking probability of the reactant molecules. In this case, there is a less sharp front between coated and as yet uncoated parts of the feature. For plasma-enhanced ALD, Knoops et al.⁶⁶ introduced a recombination limited growth type, where saturation is not limited by the diffusion rate or the sticking probability but by the recombination loss during collisions of the radicals with the side-walls of the feature. These different growth types are illustrated in Fig. 4, which shows unsaturated thickness profiles. Note that both the aspect ratio of the feature and the sticking probability of the ALD reactants will determine the ALD growth type, as will be discussed in Sec. VI B 4 and in Fig. 23.

The Aspect Ratio (AR) of a structure is typically defined as

with L (m) being the depth and w (m) being the width of the structure, as indicated in Fig. 5. Achieving a conformal coating in a structure becomes more difficult with an increase in the AR of the structure. In addition, it will be easier to coat an elongated trench with depth L and width w than a cylindrical hole with the same depth L and diameter w because the opening through which the reactant can enter will be larger for the trench which facilitates the diffusion. AR is a 2D concept, based on geometrical measures of a cross-section of a 3D object, and is therefore not expected to be sufficient as a parameter to fully describe the difficulty of precursor diffusion into 3D features. Therefore, not only the AR will be important but also the length of the feature along the third dimension will have an impact on the conformality. A schematic representation of a hole and trench structure with equal AR is given in Fig. 5.

Gordon et al.⁶² proposed a generalized expression for the aspect ratio a of holes

with L (m) being the depth of the hole, p (m) being its perimeter, and A (m²) being the cross-sectional area. This expression takes into account the 3D nature (third dimension) of the feature. For cylindrical holes, Eq. (12) reduces to L/w which is equal to the depth to width AR interpretation from Eq. (11). For trenches, Eq. (12) reduces to L/(2w), which is a factor of two smaller than the conventionally used AR of Eq. (11). Gordon et al. also showed that for large a, the required exposure for conformal coating scales with a². The difference of a with a factor of two between trenches and cylinders implies that a four times larger exposure is required to conformally coat a hole in comparison with a trench with the same depth to width ratio (AR) as can be measured in a cross-sectional view. This example of trenches versus holes clearly illustrates that a depth to width ratio of a 3D feature is not a sufficient measure to estimate the difficulty in coating the 3D structure: gas flow into the real structure will depend on the full 3D geometry of the feature.

To facilitate a direct comparison between different structures and literature reports, we propose a new concept to express the aspect ratio in a structure-independent way. Analogous to the “equivalent oxide thickness” which was introduced in the field of high-k oxides to enable a straightforward comparison between different structures and materials by expressing their key functional properties with respect to a well-known reference material (SiO₂),⁷³ we propose to introduce the concept of an Equivalent Aspect Ratio (EAR) by referring to simple cylindrical holes as the reference structure.⁷⁴ The EAR of a given 3D feature can then be defined as the aspect ratio of a hypothetical cylindrical hole that would require the same reactant exposure dose during an ALD reaction as the feature of interest.

In Fig. 6, the AR and EAR are compared for arrays of holes, trenches,⁶² and pillars.⁵¹ Following expressions for the (E)AR were found:

• For circular holes: AR = L/w and EAR = L/w.

• For square holes: AR = L/w and EAR = L/w.

• For (infinite) trenches, AR = L/w and EAR = L/(2w).

• For elongated holes, AR = L/w and EAR = (L(w + z))/(2wz), with z being the length of the hole as indicated in Fig. 6.

• For squares pillars, AR = L/w and $EAR=L/(22w)$ (valid for L/w in the range of 5–50, w/w_pillar = 3).

Most expressions follow from Gordon's definition (12) and were further confirmed via Monte Carlo modelling.⁵¹ For arrays of pillars, the EAR was not derived from analytical equations, but was determined from Monte Carlo simulations, assuming an initial sticking coefficient of unity (see Sec. V). Comparing holes and trenches with the same AR, the EAR of trenches is a factor of two smaller. For L/w ratios in the range of 5–50 and w/w_pillar = 3, we found that the EAR of an array of pillars is a factor of $22$ smaller than that for holes. It should be noted that this EAR was determined for molecular flow conditions. Additional calculations would be needed to determine the EAR of an array of pillars for viscous flow conditions and hence confirm whether or not the EAR is a flow regime dependent parameter.

The 3D uniformity of a film is often discussed either in terms of step coverage or conformality (sometimes conformity). The definition of step coverage may vary from reference to reference.

Typically, Step Coverage (SC) is used for the ratio of the film thickness at the bottom of a feature to the film thickness at the top of the feature. Alternatively, it is calculated as the ratio of the film thickness at the side wall to the film thickness at the top [Fig. 7(a)]. Step coverage is typically expressed as percentage. The term is often used for thin films made by PVD⁷⁵ or (PE)CVD.⁷⁶ In ALD, the “steps” which may be challenging for PVD and (PE)CVD (e.g., EAR 5:1) are typically coated 100% uniformly, i.e., with 100% SC, and therefore, more demanding (e.g., lateral) test structures are needed for ALD (see Sec. III).

When analysed with vertical structures, conformality of ALD is often defined in a similar manner as step coverage: ratio of bottom-top or sidewall-top film thickness and is given as percentage.

An alternative way to express the conformality of ALD coatings is via the coated (E)AR. This approach is typically used when the ALD coating did not reach the bottom of the feature and is especially useful for lateral, highly demanding test structures (AR > 50:1). If a thickness profile is experimentally obtained, one determines the penetration depth at which the film thickness equals 50% of the film thickness at the top, which we propose to call the half-thickness-penetration-depth or 50%-thickness-penetration-depth, abbreviated PD^50%.⁷⁷ From the PD^50%, one can calculate the coated (E)AR. Alternatively, one can distinguish the coated versus uncoated regions of the feature in cross-sectional electron microscopy or optical images. The transition point is then used to calculate the coated (E)AR. Similar to the PD^50%, one can also define the PD^80% which stands for the penetration depth at which the film thickness is reduced to 80% of the original film thickness. The concepts of step coverage,^78–81 PD^50% and PD^80%, are illustrated in Fig. 7.

When studying the conformality of a certain ALD process, it is important to use test structures in which gas diffusion occurs according to a flow regime that is relevant for the envisioned application and reactor. Most of the high aspect ratio structures on which ALD is used for applications in micro-electronics,²⁹ batteries,³⁶ fuel cells,³¹ catalytic surfaces,⁴⁰ etc., exhibit micrometer- or nanometer-sized dimensions. Note that for porous materials, IUPAC classifies three pore types according to the pore size: micropores have a diameter below 2 nm, mesopores have a diameter in the range of 2 to 50 nm, and macropores have a diameter above 50 nm.⁸² As shown in Fig. 3, gas diffusion into structures with a characteristic width below ca. 1 μm will be determined by molecular flow when the deposition is performed in traditional flow- (low-vacuum) and pump-type (high-vacuum) ALD reactors (because the mean free path of the reactant molecules will be much larger than the structure width). Therefore, the molecular flow regime is most relevant for the majority of 3D substrates, ALD reactors, and targeted applications although viscous flow applies in some specific cases.

Table I and Fig. 8 overview different types of high aspect ratio test structures that have been used for quantifying the conformality of ALD processes, ordered according to a decreasing size. Many types of structures have been used in mm-μm-nm ranges of feature sizes. We classify them here as vertical structures, lateral structures, and porous materials. These structures often differ in the methods typically used to characterize the film after deposition. Table I includes some references that discuss the fabrication of the specific structures and that introduce a particular characterization approach for determining the thickness/conformality of the deposited ALD coating, as further detailed in Secs. III A–III C.

Large surface area substrates with vertical features such as arrays of trenches, forests of pillars, carbon nanotubes, or assemblies of pores [e.g., anodized alumina (AAO)] have been used in combination with ALD for applications in fuel cells,³¹ batteries,³⁵ and supercapacitors.¹¹⁶ Evidently, these substrates can also be used to quantify the conformality of an ALD process. Cross-sectional imaging of the structures [e.g., by scanning electron microscopy (SEM)] allows relatively straightforward visualization of the depth up to which an ALD coating has been deposited. The film thickness can be measured point-by point; accuracy depends on the sample preparation and skills of the electron microscopy operator.

Trenches etched into silicon have most often a width in the range of 100 nm¹¹⁷ to several μm and achieve an EAR of >40:1 [Fig. 8(d)]. Vertical trenches are fabricated with an anisotropic etch process which is designed so that a passivated film is deposited on the sidewalls, while the feature is being etched.¹¹⁸ With the basic Bosch Deep Reactive Ion Etching (DRIE) process, developed in the MEMS industry, one alternates etching and passivation cycles.¹¹⁹ During the passivation cycle, a protective fluorocarbon film is deposited on all the surfaces. This step is followed by an etching step during which an ion bombardment removes the protecting film from all the horizontal surfaces. This technique allows us to achieve trenches with EAR up to 80:1 for widths in the range of 250–800 nm.¹²⁰ To characterize coated trenches, Gluch et al.⁸⁹ introduced a TEM lamellae preparation using focused ion beam (FIB) to measure detailed thickness profiles in trenches.

AAO structures can be prepared¹²¹ by electrochemical anodization of aluminum films in liquid electrolytes and consist typically of a high density of well-defined parallel and uniform cylindrical pores which are arranged in a hexagonal symmetry with pore diameters between 5 and 300 nm [Fig. 8(f)]. The length of the pores can be controlled from a few tens of nm to a few hundreds of microns. After deposition, cross-sectional SEM is most often used to evaluate the penetration of the ALD coating. Elam et al.⁶⁵ introduced an alternative approach, where they polished the AAO membrane under a slight angle. In this way, one can obtain cross-sections along the entire length of the pore at different locations in one plane, simplifying the SEM analysis. This measurement technique is illustrated with a thickness profile of a ZnO coating in an AAO structure in Fig. 9(a). Perez et al.⁹⁵ dissolved the AAO structures and studied the ALD formed nanotubes directly by TEM, avoiding the need for preparing cross-sectional TEM samples from the AAO template. They also introduced an algorithm to automatically determine the wall thickness and diameter of the nanotube as a function of the depth from the TEM images. The thickness profile of a nanotube obtained out of an AAO pore is shown in Fig. 9(b). Recently, Macak and co-workers¹²² studied the conformality of ALD processes in anodic TiO₂ nanotube layers. The closely spaced nanotubes had a diameter of 110 nm and an EAR of 180:1. By depositing the TiO₂ nanotube substrate on a quartz crystal that can be mounted in a quartz crystal microbalance (QCM), they were able to perform in-situ QCM measurements during the ALD process. This enabled real-time monitoring of reactant uptake during the ALD reactions. In principle, this approach could be extended to other porous substrates.

Track-etch membranes are another type of micro- or nanoporous materials which are formed by irradiation of polymeric sheets. The diameter of the etched pores can be in the range of a few nm to mm, resulting in an EAR of 10:1–1000:1.¹²³ SEM and cross-sectional TEM are often used to investigate the conformality of the deposited ALD coating in the membranes.¹²⁴ Arrays of (silicon) pillars^93,125 can be either etched or grown through catalyzed chemical vapor deposition [Fig. 8(e)]. To ensure mechanical stability, the pillars often have a diameter and spacing of 1–10 μm and a typically height of 50 μm. Forests of carbon nanotubes (CNTs) can be grown by CVD. Multiwalled CNTs can have a diameter of 10–100 nm and a length up to several micrometers¹²⁶ [Fig. 8(g)]. To quantify the conformality of an ALD process on pillars or CNTs, EDX mapping is performed on a cross-section to quantify the amount of deposited material along the wall of the pillar or nanotube.^92,100

Dedicated lateral test structures have been designed by several groups to enable easy and accurate quantification of the penetration depth and the composition profile of the deposited coating. Often, the goal is to avoid the need of (time-consuming) cross-sectioning and electron microscopy. In the CVD literature, Yang et al.¹²⁷ used macroscopic lateral structures to analyze the CVD growth of HfO₂ thin films. More recently, Shima et al.⁸⁴ introduced parallel-plate microchannels to study CVD processes. A patterned Si wafer, fabricated by single step etching, is clamped onto a planar Si substrate. In this way, one obtains lateral trench-like features with a microscopic gap size of 1–15 μm (determined by the depth of the Si etching process) and EAR up to 1000:1. The chronological overview below discusses the most important lateral structures that have been used to characterize the conformality of ALD films.

Fused silica capillary tubes with a length of 2 mm and a diameter of 20 μm were introduced as ALD conformality test structures by Becker et al.⁸⁵ [Fig. 8(b)]. After ALD, the coating on the exterior of the tubes was burned off and the inside was filled with a liquid with a similar refracting index as fused silica. In this way, one could visually determine the penetration depth of the ALD coating on the interior of the tubes using an optical microscope.

Macroscopic lateral structures were introduced by Dendooven et al.⁶⁷ [Fig. 8(a)] and were created by cutting a rectangular shaped structure (typically 0.5 cm × 2 cm) from a sheet of aluminum foil (thickness of 100–500 μm) and clamping the resulting foil in-between two silicon wafers [Fig. 10(a)]. Because of the design of the structure, exposed regions of the clamped Si wafer are effectively turned into the sidewalls of a lateral trench. By using aluminum foils with different thicknesses and by cutting different shapes, one can easily produce structures with EARs in the range of 1:1–100:1. After ALD deposition, the clamped structure can be disassembled, resulting in two planar Si wafers. The penetration depth of the ALD coating can often be observed with the naked eye. Since the lateral size of the structures is on the order of cm, any technique for characterization of the layer thickness or composition with an intrinsic lateral resolution of the order of 1 mm can be used for obtaining an accurate profile of the thickness and composition of the ALD coating along the sidewalls of the test structure. Quantitative thickness profiles can be obtained, e.g., by ellipsometry (SE), x-ray fluorescence (XRF), or Rutherford backscattering spectroscopy (RBS) mapping of the regions of the Si wafers that constituted the sidewalls in the clamped structure. Not only the film thickness as a function of the depth but also the change in the composition of the deposited film can be monitored. This can be particularly important for complex coatings such as, e.g., ternary oxides or LiPON for 3D battery applications. An advantage of the macroscopic lateral structures is that the small thickness of the deposited film (nms) as compared to the total width of the trench (mms) implies that the EAR of the trench remains essentially constant during deposition, which simplifies, e.g., modelling. The main disadvantage is that their use is limited to pump-type reactors working at high vacuum if the molecular-flow assumption needs to be valid.

Air wedge structures were used by Gabriel et al.⁸³ to quantify the conformality of optical coatings deposited by ALD. These structures consist of two square silicon wafers with a side of 7 cm. The two wafers are in contact along one edge and open at the opposite edge with an air gap of 1560 μm [Fig. 10(b)]. By using such wedge structures, one can simultaneously estimate the penetration depth for a range of EARs (38:1–1410:1).

Musschoot et al.⁶⁹ extended the method of Dendooven et al. to investigate the penetration of thermal and plasma-enhanced ALD into fibrous materials. They used a hole, made of Teflon (1 cm × 1 cm × 5 cm), and filled it with non-woven polyester. The hole structure was clamped between the substrate holder and a flat Teflon surface. In this way, the ALD reactants could enter the fibrous material from only one side and precursor penetration into the non-woven polyester could be studied in a systematic way.

Puurunen and co-workers^86,87 fabricated microscopic lateral high aspect ratio (LHAR) trenches, code named PillarHall structures, with a gap height of 200–1000 nm and EARs up to 12 500:1, using micro-fabrication techniques commonly used for MEMS [Figs. 10(c) and 8(c)]. In recent work, the gap height range was expanded to 100–2000 nm.⁷⁷ The lateral structures consist of a Si wafer (as bottom surface) and a suspended polysilicon membrane that is locally supported by a network of Si or SiO₂ pillars. Elongated openings are etched through the top polysilicon membrane to define the access points for the gas into the lateral structure. After ALD, the film penetration can be investigated non-destructively through the membrane using, e.g., infrared spectroscopy or in some cases using a laboratory microscope. After the removal of the membrane, e.g., by using an adhesive tape, one can measure the penetration depth by microscopy and quantify the thickness of the ALD coating deposited onto the Si wafer (i.e., onto the bottom part of the lateral test structures) by using small-spot size techniques such as reflectometry or SEM/EDX. As indicated in Fig. 3, these LHAR structures with a typical 500 nm gap can be used in flow-type ALD reactors in the molecular flow gas transport regime up to pressures of 1000 Pa.

Recently, Schwille et al.^128,129 studied the conformality of ALD in microscopic rectangular cavities with lateral dimensions of 2000 μm, a height of 4.5 μm, and a central access hole with a diameter in the range of 4–60 μm [Fig. 10(d)]. The centrosymmetric nature of the structure slightly complicates the analysis as the equations for trenches do not apply directly because of the different symmetry. These structures resemble those encountered in real MEMS processing.

To tentatively demonstrate that the conformality of an ALD process is indeed determined by the flow type and EAR and not by the absolute dimension, a test was made for this review where a macroscopic lateral test structure reported by Dendooven et al.⁶⁷ with EAR of 200:1 and LHAR structures reported by Puurunen and co-workers (Generation 1, Ref. 86) were used to compare the quantification of conformality for the same ALD process in the same ALD run. Both substrates were coated in a pump-type reactor during a TMA/H₂O process to deposit an Al₂O₃ film. The process parameters were identical for both substrates: T = 100 °C, P_TMA = 0.28 Pa, and $PH2O=0.25 Pa$⁠. During the process, 1000 ALD cycles were applied with the following pulse/pump times: TMA (20 s) - pump (60 s) - H₂O (20 s) - pump (60 s). The film thickness was measured with SE in the case of the macroscopic lateral structure, while the coating was visualized using an optical microscope for the microscopic LHAR substrate. For both substrates, a coated EAR of roughly 100:1 was found as can be seen in Fig. 11.

Most conformality research has focused on ALD coatings in materials with critical dimensions > 30 nm, e.g., using the above-mentioned Si-based trench structures, AAO, and lateral structures. Fewer studies have focused on ALD coatings in sub-30 nm pores.

George and co-workers investigated ALD of Al₂O₃, TiO₂, and SiO₂ in tubular alumina membranes with a pore diameter of 5 nm.⁴⁷ After each ALD growth reaction, the pore diameter was derived from in situ N₂ conductance measurements (assuming molecular flow in the pores). The pore size was smaller after a metal reactant exposure than after the subsequent H₂O exposure, in accordance with the replacement of the bulkier metal reactant ligands on the pore walls by the smaller OH groups during the H₂O step. The pore diameter was successfully reduced to molecular diameters (estimated in the range of 3–10 Å), demonstrating the potential of ALD in tailoring nanoporous membranes for specific gas separation purposes.

To quantify the penetration of ALD coatings into nanosized pores, Dendooven et al.^106,115 developed an approach based on mesoporous SiO₂ and TiO₂ films that were deposited onto silicon substrates. The mesoporous SiO₂ films had randomly ordered channel-like pores with an average diameter of ca. 6.5 nm. The mesoporous titania thin films contained ink-bottle shaped pores [Fig. 8(i)], i.e., spherical cages with a diameter in the range of 4–7 nm connected to each other via smaller pore necks (3–5 nm). The amount of deposited material during the ALD process was monitored using in situ XRF¹⁰⁵ and the remaining porosity using in situ grazing incidence small angle x-ray scattering (GISAXS)¹¹⁵ and ellipsometric porosimetry (EP).¹⁰⁶ Since the thickness of the deposited film was of the same order as the gap size through which the gas had to enter the pore, the equivalent aspect ratio was not constant, but was gradually increasing as the thickness of the deposited film increased. They demonstrated tuning of the pore size up to near molecular dimensions for both channel-like and ink-bottle shaped mesopores, indicating that the key limiting factor in ALD deposition into nanopores is the diameter of the reactant molecules used during the process, e.g., a TDMAT molecule has a molecular diameter of about 0.7 nm.¹³⁰ 

Opal structures consist mostly of close-packed silica or polystyrene spheres with a diameter of several hundred nm [Fig. 8(h)]. With ALD, one can deposit a coating into the void space between the spheres.^101–104 After the deposition, the original spheres can be removed to obtain an inverse opal structure, with potential applications in photonics. The conformality of ALD in these structures can be evaluated by EDX mapping of cross-sections of the opal structure to measure the penetration depth. After the removal of the opal template, cross-sectional SEM can be used to measure the thickness and lateral uniformity of the inverse opal structure.

Besides conformality in mesoporous thin films and opal structures, ALD infiltration in mesoporous powders, including silica gel,¹⁰⁹ Zeotile-4^111,131 [Fig. 8(j)], metal organic frameworks (MOFs),^110,132 and alumina and silica powder with a size of several hundred microns,^112,133 has also been a subject of research. ALD functionalization of porous powders is mainly explored for applications in the field of catalysis. The diffusion into the interior portions of these materials is considered to be highly challenging, not only because of the high EAR (few nm wide pores of sometimes several microns in length) but also because of the very large surface areas (up to 2500 m²/g)¹¹⁰ that need to be covered. Because of these large surface areas, the conformality of ALD is governed not only by the diffusion of the molecules but also by the reactant supply.¹⁰⁹ 

In this section, we aim to provide a systematic overview of experimental data on the conformality of ALD processes. This overview does not aim to include all reported ALD processes but rather concentrates on those reports in which conformality has been investigated in detail. We distinguish three types of ALD processes: thermal, ozone-based, and plasma-enhanced processes. In thermal ALD, the conformality is influenced by the molar mass and reactivity of the ALD reactants as well as by the partial pressures and exposure times. Reactant decomposition (not occurring in theoretical ALD, but sometimes occurring in real processes) and too short purge/evacuation steps may decrease the conformality. For ozone-based and plasma-enhanced ALD, the conformality additionally depends on the recombination of radicals (or ozone) which causes the flux of radicals (ozone) to decrease inside trenches and hence leads to decreased conformality. Therefore, in general, it is expected that better conformality will be achieved for thermal ALD than for ozone-based or plasma-enhanced ALD processes.

Tables II–IV overview experimental results on the conformality of thermal, ozone-based, and plasma-enhanced ALD processes, respectively, as reported in the literature. The indicated EAR has been calculated according to the definition given in Sec. II based on the feature dimensions reported in the corresponding reference. For AAO pores that are accessible from both sides, the tabulated EAR is calculated using half of the pore length. Unless stated otherwise in the table footnotes, the given exposure corresponds to the metal reactant (reactant A) of the ALD process. The value has been calculated from the reactant partial pressures and pulse times reported in the referenced papers (where available). The coated EAR equals the EAR of the structure in the case of complete coverage or is determined by PD^50% in the case of incomplete coverage as described in Sec. II and indicated with *. If the results were presented in a thickness profile (film thickness as a function of the depth of the structure), “D” is noted after the coated EAR. Not all fields in the tables could be filled because the process parameters were not always reported. For the PE-ALD processes shown in Table IV, the type of plasma configuration is noted. The influence of the type of configuration will be discussed in Sec. IV C.