What is the atomic layer deposition process, or what is ALD coating? 

Because each precursor pulse creates one atomic layer, and many pulses deposit a coating, this technique is called Atomic Layer Deposition. So ALD is therefore based on the self-limiting nature of sequential reactions and this unique growth technique provides atomic layer control.

When was atomic layer deposition invented?

Technology preceding the multilayer ALD coating process, related to the vapor growth and analysis of single or double atomic layers, was investigated in Russia during the 1960s. Later, it was developed independently in Finland, as a multi cycle coating technology (1974, patented by Prof. Tuomo Suntola). While in Russia it was, and still is, called molecular layering, in Finland it was called Atomic Layer Epitaxy. Later is was renamed to the the more general term Atomic Layer Deposition, and the term Atomic Layer Epitaxy is now reserved for (high temperature) epitaxial ALD (see below). Take a look at this review paper for an in depth report on the history. 
 

The Fundamentals of the ALD process:

Atomic Layer Deposition typically involves a cycle of 4 steps that is repeated as many times as necessary to achieve the required coating thickness. During growth, the surface is alternately exposed to two complementary chemical precursors. In this case, each precursor is fed individually into the reactor.

To remove any residual gas chemically active or by-products before introducing another precursor into the reactor, inert gas purging or pump-down steps are used between exposure steps (Fig 1). The example shows ALD of Al₂O₃ using Al(CH₃), (trimethyl aluminum, TMA) and H₂O or oxygen plasma.

Fig 1: Atomic Layer Deposition Cycles.

During each cycle, the following steps are performed (Fig. 1): 

1. Flow of first precursor TMA, which adsorbs on and reacts with OH groups on a surface. With the correct choice of precursor and parameters, this reaction is self-limiting. 
2. Purge, usually with nitrogen gas, removes any residual precursor from the previous step. 
3. Flow of second precursor (water or oxygen). The reaction of H₂O (thermal ALD) or Oxygen plasma radicals (plasma ALD) oxidizes the surface and removes surface ligands. This reaction is also self-limiting.
4. Purge. Continue with step 1.

In the case of the deposition of Al2O3 using Tri-methyl aluminum and water:

1. Al(CH3)3 + OH => O-Al-(CH3)2 + CH4 
2. Remove remaining Al(CH3)3 and CH4 using N2 purge
3. O-Al-(CH3)2 + H2O => O-Al-OH(2) + (O)2-Al-CH3 + CH4
4. Remove remaining H2O and CH4 using N2 purge. Continue with step 1.

Surface sites are saturated to one monolayer as a result of each exposure step. As soon as the surface has been saturated, no further reaction occurs due to the precursor chemistry and process conditions.

In order to prevent the precursors from reacting with each other in any place other than on the surface, which would lead to chemical vapor deposition (CVD), the steps must be separated well by nitrogen purges.

YouTube video:

You can watch a very nice YouTube video of the ALD process explained and in action: Atomic Layer Deposition of copper - If you like sputtering, you'll love this!. 

Advantages of ALD:

As many researchers and industries have already discovered, ALD has many advantages:

1. By controlling the number of deposition cycles, the film thickness can be controlled with sub-nm precision, with excellent repeatability.
2. The coatings have very low roughness, and substrate curvature is followed exactly.
3. Perfect 3D conformality and 100% step coverage: uniform smooth coatings on flat, inside porous, and around particle samples.
4. The coating even grows underneath dust particles laying on a substrate, preventing pinholes.
5. Excellent adhesion due to covalent bonds with surfaces or sometimes even infiltration (polymers). It sticks even to Teflon!
6. Easy batch scalability (many substrates can be stacked and coated at once with perfect coating thickness uniformity).
7. Large area thickness uniformity, even over-meter sizes.
8. Gentle deposition process for sensitive substrates, usually no plasma needed.
9. Wide process windows (no sensitivity to temperature or precursor dose variations).
10. Low-defect density
11. It can be Amorphous or crystalline, depending on substrate and temperature
12. Tailored material properties via digital control of sandwiches, hetero-structures, nanolaminates, mixed oxides, graded layers, and doping.
13. Standard, easily reproduced recipes for oxides, nitrides, metals,  semiconductors, and more.
14. All types of objects can be coated: wafers, 3D parts, film rolls, porous materials, and even powders, from nano to meter size.
15. Coating equipment is robust, easy to operate, and scalable without the need for ultra-high vacuum. Even atmospheric ALD is possible.
16.  

How long does atomic layer deposition take?

The ALD process growth process is known to be rather slow, about 1 atomic layer per second. However, some variants are much quicker, in particular fast optimized flow reactors (1-5 nm/sec) and spatial ALD (1-10 nm/sec). However, because of its inherent self passivation, thousands of substrates can be loaded in a reactor, making the coating speed per part extremely fast, uniform and reproducible! Alternatively, roll to roll ALD is possible, where the roll speed can be high when many coating heads are used (comparable to spatial ALD).

What is the difference between CVD and ALD and how does a coating form in CVD?

What temperature is needed for deposition to occur? 
 

In ALD, the substrate temperatures suitable for deposition, range from room temperature to 800C. Most depositions take place at around 100-200C. Above 100C, water vapor which is often used as one of the reactants, evaporates quickly from the substrate and walls, and so the cycle speed between precursors can be faster using temperatures above 100C. At high temperatures, it's possible to achieve epitaxial growth for certain materials, where deposited layers conform the the substrate crystal surface, and form a single crystal coating. This is called atomic layer epitaxy!

Type of Materials That Can Be Deposited With ALD:

The technology industry and academics have done extensive research on materials that can be used in ALD. The list keeps being updated every year. Below we give you a selection of materials used:

• Oxides: Al₂O₃, CaO, CuO, Er2O3, Ga₂O₃, HfO₂, La₂O₃, MgO, Nb₂O₅, Sc₂O₃, SiO₂, Ta₂O₅, TiO₂, VXOY, Y₂O₃, Yb₂O₃, ZnO, and others!
• Nitrides: AlN, GaN, TaNX, TiAlN, TiNX, etc.
• Carbides: TaC, TiC, etc.
• Metals: Ir, Pd, Pt, Ru, etc.
• Sulfides: ZnS, SrS, etc.
• Fluorides: CaF₂, LaF₃, MgF₂, SrF₂, etc.
• Biomaterials: Ca₁₀(PO₄)₆(OH)₂ (hydroxyapatite)
• Polymers: PMDA–DAH, PMDA–ODA, etc.

It is also possible to do doping and mix different structures with ALD. Material combinations are endless with it.

A demonstration of  perfect high aspect ratio conformality of ALD coatings. (a) Cross sectional SEM picture of 300 nm Al2O3 deposited on silicon wafer with trench structures. (b) Cross sectional TEM of conformal ruthenium ALD coating on a trench patterned substrate

Types of ALD

While ALD was developing, new variants of the technique came to place. Each one with a specific characteristic, which can be advantageous depending on the type of material you are working with or the type of material you want as a product. We explain the most important ones and try to answer, what is ALD equipment, and what different types exist:

Thermal ALD: In most ALD methods, atomic vaporization is controlled; however, higher temperatures are sometimes required. Molecules containing aluminum, in particular, often require high temperatures. Thermal ALD typically uses temperatures between 150 and 350 degrees Celsius. A common example is the formation of Al2O3 from water and trimethylaluminum on a substrate surface.

Metal ALD: By utilizing elimination reactions between halogen-functionalized metallic molecules, commonly metal fluorides and silicon-based molecules. Fluorosilane elimination reactions are most commonly used in metal ALD processes, which deposit metal on the surface of the substrate. A wide range of metals can be deposited onto a substrate surface using this technique.

Particle ALD: This technique is similar to conventional ALD. Unlike most ALD methods, particle ALD coats the entire surface of a particle (including the surface of nanoparticles). The coating of many materials on a particle surface can be done uniformly and conformably without missing any parts of the particle. 

Plasma ALD: In plasma-enhanced ALD (PEALD), Plasma is used to reduce the molecular vaporization temperature and can be used with a variety of precursor materials. PEALD allows for a wider choice of precursor chemistry with enhanced film quality:

• Plasma enables low-temperature ALD processes, and the remote source maintains low plasma damage
• Eliminates the need for water as a precursor, reducing purge times between ALD cycles - especially for low temperatures
• Effective metal chemistry through the use of hydrogen plasma
• Higher quality films through improved removal of impurities
• Reduced nucleation delay
• Ability to control stoichiometry/phase
• Plasma surface treatment, functionalization, and activation
• Plasma cleaning of the chamber is possible for some materials

Roll-to-roll: Continuous roll-to-roll is significant because it allows application in many large-area substrates in a cost-effective manner.

The idea implies moving the substrate through zones of constant precursor flows. Chemically reactive zones are alternated with purge zones on the substrate during the process.

Inert gas purging and vacuum zones separate the orifices on the gas delivery heads where the precursors continuously flow. By separating the precursor delivery regions with vacuum pumping zones as well as inert gas curtains, gas phase reactions are prevented between the zones.Atmospheric Spatial ALD: Spatial ALD (SALD) is a variation of ALD in which precursors are continuously transported through different precursor locations (spaces). The precursors in these locations are kept apart by an inert gas region, usually nitrogen. 

Moving the substrate to locations containing the different precursor's results in film growth. Since this process doesn't require filling and emptying a chamber with precursor or nitrogen, the spatial ALD process is quicker, and is compatible with fast-throughput techniques like roll-to-roll film coating. It does not require a vacuum pump, however, it is limited to flat substrates (although they may have micron sized complex surfaces). 

It is used for very fast deposition of silicon wafers, glass panels and rolls of film. In the case of silicon wafers, the wafers are either moved back and forth, or rotate on a platter, where they move through different precursor zones. In the case of roll-to-roll film coating, the film is passed through spatial ALD heads with multiple precursor nozzles, separated by nitrogen nozzles. This way, the film substrate sees precursor 1, nitrogen, precursor 2, nitrogen, etc.

Vacuum Spatial ALD: Similarly to Atmospheric Spatial ALD, Vacuum Spatial ALD supplies the precursors in different locations. However, in this case, the system is under a partial vacuum. This uses less nitrogen, but has more complex mechanics and requires vacuum pumps.

Photo-assisted ALD: activates surface reactions by using ultraviolet (UV) light rather than high temperatures to trigger reactions. In this case, surface reactions can be tuned by varying UV light wavelength, intensity, and illumination time.

ALD for High Aspect Ratios and 3D Objects: ALD for infusing high aspect ratio objects, like porous materials and plastics, requires long dosing times of precursors so that the material pores can be saturated. It requires a "stop valve" or "shut off valve" that allows the precursor to have a long residence time in the reactor, e.g., for more than 10 seconds. This can go up to hours when infusing into plastics.

In the case of 3D objects, bigger reaction chambers are required. 

Processes closely related to ALD

Several chemical processes related to ALD also take advantage of the sequentially passivated vapor exposure process: molecular layer deposition, atomic layer epitaxy, and vapor phase infiltration. These are usually included under the umbrella of the ALD denomination.

Molecular layer deposition: 

Molecular layer deposition (MLD) is a technique that can utilize small, bifunctional organic molecules as precursors. 

The uniqueness of MLD is that organic-only or hybrid materials can be synthesized by working with organic precursors.

When considering the process cycles in MLD, an organic precursor is initially pulsed into a chamber and reacts with the material surface. Precursor excess and byproducts are removed by purging or pumping. 

An organic precursor is introduced in the second step, reacting with the first precursor molecules anchored to the surface. This surface reaction is again self-saturated, followed by purging/pumping of the reactor.  

MLD makes it possible to upscale the production of organic and hybrid materials and the material coverage with organic layers. Moreover, molecular layer deposition is also known for its thickness control at Angstrom-level precision due to its slow, cyclical approach.

Atomic layer etching: 

Atomic layer etching is an emerging technique where individual atomic layers can be removed by changing only the top atomic layers of the wafer. 

In ALE, two chemical precursor exposures are used to etch: one to create a surface compound and a second that can etch this compound. 

When ions are used, ALE permits narrow and deep material structures, granting the possibility of directional etching manipulation. 

The initial step is to form a reactive layer, followed by the removal of this layer. In each cycle, uniform layers of material are removed with consistent thickness. 

In addition to improving uniformity and reducing damage, ALE also increases selectivity and minimizes aspect ratio-dependent etching (ARDE). 

Vapor phase infiltration: 

Vapor phase infiltration (VPI) is a new gas-phase synthesis method based on atomic layer deposition (ALD). 

VPI enables the growth and nucleation of inorganic materials in the free volume of polymers. This is made possible by placing the material to be covered in a vacuum chamber, exposing it to a first precursor for as long as necessary, purge, and then exposing it to a second precursor.

The cycle allows the diffusion of precursors and consequent coverage of the porous material. 

ALD Application Landscape:

Technology advancements have been made possible as new techniques for material development emerge. The electronic industry, as well as industries such as green energy, medical and pharmaceuticals, and precision manufacturing, have taken advantage of ALD.

Below you can find a list of products and industries where ALD is applied:

• IC applications: High-k oxide films, Spacer oxide films, Inter-poly dielectric oxide films, Tunneling oxide films, Blocking oxide films, Gap-filling oxide films, Passivation films, Capping layer films, Copper barrier and seed films, Adhesion layer films, Conductive metal films for interconnects (through-silicon-vias, TSV), Diffusion barrier films, Ferroelectric materials, Paramagnetic materials
• MEMS: Diffusion barriers, Adhesion layers, Charge dissipative layers, Layers lowering frictional wear, Optical layers, Films to close nanoscale pores, Biocompatible coatings, Coatings for hermetical sealing, Hydrophobic layers to decrease stiction, Conformal, thermally conductive layers, Conductive seed layers for plating purposes, Etch masks and etch stop layers, Conformal, electrically insulating layers
• Medical: Biocompatible coatings for medical applications
• Transistors: The largest industrial application of ALD has been its use for depositing ultra-thin high-k gate dielectrics such as HfO2, TiO2, Ta2O5, ZrO2, SrTiO3, and HfxSiyO, and other oxide films for nanometer size transistors in integrated circuits, particularly logic devices in computer chips. 
• Metal layers in ICs: Interconnecting the active layers in a 3D-integrated system requires highly conformal, ultra-thin metallization film that is able to cover uniformly deep trench structures with very high aspect ratios. Deep trench capacitors for memory devices also require high-quality metallization layers. ALD is the ideal method for producing these metal layers, thus enabling continuous shrinkage of device features. 
• Memory devices: ALD is a major player in memory devices. ALD is used to engineer the bandgap of high-k dielectric materials and ensure low leakage current and suitable cell capacitance even in the ~5–7nm thickness range. 
• Image sensors: ALD is primarily used as a passivation layer in image sensors. Since films with different refractive indices can be stacked in a single process run, ALD layers can be used as anti-reflection layers and collect more radiation
• OLEDs: The organic LED layers are very sensitive to moisture, while ALD films are moisture-resistant. This makes ALD a powerful tool to protect OLEDs.
• Power devices: ALD makes deposition of efficient gate insulation and surface passivation layers increase devices viability as a technology for large-scale utilization.
• Batteries: The engineering of the material in high-performance batteries is made possible with ALD. Batteries using Pt, TiN, Li-compounds, and electrode metal oxides are taken to the optimal level using layer deposition.
• Fuel cells: Electrolytes and electrode materials can be deposited as well as anticorrosive protective layers on the metal interconnect using ALD. Low resistance and faster reaction kinetics can also be achieved with thin ALD layers.
• Catalysts: Powder-based catalyst materials
• Solar energy: A thin layer of ALD Al₂O₃ is used to prevent the recombination of charge carriers in Si solar cells manufactured on a large scale. ALD of Al₂O₃ passivated cells adds up to the most efficient large-scale manufactured solar cells.
• Orthopedic implants: The uniform fit of the ALD coating means improved tissue attachment, a longer lifetime, and a higher level of reliability for orthopedic implants. Consequently, patient safety improves, and corrective or replacement surgeries can potentially be avoided.
• Electronic implants: Electro-active implants such as cardiac pacemakers can be insulated with ALD coatings that fit seamlessly around the device, even without bulky metal casings.
• Pharmaceuticals: By creating a nanometer-scale barrier, ALD can provide unprecedented control over the release of active drug molecules after administration.
• Optics Machine parts: In addition to moisture barriers and insulators, ALD can also be used for phosphor layers, antireflective coatings, waveguides, optical filters, and tuning of colors.
• Decorative coatings: long-lasting color coatings in jewelry, especially parts of swiss watches.
• Luxury products: Several companies in the global minting and luxury products industries already use ALD for anti-tarnishing, passivation, and improving surface mechanical properties.
• Packaging: ALD can be used for environmentally friendly moisture barrier layers in packaging applications.

Further reading:

For a more in-depth introduction to ALD and its related processes, take a look at the following useful links:

• Introduction to atomic layer deposition (ALD): principles, applications, future. A podcast video by Prof. Riika Puurunen.
• Introductory article by Knoops et al (2020).
• Atomic layer deposition, molecular layer deposition and atomic layer etching Review papers, Books, Theses, Podcasts, Lectures and Market Research.