Thin-film deposition technique that deposits one 1-atom thick layer at a time
Atomic layer deposition (ALD) is a thin-film deposition technique based on the sequential use of a gas-phase chemical process; it is a subclass of chemical vapour deposition. The majority of ALD reactions use two chemicals called precursors (also called "reactants"). These precursors react with the surface of a material one at a time in a sequential, self-limiting, manner. A thin film is slowly deposited through repeated exposure to separate precursors. ALD is a key process in fabricating semiconductor devices, and part of the set of tools for synthesizing nanomaterials.
Introduction.
During atomic layer deposition, a film is grown on a substrate by exposing its surface to alternate gaseous species (typically referred to as precursors or reactants). In contrast to chemical vapor deposition, the precursors are never present simultaneously in the reactor, but they are inserted as a series of sequential, non-overlapping pulses. In each of these pulses the precursor molecules react with the surface in a self-limiting way, so that the reaction terminates once all the available sites on the surface are consumed. Consequently, the maximum amount of material deposited on the surface after a single exposure to all of the precursors (a so-called ALD cycle) is determined by the nature of the precursor-surface interaction. By varying the number of cycles it is possible to grow materials uniformly and with high precision on arbitrarily complex and large substrates.
ALD is a deposition method with great potential for producing very thin, conformal films with control of the thickness and composition of the films possible at the atomic level. A major driving force for the recent interest is the prospective seen for ALD in scaling down microelectronic devices according to Moore's law. ALD is an active field of research, with hundreds of different processes published in the scientific literature, though some of them exhibit behaviors that depart from that of an ideal ALD process. Currently there are several comprehensive review papers that give a summary of the published ALD processes, including the work of Puurunen, Miikkulainen "et al.", 
Knoops "et al.", and Mackus &amp; Schneider "et al.". An interactive, community driven database of ALD processes is also available online which generates an up-to-date overview in the form of an annotated periodic table.
The sister technique of atomic layer deposition, molecular layer deposition (MLD), uses organic precursors to deposit polymers. By combining the ALD/MLD techniques, it is possible to make highly conformal and pure hybrid films for many applications.
Another technology related to ALD is sequential infiltration synthesis (SIS) which uses alternating precursor vapor exposures to infiltrate and modify polymers. SIS is also referred to as vapor phase infiltration (VPI) and sequential vapor infiltration (SVI).
History.
1960s.
In the 1960s, Stanislav Koltsov together with Valentin Aleskovsky and colleagues experimentally developed the principles of ALD at Leningrad Technological Institute (LTI) in the Soviet Union. The purpose was to experimentally build upon the theoretical considerations of the "framework hypothesis" coined by Aleskovsky in his 1952 habilitation thesis. The experiments started with metal chloride reactions and water with porous silica, soon extending to other substrate materials and planar thin films. Aleskovskii and Koltsov together proposed the name "Molecular Layering" for the new technique in 1965. The principles of Molecular Layering were summarized in the doctoral thesis ("professor's thesis") of Koltsov in 1971. Research activities of molecular layering covered a broad scope, from fundamental chemistry research to applied research with porous catalysts, sorbents and fillers to microelectronics and beyond.
In 1974, when starting the development of thin-film electroluminescent displays (TFEL) at Instrumentarium Oy in Finland, Tuomo Suntola devised ALD as an advanced thin-film technology. Suntola named it atomic layer epitaxy (ALE) based on the meaning of "epitaxy" in Greek language, "arrangement upon". The first experiments were made with elemental Zn and S to grow ZnS. ALE as a means for growth of thin films was internationally patented in more than 20 countries. A breakthrough occurred, when Suntola and co-workers switched from high vacuum reactors to inert gas reactors which enabled the use of compound reactants like metal chlorides, hydrogen sulfide and water vapor for performing the ALE process. The technology was first disclosed in 1980 SID conference. The TFEL display prototype presented consisted of a ZnS layer between two aluminum oxide dielectric layers, all made in an ALE process using ZnCl2 + H2S and AlCl3 + H2O as the reactants. The first large-scale proof-of-concept of ALE-EL displays were the flight information boards installed in the Helsinki-Vantaa airport in 1983. TFEL flat panel display production started in the mid-1980s by Lohja Oy in the Olarinluoma factory. Academic research on ALE started in Tampere University of Technology (where Suntola gave lectures on electron physics) in 1970s, and in 1980s at Helsinki University of Technology. TFEL display manufacturing remained until the 1990s the only industrial application of ALE. In 1987, Suntola started the development of the ALE technology for new applications like photovoltaic devices and heterogeneous catalysts in Microchemistry Ltd., established for that purpose by the Finnish national oil company Neste Oy. In the 1990s, ALE development in Microchemistry was directed to semiconductor applications and ALE reactors suitable for silicon wafer processing. In 1999, Microchemistry Ltd. and the ALD technology were sold to the Dutch ASM International, a major supplier of semiconductor manufacturing equipment and Microchemistry Ltd. became ASM Microchemistry Oy as ASM's Finnish daughter company. Microchemistry Ltd/ASM Microchemistry Ltd was the only manufacturer of commercial ALD-reactors in the 1990s. In the early 2000s, the expertise on ALD reactors in Finland triggered two new manufacturers, Beneq Oy and Picosun Oy, the latter started by Sven Lindfors, Suntola's close coworker since 1975. The number of reactor manufacturers increased rapidly and semiconductor applications became the industrial breakthrough of the ALD technology, as ALD became an enabling technology for the continuation of Moore's law. In 2004, Tuomo Suntola received the European SEMI award for the development of the ALD technology for semiconductor applications and in 2018 the Millennium Technology Prize.
The developers of ML and ALE met at the 1st international conference on atomic layer epitaxy, "ALE-1" in Espoo, Finland, 1990. An attempt to expose the extent of molecular layering works was made in a scientific ALD review article in 2005 and later in the VPHA-related publications.
The name "atomic layer deposition" was apparently proposed for the first time in writing as an alternative to ALE in analogy with CVD by Markku Leskelä (professor at the University of Helsinki) at the ALE-1 conference, Espoo, Finland. It took about a decade before the name gained general acceptance with the onset of the international conference series on ALD by American Vacuum Society.
2000s.
In 2000, Gurtej Singh Sandhu and Trung T. Doan of Micron Technology initiated the development of atomic layer deposition high-κ films for DRAM memory devices. This helped drive cost-effective implementation of semiconductor memory, starting with 90-nm node DRAM. Intel Corporation has reported using ALD to deposit high-κ gate dielectric for its 45 nm CMOS technology.
ALD has been developed in two independent discoveries under names atomic layer epitaxy (ALE, Finland) and molecular layering (ML, Soviet Union). To clarify the early history, the Virtual Project on the History of ALD (VPHA) has been set up in summer 2013. it resulted in several publications reviewing the historical development of ALD under the names ALE and ML.
2010s.
In 2010, sequential infiltration synthesis (SIS), first reported by researchers at Argonne National Laboratory, was added to the family of ALD-derived techniques.
Surface reaction mechanisms.
In a prototypical ALD process, a substrate is exposed to two reactants A and B in a sequential, non-overlapping way. In contrast to other techniques such as chemical vapor deposition (CVD), where thin-film growth proceeds on a steady-state fashion, in ALD each reactant reacts with the surface in a self-limited way: the reactant molecules can react only with a finite number of reactive sites on the surface. Once all those sites have been consumed in the reactor, the growth stops. The remaining reactant molecules are flushed away and only then reactant B is inserted into the reactor. By alternating exposures of A and B, a thin film is deposited. This process is shown in the side figure. Consequently, when describing an ALD process one refers to both dose times (the time a surface is being exposed to a precursor) and purge times (the time left in between doses for the precursor to evacuate the chamber) for each precursor. The dose-purge-dose-purge sequence of a binary ALD process constitutes an ALD cycle. Also, rather than using the concept of growth rate, ALD processes are described in terms of their growth per cycle.
In ALD, enough time must be allowed in each reaction step so that a full adsorption density can be achieved. When this happens the process has reached saturation. This time will depend on two key factors: the precursor pressure, and the sticking probability. Therefore, the rate of adsorption per unit of surface area can be expressed as:
formula_1
Where R is the rate of adsorption, S is the sticking probability, and F is the incident molar flux. However, a key characteristic of ALD is the S will change with time, as more molecules have reacted with the surface this sticking probability will become smaller until reaching a value of zero once saturation is reached.
The specific details on the reaction mechanisms are strongly dependent on the particular ALD process. With hundreds of process available to deposit oxide, metals, nitrides, sulfides, chalcogenides, and fluoride materials, the unraveling of the mechanistic aspects of ALD processes is an active field of research. Some representative examples are shown below.
Thermal ALD.
Thermal ALD requires temperatures ranging from room temperature (~20°C) to 350°C for ligand exchange or combustion type surface reactions. It occurs through surface reactions, which enables accurate thickness control no matter the substrate geometry (subject to aspect ratio) and reactor design.
The synthesis of Al2O3 from trimethylaluminum (TMA) and water is one of the best known thermal ALD examples. During the TMA exposure, TMA dissociatively chemisorbs on the substrate surface and any remaining TMA is pumped out of the chamber. The dissociative chemisorption of TMA leaves a surface covered with AlCH3.
The surface is then exposed to H2O vapor, which reacts with the surface –CH3 forming CH4 as a reaction byproduct and resulting in a hydroxylated Al2O3 surface.
Plasma ALD.
In plasma-assisted ALD (PA-ALD), the high reactivity of the plasma species allows to reduce the deposition temperature without compromising the film quality; also, a wider range of precursors can be used and thus a wider range of materials can be deposited as compared to thermal ALD.
Spatial ALD.
In temporal ALD the separate precursor and co-reactant doses are separated from each other in "time" by a purge step. In contrast, in spatial ALD (s-ALD), these gases are delivered at different locations, so they are separated in "space". In atmospheric pressure s-ALD the precursor and co-reactant are delivered continuously and they are separated from each other by a gas curtain to prevent gas phase reactions. Such gas curtain typically consists of nitrogen injection and exhaust positions, see Figure 1. As a substrates moves through the different gas zones, self-limiting reactions take place at the substrate surface and the ALD process takes place. As this process can easily be accelerated, the deposition rate for spatial ALD can be much higher than for conventional ALD. For example, for ALD of Al2O3 the deposition rate increases from 100-300 nm per hour to 60 nm per minute. 
The inline nature of spatial ALD makes it suitable for high volume production lines and roll-to-roll production. In general, s-ALD has been employed to apply moisture permeation barriers, passivation layers in silicon solar cells and functional layers in batteries. The chemistry for spatial ALD processes is comparable with typical temporal ALD processes, and materials that have been explored include inorganic metal oxides such as Al2O3, (Al- or Ga doped) ZnO, SiO2, In2O3, InZnO, LIPON, Zn(O,S), SnOx, and TiOx ","but also PMG metals (Pt, Ir, Ru) can be deposited. Additionally, organic molecules can be grown in combination with inorganic atoms to enable molecular layer deposition (MLD). Plasma- or ozon enhanced spatial ALD has been demonstrated which generally lowers the deposition temperatures required. 
Photo-assisted ALD.
In this ALD variety, UV light is used to accelerate surface reactions on the substrate. Hence reaction temperature can be reduced, as in plasma-assisted ALD. As compared to plasma-assisted ALD, the activation is weaker, but is often easier