View source for Atomic layer deposition

{{Use dmy dates|date=December 2017}}
[[File:ALD schematics.jpg|thumb|300px|Schematic of ALD. In panel A, precursor 1 (blue) is added to the reaction chamber containing the material surface to be coated by ALD. After precursor 1 has adsorbed on the surface, any excess is removed from the reaction chamber. Precursor 2 (red) is added (B) and reacts with precursor 1 to create another layer on the surface (C). Precursor 2 is then cleared from the reaction chamber and this process is repeated until a desired thickness is achieved and the resulting product resembles panel D.]]
'''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 [[wiktionary:precursor|precursor]]s. These precursors react with the surface of a material one at a time in a sequential, self-limiting, manner. Through the repeated exposure to separate precursors, a thin film is slowly deposited. ALD is a key process in the [[semiconductor device fabrication|fabrication of semiconductor devices]], and part of the set of tools
available for the synthesis of [[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 [[Precursor (chemistry)|precursor]]s). 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 reactive 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.<ref name=stam/><ref name=":4">{{Cite journal|title = Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process|journal = Journal of Applied Physics|date = 15 June 2005|pages = 121301–121301–52|volume = 97|issue = 12|doi = 10.1063/1.1940727|first = Riikka L.|last = Puurunen|bibcode = 2005JAP....97l1301P}}</ref> By varying the number of cycles it is possible to grow materials uniformly and with high precision on arbitrarily complex and large substrates.

[[File:ALD Periodic Table - May 2019.png|thumb|300px|An overview of the materials synthesized by ALD with 1 or more publications describing the process, an up-to-date figure can be obtained online <ref name="ALDDatabase">{{cite journal |url=https://atomiclimits.com/alddatabase/ |doi=10.6100/alddatabase | title=ALD Database |year=2019 |last1=Kessels |first1=W.M.M. }}</ref> under the [[Creative Commons license]] for general use.]]
ALD is considered one 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,<ref name=stam/><ref name=":4" /><ref name=":5">{{Cite journal|title = Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends|journal = Journal of Applied Physics|date = 14 January 2013|pages = 021301–021301–101|volume = 113|issue = 2|doi = 10.1063/1.4757907|first = Ville|last = Miikkulainen|first2 = Markku|last2 = Leskelä|first3 = Mikko|last3 = Ritala|first4 = Riikka L.|last4 = Puurunen|bibcode = 2013JAP...113b1301M}}</ref> though some of them exhibit behaviors that depart from that of an ideal ALD process.<ref name=":5" /> Currently there are several comprehensive review papers that give a summary of the published ALD processes, including the work of Puurunen,<ref>{{cite journal |last1=Puurunen |first1=Riikka L. |title=Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process |journal=Journal of Applied Physics |date=15 June 2005 |volume=97 |issue=12 |pages=121301 |doi=10.1063/1.1940727}}</ref> Miikkulainen ''et al.'',<ref name=":5"/> 
Knoops ''et al.'',<ref>{{cite journal |last1=Knoops |first1=Harm C. M. |last2=Faraz |first2=Tahsin |last3=Arts |first3=Karsten |last4=Kessels |first4=Wilhelmus M. M. (Erwin) |title=Status and prospects of plasma-assisted atomic layer deposition |journal=Journal of Vacuum Science & Technology A |date=May 2019 |volume=37 |issue=3 |pages=030902 |doi=10.1116/1.5088582}}</ref> and Mackus & Schneider ''et al.''.<ref>{{cite journal |last1=Mackus |first1=Adriaan J. M. |last2=Schneider |first2=Joel R. |last3=MacIsaac |first3=Callisto |last4=Baker |first4=Jon G. |last5=Bent |first5=Stacey F. |title=Synthesis of Doped, Ternary, and Quaternary Materials by Atomic Layer Deposition: A Review |journal=Chemistry of Materials |date=10 December 2018 |volume=31 |issue=4 |pages=1142–1183 |doi=10.1021/acs.chemmater.8b02878}}</ref> An interactive, community driven database of ALD processes is also available online<ref name="ALDDatabase"/> 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), is employed when organic precursors are wished to be used. By combining the ALD/MLD techniques, it is possible to make highly conformal and pure hybrid films for many applications.

==History==
ALD has been developed in two [[Multiple discovery|independent discoveries]] under names [[atomic layer epitaxy]] (ALE, Finland) and molecular layering (ML, Soviet Union).<ref name=":6">{{Cite journal|last=Ahvenniemi|first=Esko|last2=Akbashev|first2=Andrew R.|last3=Ali|first3=Saima|last4=Bechelany|first4=Mikhael|last5=Berdova|first5=Maria|last6=Boyadjiev|first6=Stefan|last7=Cameron|first7=David C.|last8=Chen|first8=Rong|last9=Chubarov|first9=Mikhail|date=2016|title=Review Article: Recommended reading list of early publications on atomic layer deposition—Outcome of the "Virtual Project on the History of ALD"|journal=Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films|volume=35|issue=1|pages=010801|doi=10.1116/1.4971389}}</ref> To clarify the early history, the Virtual Project on the History of ALD (VPHA) has been set up in summer 2013.<ref>[http://www.vph-ald.com Virtual project on the history of ALD]. vph-ald.com</ref> it resulted in several publications reviewing the historical development of ALD under the names [[atomic layer epitaxy|ALE]] and ML.<ref name=":6" /><ref name=":1">{{Cite journal|title = A Short History of Atomic Layer Deposition: Tuomo Suntola's Atomic Layer Epitaxy|journal = Chemical Vapor Deposition|date = 2014|pages = 332–344|volume = 20|issue = 10–11–12|doi = 10.1002/cvde.201402012|first = Riikka L.|last = Puurunen}}</ref><ref name=":0">{{Cite journal|title = From V. B. Aleskovskii's "Framework" Hypothesis to the Method of Molecular Layering/Atomic Layer Deposition|journal = Chemical Vapor Deposition|date = 2015|pages = 216–240|volume = 21|issue = 10–11–12|doi = 10.1002/cvde.201502013|first = Anatolii A.|last = Malygin|first2 = Victor E.|last2 = Drozd|first3 = Anatolii A.|last3 = Malkov|first4 = Vladimir M.|last4 = Smirnov}}</ref><ref name=":7">{{Cite journal|url=https://ecsarxiv.org/exyv3/|website=ecsarxiv.org|doi=10.1149/osf.io/exyv3|access-date=2018-12-26|title=Learnings from an Open Science Effort: Virtual Project on the History of ALD|last1=Puurunen|first1=Riikka}}</ref>

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]].<ref name=":0" /> The purpose was to experimentally build upon the theoretical considerations of the "framework hypothesis" coined by Aleskovsky in his 1952 [[habilitation]] thesis.<ref name=":0" /> The experiments started with metal chloride reactions and water with porous silica, soon extending to other substrate materials and planar thin films.<ref name=":0" /> Aleskovskii and Koltsov together proposed the name "Molecular Layering" for the new technique in 1965.<ref name=":0" /> The principles of Molecular Layering were summarized in the doctoral thesis ("professor's thesis") of Koltsov in 1971.<ref name=":0" /> 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.<ref name=":0" /><ref name=":2">Aleskovskii, V. B. Zh. Prikl. Khim. 47, 2145 (1974); [J. Appl. Chem. USSR. 47, 2207, (1974)].</ref>

In 1974, when starting the development of thin-film [[electroluminescent display]]s (TFEL) at Instrumentarium Oy in [[Finland]], [[Tuomo Suntola]] devised ALD as an advanced thin-film technology.<ref name=":1"/><ref name=":3">Suntola, T. and Antson, J. (1977) {{US Patent|4058430}} "Method for producing compound thin films"</ref> Suntola named it [[atomic layer epitaxy]] (ALE) based on the meaning of "epitaxy" in Greek language, "arrangement upon".<ref name=":1"/><ref name=":3"/> The first experiments were made with elemental Zn and S to grow ZnS.<ref name=":1"/><ref name=":3"/> ALE as a means for growth of thin films was internationally patented in more than 20 countries.<ref name=":1"/> 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.<ref name=":1"/><ref>Suntola, T.; Pakkala, A. and Lindfors, S. (1983) {{US Patent|4389973}} "Apparatus for performing growth of compound thin films"</ref> The technology was first disclosed in 1980 SID conference.<ref name=":1"/> The TFEL display prototype presented consisted of a ZnS layer between two aluminum oxide dielectric layers, all made in an ALE process using ZnCl<sub>2</sub> + H<sub>2</sub>S and AlCl<sub>3</sub> + H<sub>2</sub>O 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.<ref name=":1"/> TFEL flat panel display production started in the mid-1980s by Lohja Oy in the Olarinluoma factory.<ref name=":1"/> 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]].<ref name=":1"/> 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 device]]s and [[heterogeneous catalyst]]s 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]].<ref name=":1"/> In 2004, [[Tuomo Suntola]] received the European [[Semiconductor Equipment and Materials International|SEMI]] award for the development of the ALD technology for semiconductor applications<ref name=":1"/> and in 2018 the [[Millennium Technology Prize]].<ref>{{cite press release |author=<!--Staff writer(s); no by-line.--> |title= 2018 Millennium Technology Prize for Tuomo Suntola – Finnish physicist's innovation enables manufacture and development of information technology products |url= https://taf.fi/2018/05/22/2018-millennium-technology-prize-for-tuomo-suntola-finnish-physicists-innovation-enables-manufacture-and-development-of-information-technology-products/ |location= |publisher= Technology Academy Finland |agency= |date=2018-05-22 |access-date=2018-05-22}}</ref>

The developers of ML and ALE met at the 1st international conference on atomic layer epitaxy, "ALE-1" in Espoo, Finland, 1990.<ref name=":1" /><ref name=":0" /> For some reason, knowledge of molecular layering in the growing English-speaking ALD community has remained marginal. An attempt to expose the extent of molecular layering works was made in a scientific ALD review article in 2005<ref name=":4" /> and later in the VPHA-related publications.<ref name=":6" /><ref name=":0" /><ref name=":7" />

The name "atomic layer deposition" was apparently proposed for the first time in writing as an alternative to ALE in analogy with [[Chemical vapor deposition|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]].<ref>{{Cite journal|title = History of atomic layer deposition and its relationship with the American VacuumSociety|journal = Journal of Vacuum Science & Technology A|date = 2013|pages = 050818|volume = 31|issue = 5|doi = 10.1116/1.4816548|first = Gregory N.|last = Parsons|first2 = Jeffrey W.|last2 = Elam|first3 = Steven M.|last3 = George|first4 = Suvi|last4 = Haukka|first5 = Hyeongtag|last5 = Jeon|first6 = W. M. M. (Erwin)|last6 = Kessels|first7 = Markku|last7 = Leskelä|first8 = Paul|last8 = Poodt|first9 = Mikko|last9 = Ritala|url = https://research.tue.nl/nl/publications/history-of-atomic-layer-deposition-and-its-relationship-with-the-american-vacuum-society(f98173d4-c326-4a2c-96da-f12b1a9c01c6).html|bibcode = 2013JVSTA..31e0818P}}</ref>

In 2000, [[Gurtej Sandhu|Gurtej Singh Sandhu]] and Trung T. Doan of [[Micron Technology]] initiated the development of atomic layer deposition [[high-k]] films for [[Dynamic random-access memory|DRAM]] memory devices. This helped drive cost-effective implementation of [[semiconductor memory]], starting with [[90 nanometer|90-nm]] [[Semiconductor node|node]] DRAM.<ref name="ieee">{{cite web |title=IEEE Andrew S. Grove Award Recipients |url=https://www.ieee.org/about/awards/bios/grove-recipients.html |website=[[IEEE Andrew S. Grove Award]] |publisher=[[Institute of Electrical and Electronics Engineers]] |accessdate=4 July 2019}}</ref><ref>{{cite web |last1=Sandhu |first1=Gurtej |last2=Doan |first2=Trung T. |title=Atomic layer doping apparatus and method |url=https://patents.google.com/patent/WO2002038841A3 |website=[[Google Patents]] |accessdate=5 July 2019 |date=22 August 2001}}</ref> [[Intel Corporation]] has reported using ALD to deposit high-κ gate dielectric for its [[45 nanometer|45 nm CMOS technology]].<ref>{{cite book |chapter=A 45nm Logic Technology with High-k+Metal Gate Transistors, Strained Silicon, 9 Cu Interconnect Layers, 193nm Dry Patterning, and 100% Pb-free Packaging |chapter-url=https://web.archive.org/web/20080218145031/http://download.intel.com/technology/IEDM2007/HiKMG_paper.pdf |doi=10.1109/IEDM.2007.4418914|title=2007 IEEE International Electron Devices Meeting |pages=247–250 |year=2007 |last1=Mistry |first1=K. |last2=Allen |first2=C. |last3=Auth |first3=C. |last4=Beattie |first4=B. |last5=Bergstrom |first5=D. |last6=Bost |first6=M. |last7=Brazier |first7=M. |last8=Buehler |first8=M. |last9=Cappellani |first9=A. |last10=Chau |first10=R. |last11=Choi |first11=C.-H. |last12=Ding |first12=G. |last13=Fischer |first13=K. |last14=Ghani |first14=T. |last15=Grover |first15=R. |last16=Han |first16=W. |last17=Hanken |first17=D. |last18=Hattendorf |first18=M. |last19=He |first19=J. |last20=Hicks |first20=J. |last21=Huessner |first21=R. |last22=Ingerly |first22=D. |last23=Jain |first23=P. |last24=James |first24=R. |last25=Jong |first25=L. |last26=Joshi |first26=S. |last27=Kenyon |first27=C. |last28=Kuhn |first28=K. |last29=Lee |first29=K. |last30=Liu |first30=H. |displayauthors=29 |isbn=978-1-4244-1507-6 }}</ref>

==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.<ref>[https://www.youtube.com/watch?v=KOEsgZU1sts "How Atomic Layer Deposition Works"]. Youtube (2011).</ref>

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.<ref name = book>{{cite book | author1 = Butt, Hans-Jurgen | author2 = Graf, Karlheinz | author3= Kappl, Michael | title = Physics and Chemistry of Interfaces | edition = Third, Revised | isbn = 978-3-527-41216-7 | year = 2013}}</ref> Therefore, the rate of adsorption per unit of surface area can be expressed as:

::<math> {R_{abs}} = S*F</math>

Where R is the rate of adsorption, S is the sticking probability, and F is the incident molar flux.<ref>{{cite web|url=http://www.chem.qmul.ac.uk/surfaces/scc/scat2_3.htm|title=2.3 Adsorption Kinetics – The Rate of Adsorption|website=www.chem.qmul.ac.uk}}</ref> 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 mechanism]]s are strongly dependent on the particular ALD process. With hundreds of process available to deposit oxide, metals, nitrides, sulfides, chalcogenides, and fluoride materials,<ref name=":5" /> the unraveling of the mechanistic aspects of ALD processes is an active field of research.<ref name="GeorgeOverview">{{cite journal | last1 = George | first1 = S.M. | year = 2010 | title = Atomic Layer Deposition: An Overview | url = | journal = Chem. Rev. | volume = 110 | issue = 1| pages = 111–131 | doi=10.1021/cr900056b| pmid = 19947596 }}</ref> Some representative examples are shown below.

===Thermal ALD===
[[File:Al2O3 Reaction Mechanism for ALD.png|thumb|upright=2|Proposed mechanism for Al<sub>2</sub>O<sub>3</sub> ALD during the a) TMA reaction b) H<sub>2</sub>O reaction]]

Thermal ALD requires relatively high temperatures (typically 150–350&nbsp;°C). It occurs through surface reactions, which enables accurate thickness control no matter the substrate geometry and reactor design.<ref name=stam/>

The synthesis of Al{{sub|2}}O{{sub|3}} 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 AlCH{{sub|3}}.
The surface is then exposed to H{{sub|2}}O vapor, which reacts with the surface –CH{{sub|3}} forming CH{{sub|4}} as a reaction byproduct and resulting on a hydroxylated Al{{sub|2}}O{{sub|3}} surface.<ref name=stam/>

===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.<ref name=stam/>

===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 to control by adjusting the wavelength, intensity and timing of illumination.<ref name=stam/>

===Metal ALD===
Copper metal ALD has attracted much attention due to the demand for copper as an [[Interconnects (integrated circuits)|interconnect]] material<ref>{{Citation|last=Cheng|first=Yi-Lung|title=Copper Metal for Semiconductor Interconnects|date=2018-07-04|work=Noble and Precious Metals - Properties, Nanoscale Effects and Applications|editor-last=Seehra|editor-first=Mohindar Singh|publisher=InTech|doi=10.5772/intechopen.72396|isbn=9781789232929|last2=Lee|first2=Chih-Yen|last3=Huang|first3=Yao-Liang|editor2-last=Bristow|editor2-first=Alan D.}}</ref> and the relative ease by which copper can be deposited thermally.<ref name=":8">{{Cite journal|last=Knisley|first=Thomas J.|last2=Kalutarage|first2=Lakmal C.|last3=Winter|first3=Charles H.|date=December 2013 |title=Precursors and chemistry for the atomic layer deposition of metallic first row transition metal films|journal=Coordination Chemistry Reviews|volume=257|issue=23–24|pages=3222–3231|doi=10.1016/j.ccr.2013.03.019}}</ref> Copper has a positive [[Standard electrode potential|standard electrochemical potential]]<ref>{{Cite book |title=CRC handbook of chemistry and physics : a ready-reference book of chemical and physical data|date=2011|publisher=CRC Press|editor=Haynes, William M.|isbn=9781439855119|edition=92nd |location=Boca Raton, FL.|oclc=730008390}}</ref> and is the most easily reduced metal of the first-row transition metals. Thus, numerous ALD processes have been developed, including several using hydrogen gas as the coreactant.<ref name=":8" /><ref>{{Cite journal|last=Gordon|first=Peter G.|last2=Kurek|first2=Agnieszka|last3=Barry|first3=Seán T.|date=2015|title=Trends in Copper Precursor Development for CVD and ALD Applications|journal=ECS Journal of Solid State Science and Technology|volume=4|issue=1|pages=N3188–N3197|doi=10.1149/2.0261501jss|issn=2162-8769}}</ref> Ideally, copper metal ALD should be performed at ≤100&nbsp;°C to achieve continuous films with low surface roughness,<ref>{{Cite journal|last=Knisley|first=Thomas J.|last2=Ariyasena|first2=Thiloka C.|last3=Sajavaara|first3=Timo|last4=Saly|first4=Mark J.|last5=Winter|first5=Charles H.|date=2011-10-25|title=Low Temperature Growth of High Purity, Low Resistivity Copper Films by Atomic Layer Deposition|journal=Chemistry of Materials|volume=23|issue=20|pages=4417–4419|doi=10.1021/cm202475e|issn=0897-4756}}</ref> since higher temperatures can result in agglomeration of deposited copper.<ref>{{Cite journal|last=Guo|first=Zheng|last2=Li|first2=Hao|last3=Chen|first3=Qiang|last4=Sang|first4=Lijun|last5=Yang|first5=Lizhen|last6=Liu|first6=Zhongwei|last7=Wang|first7=Xinwei|date=2015-09-08|title=Low-Temperature Atomic Layer Deposition of High Purity, Smooth, Low Resistivity Copper Films by Using Amidinate Precursor and Hydrogen Plasma|journal=Chemistry of Materials|volume=27|issue=17|pages=5988–5996|doi=10.1021/acs.chemmater.5b02137|issn=0897-4756}}</ref>

Some metals can be grown by ALD via fluorosilane [[elimination reaction]]s using a [[Metal halides|metal halide]] and a silicon precursor (e.g. [[Silane|SiH<sub>4</sub>]], [[Disilane|Si<sub>2</sub>H<sub>6</sub>]]) as the reactants. These reactions are very exothermic due to the formation of stable Si–F bonds.<ref name="GeorgeOverview" /> Metals deposited by fluorosilane elimination include tungsten<ref name=":9">{{Cite journal|last=Klaus|first=J.W|last2=Ferro|first2=S.J|last3=George|first3=S.M|date=February 2000 |title=Atomic layer deposition of tungsten using sequential surface chemistry with a sacrificial stripping reaction|journal=Thin Solid Films|volume=360|issue=1–2|pages=145–153|doi=10.1016/S0040-6090(99)01076-7}}</ref> and molybdenum.<ref>{{Cite journal|last=Seghete|first=D.|last2=Rayner|first2=G.B.|last3=Cavanagh|first3=A.S.|last4=Anderson|first4=V.R.|last5=George|first5=S.M.|date=2011-04-12|title=Molybdenum Atomic Layer Deposition Using MoF 6 and Si 2 H 6 as the Reactants|journal=Chemistry of Materials|volume=23|issue=7|pages=1668–1678|doi=10.1021/cm101673u|issn=0897-4756}}</ref> As an example, the surface reactions for tungsten metal ALD using WF<sub>6</sub> and Si<sub>2</sub>H<sub>6</sub> as the reactants can be expressed as<ref name=":9" /><ref>{{Cite journal|last=Grubbs|first=R. K.|last2=Steinmetz|first2=N. J.|last3=George|first3=S. M.|date=2004|title=Gas phase reaction products during tungsten atomic layer deposition using WF[sub 6] and Si[sub 2]H[sub 6]|journal=Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures|volume=22|issue=4|pages=1811|doi=10.1116/1.1767105}}</ref>

:WSiF{{sub|2}}H* + WF{{sub|6}} → WWF{{sub|5}}* + SiF{{sub|3}}H
:WF{{sub|5}}* + Si{{sub|2}}H{{sub|6}} → WSiF{{sub|2}}H* + SiF{{sub|3}}H + 2 H{{sub|2}}

The overall ALD reaction is<ref name="GeorgeOverview" />

:WF{{sub|6}} + Si{{sub|2}}H{{sub|6}} → W + SiF{{sub|3}}H + 2 H{{sub|2}}, ∆H = –181 kcal

The growth rate can vary from 4 to 7 Å/cycle depending on the deposition temperature (177 to 325&nbsp;°C) and Si<sub>2</sub>H<sub>6</sub> reactant exposure (∼10<sup>4</sup> to 10<sup>6</sup> L), factors that may influence Si<sub>2</sub>H<sub>6</sub> [[Insertion reaction|insertion]] into Si–H bonds<ref>{{Cite journal|last=Fabreguette|first=F.H.|last2=Sechrist|first2=Z.A.|last3=Elam|first3=J.W.|last4=George|first4=S.M.|date=September 2005|title=Quartz crystal microbalance study of tungsten atomic layer deposition using WF6 and Si2H6|journal=Thin Solid Films|volume=488|issue=1–2|pages=103–110|doi=10.1016/j.tsf.2005.04.114}}</ref><ref>{{Cite journal|last=Elam|first=J.W|last2=Nelson|first2=C.E|last3=Grubbs|first3=R.K|last4=George|first4=S.M|date=May 2001|title=Kinetics of the WF6 and Si2H6 surface reactions during tungsten atomic layer deposition|journal=Surface Science|volume=479|issue=1–3|pages=121–135|doi=10.1016/S0039-6028(01)00969-4}}</ref> and result in a silicon CVD contribution to the tungsten ALD growth.<ref name="GeorgeOverview" />

The thermal ALD of many other metals is challenging (or presently impossible) due to their very negative electrochemical potentials. Recently, the application of novel strong [[reducing agent]]s has led to the first reports of low-temperature thermal ALD processes for several electropositive metals. Chromium metal was deposited using a chromium [[alkoxide]] precursor and BH<sub>3</sub>(NHMe<sub>2</sub>).<ref>{{Cite journal|last=Kalutarage|first=Lakmal C.|last2=Martin|first2=Philip D.|last3=Heeg|first3=Mary Jane|last4=Winter|first4=Charles H.|date=2013-08-28|title=Volatile and Thermally Stable Mid to Late Transition Metal Complexes Containing α-Imino Alkoxide Ligands, a New Strongly Reducing Coreagent, and Thermal Atomic Layer Deposition of Ni, Co, Fe, and Cr Metal Films|journal=Journal of the American Chemical Society|volume=135|issue=34|pages=12588–12591|doi=10.1021/ja407014w|issn=0002-7863}}</ref> Titanium and tin metals were grown from their respective metal chlorides (MCl<sub>4</sub>, M = Ti, Sn) and a bis([[trimethylsilyl]]) six-membered ring compound.<ref>{{Cite journal|last=Klesko|first=Joseph P.|last2=Thrush|first2=Christopher M.|last3=Winter|first3=Charles H.|date=2015-07-28|title=Thermal Atomic Layer Deposition of Titanium Films Using Titanium Tetrachloride and 2-Methyl-1,4-bis(trimethylsilyl)-2,5-cyclohexadiene or 1,4-Bis(trimethylsilyl)-1,4-dihydropyrazine|journal=Chemistry of Materials|volume=27|issue=14|pages=4918–4921|doi=10.1021/acs.chemmater.5b01707|issn=0897-4756}}</ref><ref>{{Cite journal|last=Stevens|first=Eric C.|last2=Mousa|first2=Moataz Bellah M.|last3=Parsons|first3=Gregory N.|date=November 2018 |title=Thermal atomic layer deposition of Sn metal using SnCl 4 and a vapor phase silyl dihydropyrazine reducing agent|journal=Journal of Vacuum Science & Technology A|volume=36|issue=6|pages=06A106|doi=10.1116/1.5055212|issn=0734-2101}}</ref> Aluminum metal was deposited using an aluminum [[Hydride|dihydride]] precursor and [[Aluminium chloride|AlCl<sub>3</sub>]].<ref>{{Cite journal|last=Blakeney|first=Kyle J.|last2=Winter|first2=Charles H.|date=2018-03-27|title=Atomic Layer Deposition of Aluminum Metal Films Using a Thermally Stable Aluminum Hydride Reducing Agent|journal=Chemistry of Materials|volume=30|issue=6|pages=1844–1848|doi=10.1021/acs.chemmater.8b00445|issn=0897-4756}}</ref>

===Catalytic SiO{{sub|2}} ALD===
The use of catalysts is of paramount importance in delivering reliable methods of SiO{{sub|2}} ALD. Without [[catalysis|catalysts]], surface reactions leading to the formation of SiO{{sub|2}} are generally very slow and only occur at exceptionally high temperatures. Typical catalysts for SiO{{sub|2}} ALD include Lewis bases such as NH{{sub|3}} or pyridine and SiO{{sub|2}}; ALD can also be initiated when these [[Lewis bases]] are coupled with other silicon precursors such as [[tetraethoxysilane]] (TEOS).<ref name="GeorgeOverview" /> [[Hydrogen bonding]] is believed to occur between the Lewis base and the SiOH* surface species or between the H{{sub|2}}O based reactant and the Lewis base. Oxygen becomes a stronger [[nucleophile]] when the Lewis base hydrogen bonds with the SiOH* surface species because the SiO-H bond is effectively weakened. As such, the electropositive Si atom in the SiCl{{sub|4}} reactant is more susceptible to nucleophilic attack. Similarly, hydrogen bonding between a Lewis base and an H{{sub|2}}O reactant make the electronegative O in H{{sub|2}}O a strong nucleophile that is able to attack the Si in an existing SiCl* surface species.<ref>{{cite journal | last1 = Brown | first1 = S. | last2 = Howe | first2 = J.Y. | last3 = Ma | first3 = Z. | last4 = Et | year = 2008 | title = Surface Modification of Au/TiO<sub>2</sub> Catalysts by SiO<sub>2</sub> via Atomic Layer Deposition | journal = The Journal of Physical Chemistry C | volume = 112 | issue = 25| pages = 9448–9457 | doi=10.1021/jp801484h}}</ref> The use of a Lewis base catalyst is more or less a requirement for SiO{{sub|2}} ALD, as without a Lewis base catalyst, reaction temperatures must exceed 325&nbsp;°C and pressures must exceed 10{{sup|3}} torr. Generally, the most favorable temperature to perform SiO{{sub|2}} ALD is at 32&nbsp;°C and a common deposition rate is 1.35 angstroms per binary reaction sequence. Two surface reactions for SiO{{sub|2}} ALD, an overall reaction, and a schematic illustrating Lewis base catalysis in SiO{{sub|2}} ALD are provided below.

:Primary reactions at surface:
::SiOH* + SiCl{{sub|4}} → SiOSiCl{{sub|3}}* + HCl
::SiCl* + H{{sub|2}}O → SiOH* + HCl
:Overall ALD reaction:
::SiCl{{sub|4}} + 2H{{sub|2}}O → SiO{{sub|2}} + 4 HCl

[[File:SiO2 Reaction Mechanism.png|thumb|Proposed mechanism of Lewis base catalysis of SiO<sub>2</sub> ALD during a) an SiCl<sub>4</sub> reaction and b) an H<sub>2</sub>O reaction]]

{| class="wikitable" style="text-align:center;"
|+ ALD reaction mechanisms 
|-
! Type of ALD !! Temperature range !! Viable precursors !! Reactants !! Applications
|-
| Catalytic ALD || >32&nbsp;°C with Lewis base catalyst<ref name="GeorgeOverview" />|| Metal oxides (i.e. TiO{{sub|2}}, ZrO{{sub|2}},SnO2{{sub|2}})<ref name="GeorgeOverview" />|| (Metal)Cl{{sub|4}}, H{{sub|2}}O<ref name="GeorgeOverview" />|| High k-dielectric layers, protective layers, anti-reflective layers, etc.<ref name="GeorgeOverview" />
|-
| Al{{sub|2}}O{{sub|3}} ALD || 30–300&nbsp;°C || Al{{sub|2}}O{{sub|3}}, metal oxides<ref name=Florian>{{cite journal | author1 = Werner, Florian | author2 = Stals, Walter | author3= Gortzen, Roger | author4 = Veith, Boris | author5 = Brendel, Rolf | author6 = Schmidt, Jan | title = High-rate atomic layer deposition of Al{{sub|2}}O{{sub|3}} for the surface passivation of Si solar cells | journal = Energy Procedia | year = 2011 | volume = 8 | pages = 1301–306|doi=10.1016/j.egypro.2011.06.140 }}</ref>|| (Metal)Cl{{sub|4}}, H{{sub|2}}O, Ti(OiPr){{sub|4}}, (Metal)(Et){{sub|2}}<ref name="GeorgeOverview" />|| Dielectric layers, insulating layers, etc., Solar Cell surface passivations<ref name=Florian/>
|-
| Metal ALD using thermal chemistry || 175–400&nbsp;°C<ref name = kimreview>{{cite journal | author1 = Kim, H. | title = Atomic layer deposition of metal and nitride thin films: Current research efforts and applications for semiconductor device processing | journal = Journal of Vacuum Science and Technology | year = 2003 | volume = 21| issue = 6 | doi =10.1116/1.1622676 | page=2231| bibcode = 2003JVSTB..21.2231K }}</ref>|| Metal Fluorides, organometallics, catalytic metals<ref name=kimreview/>|| M(C{{sub|5}}H{{sub|5}}){{sub|2}}, (CH{{sub|3}}C{{sub|5}}H{{sub|4}})M(CH{{sub|3}}){{sub|3}} ,Cu(thd){{sub|2}}, Pd(hfac){{sub|2}}, Ni(acac){{sub|2}}, H{{sub|2}}<ref name=kimreview/>|| Conductive pathways, catalytic surfaces, MOS devices<ref name=kimreview/>
|-
| ALD on polymers || 25–100&nbsp;°C<ref name="GeorgeOverview" />|| Common polymers (polyethylene, PMMA, PP, PS, PVC, PVA, etc.)<ref name="GeorgeOverview" /> || Al(CH{{sub|3}}){{sub|3}}, H{{sub|2}}O, M(CH{{sub|3}}){{sub|3}}<ref name="GeorgeOverview" />|| Polymer surface functionalization, creation of composites, diffusion barriers, etc.<ref name="GeorgeOverview" />
|-
| ALD on particles || 25–100&nbsp;°C for polymer particles, 100–400&nbsp;°C for metal/alloy particles<ref name="GeorgeOverview" />|| BN, ZrO{{sub|2}}, CNTs, polymer particles || Various gases: [[Fluidized bed reactor]]s are used to allow coating of individual particles<ref name="GeorgeOverview" />|| Deposition of protective and insulative coatings, optical and mechanical property modification, formation of composite structures, conductive mediums
|-
| Plasma or radical-enhanced ALD for single element ALD materials || 20–800&nbsp;°C<ref>{{cite journal|last1=Mackus|first1=Adriaan J. M.|last2=Garcia-Alonso|first2=Diana|last3=Knoops|first3=Harm C. M.|last4=Bol|first4=Ageeth A.|last5=Kessels|first5=Wilhelmus M. M.|title=Room-Temperature Atomic Layer Deposition of Platinum|journal=Chemistry of Materials|date=2013|volume=25|issue=9|pages=1769–1774|doi=10.1021/cm400274n}}</ref><ref name="GeorgeOverview" /> || Pure metals (i.e. Ta, Ti, Si, Ge, Ru, Pt), metal nitrides (i.e. TiN, TaN, etc.)<ref name="GeorgeOverview" /> || Organometallics, MH{{sub|2}}Cl{{sub|2}}, tertbutylimidotris(diethylamido)tantalum (TBTDET), bis(ethylcyclopentadienyl)ruthenium), NH{{sub|3}}<ref name="GeorgeOverview" /> || DRAM structures, [[MOSFET]] and semiconductor devices, capacitors<ref>Greer, Frank; Fraser, D.; Coburn, J.W. and Graves, David B. (2002) [https://web.archive.org/web/20140714142300/http://www.avsusergroups.org/tfug_pdfs/TFUG_12_2002_Greer.pdf "Fundamental Vacuum Beam Studies of Radical Enhanced Atomic Layer Chemical Vapor Deposition (REAL-CVD) of TiN"]. NCCAVS.</ref>
|-
| Plasma-enhanced ALD of metal oxides and nitrides || 20–300&nbsp;°C || Al{{sub|2}}O{{sub|3}}, SiO{{sub|2}}, ZnO<sub>x</sub>, InO<sub>x</sub>, HfO{{sub|2}}, SiN<sub>x</sub>, TaN<sub>x</sub><ref>{{cite journal|last1=Potts|first1=S.E.|last2=Kessels|first2=W.M.M.|title=Energy-enhanced atomic layer deposition for more process and precursor versatility|journal=Coordination Chemistry Reviews|date=December 2013|volume=257|issue=23–24|pages=3254–3270|doi=10.1016/j.ccr.2013.06.015}}</ref><ref>{{cite journal|last1=Knoops|first1=Harm C. M.|last2=Braeken|first2=Eline M. J.|last3=de Peuter|first3=Koen|last4=Potts|first4=Stephen E.|last5=Haukka|first5=Suvi|last6=Pore|first6=Viljami|last7=Kessels|first7=Wilhelmus M. M.|title=Atomic Layer Deposition of Silicon Nitride from Bis(''tert''-butylamino)silane and N<sub>2</sub> Plasma|journal=ACS Applied Materials & Interfaces|date=9 September 2015|volume=7|issue=35|pages=19857–19862|doi=10.1021/acsami.5b06833|pmid=26305370}}</ref><ref>{{cite journal|last1=Langereis|first1=E.|last2=Knoops|first2=H. C. M.|last3=Mackus|first3=A. J. M.|last4=Roozeboom|first4=F.|last5=van de Sanden|first5=M. C. M.|last6=Kessels|first6=W. M. M.|title=Synthesis and in situ characterization of low-resistivity TaN<sub>x</sub> films by remote plasma atomic layer deposition|journal=Journal of Applied Physics|date=15 October 2007|volume=102|issue=8|pages=083517–083517–11|doi=10.1063/1.2798598|url=https://research.tue.nl/nl/publications/synthesis-and-in-situ-characterization-of-lowresistivity-tanx-films-by-remote-plasma-atomic-layer-deposition(a90b2cfe-2c78-4c12-8a2c-c4872f23466e).html|bibcode=2007JAP...102h3517L}}</ref>
|| Similar to thermal ALD ||
|}

==Applications==
===Microelectronics applications===
ALD is a useful process for the fabrication of microelectronics due to its ability to produce accurate thicknesses and uniform surfaces in addition to high quality film production using various different materials. In microelectronics, ALD is studied as a potential technique to deposit [[kigh-κ dielectric|high-κ]] (high [[permittivity]]) gate oxides, high-κ memory capacitor dielectrics, ferroelectrics, and metals and nitrides for electrodes and [[Interconnects (integrated circuits)|interconnects]]. In high-κ gate oxides, where the control of ultra thin films is essential, ALD is only likely to come into wider use at the 45&nbsp;nm technology. In metallizations, conformal films are required; currently it is expected that ALD will be used in mainstream production at the 65&nbsp;nm node. In [[dynamic random access memory|dynamic random access memories]] (DRAMs), the conformality requirements are even higher and ALD is the only method that can be used when feature sizes become smaller than 100&nbsp;nm. Several products that use ALD include [[recording head|magnetic recording heads]], [[MOSFET]] gate stacks, [[Dynamic random-access memory|DRAM]] capacitors, nonvolatile ferroelectric memories, and many others.

====Gate oxides====
Deposition of the [[high-κ]] oxides [[Aluminium oxide|Al<sub>2</sub>O<sub>3</sub>]], [[Zirconium dioxide|ZrO<sub>2</sub>]], and [[Hafnium(IV) oxide|HfO<sub>2</sub>]] has been one of the most widely examined areas of ALD. The motivation for high-κ oxides comes from the problem of high tunneling current through the commonly used [[Silicon dioxide|SiO<sub>2</sub>]] gate dielectric in MOSFETs when it is downscaled to a thickness of 1.0&nbsp;nm and below. With the high-κ oxide, a thicker gate dielectric can be made for the required capacitance density, thus the tunneling current can be reduced through the structure.

====Transition-metal nitrides====
Transition-metal [[nitride]]s, such as [[Titanium nitride|TiN]] and [[Tantalum nitride|TaN]], find potential use both as [[Barrier metal|metal barriers]] and as [[metal gate|gate metals]]. Metal barriers are used to encase the [[copper interconnects]] used in modern integrated circuits to avoid diffusion of Cu into the surrounding materials, such as insulators and the silicon substrate, and also, to prevent Cu contamination by elements diffusing from the insulators by surrounding every Cu interconnect with a layer of metal barriers. The metal barriers have strict demands: they should be pure; dense; conductive; conformal; thin; have good adhesion towards metals and insulators. The requirements concerning process technique can be fulfilled by ALD. The most studied ALD nitride is TiN which is deposited from TiCl<sub>4</sub> and NH<sub>3</sub>.<ref>{{cite journal|doi=10.1002/1521-3862(20020704)8:4<149::AID-CVDE149>3.0.CO;2-F|title=Diffusion Barrier Deposition on a Copper Surface by Atomic Layer Deposition|journal=Chemical Vapor Deposition|volume=8|issue=4|pages=149|year=2002|last1=Elers|first1=K.-E.|last2=Saanila|first2=V.|last3=Soininen|first3=P.J.|last4=Li|first4=W.-M.|last5=Kostamo|first5=J.T.|last6=Haukka|first6=S.|last7=Juhanoja|first7=J.|last8=Besling|first8=W.F.A.}}</ref>

====Metal films====
Motivations of an interest in metal ALD are:

# [[Cu interconnects]] and W plugs, or at least Cu seed layers<ref>{{cite journal | last1 = Waechtler | first1 = T. | display-authors = etal | year = 2011 | title = ALD-grown seed layers for electrochemical copper deposition integrated with different diffusion barrier systems| url = http://www.qucosa.de/fileadmin/data/qucosa/documents/6804/Waechtler_MEE2011_Preprint.pdf| journal = Microelectron. Eng. | volume = 88 | issue = 5| pages = 684–689 | doi = 10.1016/j.mee.2010.07.004 }}</ref> for Cu electrodeposition and W seeds for W CVD, 
# transition-metal nitrides (e.g. TiN, TaN, WN) for Cu interconnect barriers
# noble metals for [[Ferroelectric RAM|ferroelectric random access memory]] (FRAM) and DRAM capacitor electrodes
# high- and low-[[work function]] metals for [[multigate device|dual-gate]] [[MOSFET]]s.

====Magnetic recording heads====
Magnetic recording heads utilize electric fields to polarize particles and leave a magnetized pattern on a hard disk.<ref>[https://web.archive.org/web/20100711205358/https://ccrma.stanford.edu/courses/192a/6-Magnetic_Recording.pdf Magnetic Recording Theory]. stanford.edu</ref> Al{{sub|2}}O{{sub|3}} ALD is used to create uniform, thin layers of insulation.<ref name =miics>{{cite web|url=https://web.archive.org/web/20120530034701/http://www.miics.net/archive/getfile.php?file=162|title=Industrial Applications of Atomic Layer Deposition (ALD)|publisher=10th MIICS Conference|year=2010|author=Leskelä, Markku }}</ref> By using ALD, it is possible to control the insulation thickness to a high level of accuracy. This allows for more accurate patterns of magnetized particles and thus higher quality recordings.

====DRAM capacitors====
DRAM capacitors are yet another application of ALD. An individual DRAM cell can store a single bit of data and consists of a single [[MOSFET|MOS transistor]] and a [[capacitor]]. Major efforts are being put into reducing the size of the capacitor which will effectively allow for greater memory density. In order to change the capacitor size without affecting the capacitance, different cell orientations are being used. Some of these include stacked or trench capacitors.<ref>{{cite book|chapter-url=http://smithsonianchips.si.edu/ice/cd/MEMORY97/SEC07.PDF|author=Matas, Brian |author2=de Suberbasaux, Christian |title=Memory, 1997: Complete Coverage of DRAM, Sram, EPROM, and Flash Memory IC's|publisher=Integrated Circuit Engineering Corp. |place=Scottsdale, AZ, USA |year=1997|isbn=978-1877750595 |chapter=Ch. 7. DRAM Technology}}</ref> With the emergence of trench capacitors, the problem of fabricating these capacitors comes into play, especially as the size of [[semiconductor]]s decreases. ALD allows trench features to be scaled to beyond 100&nbsp;nm. The ability to deposit single layers of material allows for a great deal of control over the material. Except for some issues of incomplete film growth (largely due to insufficient amount or low temperature substrates), ALD provides an effective means of depositing thin films like dielectrics or barriers.<ref>{{cite journal|url=https://web.archive.org/web/20050908182325/http://www.future-fab.com/documents.asp?d_ID=1669|author=Gutsche, M. |title=Atomic Layer Deposition for Advanced DRAM Applications|journal=Future Fab Intl.|volume =14|year=2003}}</ref>

===Biomedical applications===
Understanding and being able to specify the surface properties on [[biomedical]] devices is critical in the biomedical industry, especially regarding devices that are implanted in the body. A material interacts with the environment at its surface, so the surface properties largely direct the interactions of the material with its environment. [[Surface chemistry]] and surface topography affect [[protein adsorption]], cellular interactions, and the [[immune response]].<ref>{{cite journal | author1 = Chen, Hong | author2 = Yuan, Lin | author3= Song, Wei | author4 = Wu, Shongkui | author5 = Li, Dan | title = Biocompatible Polymer Materials: Role of Protein-surface Interactions | journal = Progress in Polymer Science | year = 2008 | volume = 33 | issue = 11 | pages = 1059–1087 | doi = 10.1016/j.progpolymsci.2008.07.006}}</ref>

Some current uses in biomedical applications include creating flexible sensors, modifying nanoporous membranes, polymer ALD, and creating thin [[wiktionary:biocompatible|biocompatible]] coatings. ALD has been used to deposit TiO{{sub|2}} films to create optical waveguide sensors as diagnostic tools.<ref>{{cite journal | author1 = Purniawan, A. | author2 = French, P.J. | author3 = Pandraud, G. | author4 = Sarro, P.M. | title = TiO{{sub|2}} ALD nanolayer as evanescent waveguide for biomedical sensor applications | journal = Procedia Engineering | volume = 5 | pages = 1131–1135 | year = 2010 | doi = 10.1016/j.proeng.2010.09.310| title-link = evanescent wave }}</ref> Also, ALD is beneficial in creating flexible sensing devices that can be used, for example, in the clothing of athletes to detect movement or heart rate. ALD is one possible manufacturing process for flexible organic field-effect transistors (OFETs) because it is a low-temperature deposition method.<ref>{{cite journal | author1 = Pang, Changhyun | author2 = Lee, Chanseok | author3 = Suh, Kahp-Yang | title = Recent Advances in Flexible Sensors for Wearable and Implantable Devices | journal = Applied Polymer Science | year = 2013 | doi = 10.1002/app.39461 | pages = 1429–1441 | volume=130| issue = 3 }}</ref>

[[wiktionary:nanoporous|Nanoporous]] materials are emerging throughout the biomedical industry in drug delivery, implants, and tissue engineering. The benefit of using ALD to modify the surfaces of nanoporous materials is that, unlike many other methods, the saturation and self-limiting nature of the reactions means that even deeply embedded surfaces and interfaces are coated with a uniform film.<ref name=stam/> Nanoporous surfaces can have their pore size reduced further in the ALD process because the conformal coating will completely coat the insides of the pores. This reduction in pore size may be advantageous in certain applications.<ref>{{cite journal | doi=10.1007/s11837-008-0028-9 | title = Nanoporous Materials for Biomedical Devices | journal = Biological Materials Science | year = 2008 | pages = 26–32 | volume = 60 | issue = 3| last1 = Adiga | first1 = Shashishekar P. | last2 = Curtiss | first2 = Larry A. | last3 = Elam | first3 = Jeffrey W. | last4 = Pellin | first4 = Michael J. | last5 = Shih | first5 = Chun-Che | last6 = Shih | first6 = Chun-Ming | last7 = Lin | first7 = Shing-Jong | last8 = Su | first8 = Yea-Yang | last9 = Gittard | first9 = Shaun D. | last10 = Zhang | first10 = Junping | last11 = Narayan | first11 = Roger J. | bibcode = 2008JOM....60c..26A }}</ref>

===Quality and its control===
The quality of an ALD process can be monitored using several different imaging techniques to make sure that the ALD process is occurring smoothly and producing a conformal layer over a surface. One option is the use of cross-sectional scanning electron microscopy (SEM) or transmission electron microscopy (TEM). High magnification of images is pertinent for assessing the quality of an ALD layer. [[X-ray reflectivity]] (XRR) is a technique that measures thin-film properties including thickness, density, and surface roughness.<ref>{{cite journal |url=https://www.rigaku.com/downloads/journal/RJ26-2/RJ26-2.pdf| journal = The Rigaku Journal | volume = 26 | issue = 2 | year = 2010 | pages = 1–9 | title = X-ray thin-film measurement techniques | author = Yasaka, Miho}}</ref> Another optical quality evaluation tool is spectroscopic [[ellipsometry]]. Its application between the depositions of each layer by ALD provides information on the growth rate and material characteristics of the film.<ref name=insitu>{{cite journal | journal = Journal of Physics D: Applied Physics | title = In situ spectroscopic ellipsometry as a versatile tool for studying atomic layer deposition | doi = 10.1088/0022-3727/42/7/073001 | volume = 42 | issue = 7 | year = 2009 | pages = 1–19| last1 = Langereis | first1 = E. | last2 = Heil | first2 = S B S. | last3 = Knoops | first3 = H C M. | last4 = Keuning | first4 = W. | last5 = Van De Sanden | first5 = M C M. | last6 = Kessels | first6 = W M M. }}</ref>

Applying this analysis tool during the ALD process, sometimes referred to as ''in situ'' spectroscopic ellipsometry, allows for greater control over the growth rate of the films during the ALD process. This type of quality control occurs during the ALD process rather than assessing the films afterwards as in TEM imaging, or XRR. Additionally, [[Rutherford backscattering spectroscopy]] (RBS), [[X-ray photoelectron spectroscopy]] (XPS), [[Auger electron spectroscopy]] (AES), and [[four-terminal sensing]] can be used to provide quality control information with regards to thin films deposited by ALD.<ref name=insitu />

==Advantages and limitations==

===Advantages===
ALD provides a very controlled method to produce a film to an atomically specified thickness. Also, the growth of different multilayer structures is straightforward. Because of the sensitivity and precision of the equipment, it is very beneficial to those in the field of microelectronics and nanotechnology in producing small, but efficient semiconductors. ALD typically involves the use of relatively low temperatures and a catalyst, which is thermochemically favored. The lower temperature is beneficial when working with soft substrates, such as organic and biological samples. Some precursors that are thermally unstable still may be used so long as their decomposition rate is relatively slow.<ref name=stam/><ref name="GeorgeOverview" />

===Disadvantages===
High purity of the substrates is very important, and as such, high costs will ensue. Although this cost may not be much relative to the cost of the equipment needed, one may need to run several trials before finding conditions that favor their desired product. Once the layer has been made and the process is complete, there may be a requirement of needing to remove excess precursors from the final product. In some final products there are less than 1% of impurities present.<ref name="Instrument">{{Cite journal |url=https://web.archive.org/web/20140714150213/http://files.instrument.com.cn/FilesCenter/20060216/18273.pdf |pmid=14639717 |year=2003 |last1=Leskelä |first1=M. |title=Atomic layer deposition chemistry: Recent developments and future challenges |journal=Angewandte Chemie |volume=42 |issue=45 |pages=5548–54 |last2=Ritala |first2=M. |doi=10.1002/anie.200301652 }}</ref>

====Economic viability====
Atomic layer deposition instruments can range anywhere from $200,000 to $800,000 based on the quality and efficiency of the instrument. There is no set cost for running a cycle of these instruments; the cost varies depending on the quality and purity of the substrates used, as well as the temperature and time of machine operation. Some substrates are less available than others and require special conditions, as some are very sensitive to oxygen and may then increase the rate of decomposition. Multicomponent oxides and certain metals traditionally needed in the microelectronics industry are generally not cost efficient.<ref>{{cite web|url=http://www.svta.com/|title=Molecular Beam Epitaxy & Atomic Layer Deposition Systems |publisher=SVT Associates}}</ref>

====Reaction time====
The process of ALD is very slow and this is known to be its major limitation. For example, Al<sub>2</sub>O<sub>3</sub> is deposited at a rate of 0.11&nbsp;nm per cycle,<ref name=":4" /> which can correspond to an average deposition rate of 100–300&nbsp;nm per hour, depending on cycle duration and pumping speed. ALD is typically used to produce substrates for microelectronics and nanotechnology, and therefore, thick atomic layers are not needed. Many substrates cannot be used because of their fragility or impurity. Impurities are typically found on the 0.1–1 at.% because of some of the carrier gases are known to leave residue and are also sensitive to oxygen.<ref name="Instrument" />

====Chemical limitations====
Precursors must be volatile, but not subject to decomposition, as most precursors are very sensitive to oxygen/air, thus causing a limitation on the substrates that may be used. Some biological substrates are very sensitive to heat and may have fast decomposition rates that are not favored and yield larger impurity levels. There are a multitude of thin-film substrate materials available, but the important substrates needed for use in microelectronics can be hard to obtain and may be very expensive.<ref name="Instrument" />

==References==
{{reflist|colwidth=30em|refs=
<ref name=stam>{{cite journal|title=New Development of Atomic Layer Deposition: Processes, Methods, and Applications|journal=Science and Technology of Advanced Materials|year=2019|volume=20|issue=ja|pages=465–496|doi=10.1080/14686996.2019.1599694|pmid=31164953|pmc=6534251|last1=Oviroh|first1=Peter Ozaveshe|last2=Akbarzadeh|first2=Rokhsareh|last3=Pan|first3=Dongqing|last4=Coetzee|first4=Rigardt Alfred Maarten|last5=Jen|first5=Tien-Chien}}{{open access}}</ref>
}}

==External links==
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* [https://www.youtube.com/watch?v=9p2wwOTpCCI ALD animation ] 
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[[Category:Thin film deposition]]
[[Category:Semiconductor device fabrication]]
[[Category:Chemical processes]]
[[Category:Finnish inventions]]
[[Category:Soviet inventions]]