Chemical vapor deposition
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[[Image:PICT0111.JPG|thumb|right|200px|[[direct current|DC]] [[plasma physics|plasma]] (violet) enhances the growth of [[carbon nanotube]]s in this laboratory-scale [[Plasma-enhanced chemical vapor deposition|PECVD]] apparatus.]]
'''Chemical vapor deposition (CVD)''' is a [[chemical process]] used to produce high-purity, high-performance solid materials. The process is often used in the [[semiconductor industry]] to produce [[thin film]]s. In a typical CVD process, the [[wafer (electronics)|wafer]] (substrate) is exposed to one or more volatile [[wiktionary:precursor|precursor]]s, which [[chemical reaction|react]] and/or [[chemical decomposition|decompose]] on the substrate surface to produce the desired deposit. Frequently, volatile [[by-product]]s are also produced, which are removed by gas flow through the reaction chamber.
[[Microfabrication]] processes widely use CVD to deposit materials in various forms, including: [[monocrystalline]], [[polycrystalline]], [[amorphous]], and [[Epitaxy|epitaxial]]. These materials include: [[silicon]], [[carbon fiber]], [[carbon nanofibers]], [[Electrical filament|filaments]], [[carbon nanotube]]s, [[Silicon dioxide|SiO<sub>2</sub>]], [[silicon-germanium]], [[tungsten]], [[silicon carbide]], [[silicon nitride]], [[silicon oxynitride]], [[titanium nitride]], and various [[high-k dielectric]]s. The CVD process is also used to produce [[synthetic diamond]]s.
==Types of chemical vapor deposition==
A number of forms of CVD are in wide use and are frequently referenced in the literature. These processes differ in the means by which chemical reactions are initiated (e.g., activation process) and process conditions.
* Classified by operating pressure
** ''Atmospheric pressure CVD'' (APCVD) - CVD processes at atmospheric pressure.
** ''Low-pressure CVD'' (LPCVD) - CVD processes at subatmospheric pressures. Reduced pressures tend to reduce unwanted gas-phase reactions and improve film uniformity across the wafer. Most modern CVD process are either LPCVD or UHVCVD.
** ''Ultrahigh vacuum CVD'' (UHVCVD) - CVD processes at a very low pressure, typically below 10<sup>-6</sup> [[Pascal (unit)|Pa]] (~ 10<sup>-8</sup> [[torr]]). Caution: in other fields, a lower division between high and [[ultra-high vacuum]] is common, often 10<sup>-7</sup> Pa.
* Classified by physical characteristics of vapor
** ''Aerosol assisted CVD'' (AACVD) - A CVD process in which the precursors are transported to the substrate by means of a liquid/gas aerosol, which can be generated ultrasonically. This technique is suitable for use with involatile precursors.
** ''Direct liquid injection CVD'' (DLICVD) - A CVD process in which the precursors are in liquid form (liquid or solid dissolved in a convenient solvent). Liquid solutions are injected in a vaporization chamber towards injectors (typically car injectors). Then the precursors vapours are transported to the substrate as in classical CVD process. This technique is suitable for use on liquid or solid precursors. High growth rates can be reached using this technique.
* Plasma methods (see also [[Plasma processing]])
** ''Microwave plasma-assisted CVD'' (MPCVD)
** ''[[Plasma-enhanced chemical vapor deposition|Plasma-Enhanced CVD]]'' (PECVD) - CVD processes that utilize a [[Plasma (physics)|plasma]] to enhance chemical reaction rates of the precursors. PECVD processing allows deposition at lower temperatures, which is often critical in the manufacture of semiconductors.
** ''Remote plasma-enhanced CVD'' (RPECVD) - Similar to PECVD except that the wafer substrate is not directly in the plasma discharge region. Removing the wafer from the plasma region allows processing temperatures down to room temperature.
*''Atomic layer CVD'' (ALCVD) – Deposits successive layers of different substances to produce layered, [[crystal]]line films. See [[Atomic layer epitaxy]].
*''Hot wire CVD'' (HWCVD) - Also known as Catalytic CVD (Cat-CVD) or hot filament CVD (HFCVD). Uses a hot filament to chemically decompose the source gases.<ref>{{cite conference | first=R.E.I. | last=Schropp | coauthors=B. Stannowski, A.M. Brockhoff, P.A.T.T. van Veenendaal and J.K. Rath | title=Hot wire CVD of heterogeneous and polycrystalline silicon semiconducting thin films for application in thin film transistors and solar cells | booktitle=Materials Physics and Mechanics | pages=73–82 | url=http://www.ipme.ru/e-journals/MPM/no_2100/schropp/schropp.pdf | format=PDF}}</ref>
*''[[Metalorganic chemical vapor deposition]]'' (MOCVD) - CVD processes based on [[metalorganic]] precursors.
*''[[Hybrid Physical-Chemical Vapor Deposition]]'' (HPCVD) - Vapor deposition processes that involve both [[chemical decomposition]] of precursor gas and [[vaporization]] of solid a source.
*''Rapid thermal CVD'' (RTCVD) - CVD processes that use heating lamps or other methods to rapidly heat the [[Wafer (electronics)|wafer substrate]]. Heating only the substrate rather than the gas or chamber walls helps reduce unwanted gas phase reactions that can lead to [[particle (ecology)|particle]] formation.
*''Vapor phase [[epitaxy]]'' (VPE)
==Substances commonly deposited for ICs==
This section discusses the CVD processes often used for [[integrated circuits]] (ICs). Particular materials are deposited best under particular conditions.
===Polysilicon===
[[Polycrystalline]] silicon is deposited from [[silane]] (SiH<sub>4</sub>), using the following reaction:
:<math>SiH_4 \to Si + 2H_2</math>
This reaction is usually performed in LPCVD systems, with either pure silane feedstock, or a solution of silane with 70-80% [[nitrogen]]. Temperatures between 600 and 650 [[Celsius|°C]] and pressures between 25 and 150 Pa yield a growth rate between 10 and 20 [[nanometre|nm]] per minute. An alternative process uses a [[hydrogen]]-based solution. The hydrogen reduces the growth rate, but the temperature is raised to 850 or even 1050 °C to compensate.
Polysilicon may be grown directly with doping, if gases such as [[phosphine]], [[arsine]] or [[diborane]] are added to the CVD chamber. Diborane increases the growth rate, but arsine and phosphine decrease it.
===Silicon dioxide===
[[Silicon dioxide]] (usually called simply "oxide" in the semiconductor industry) may be deposited by several different processes. Common source gases include silane and [[oxygen]], [[dichlorosilane]] (SiCl<sub>2</sub>H<sub>2</sub>) and [[nitrous oxide]] (N<sub>2</sub>O), or [[tetraethylorthosilicate]] (TEOS; Si(OC<sub>2</sub>H<sub>5</sub>)<sub>4</sub>). The reactions are as follows:
:SiH<sub>4</sub> + O<sub>2</sub> → SiO<sub>2</sub> + 2H<sub>2</sub>
:SiCl<sub>2</sub>H<sub>2</sub> + 2N<sub>2</sub>O → SiO<sub>2</sub> + 2N<sub>2</sub> + 2HCl
:Si(OC<sub>2</sub>H<sub>5</sub>)<sub>4</sub> → SiO<sub>2</sub> + byproducts
The choice of source gas depends on the thermal stability of the substrate; for instance, [[aluminium]] is sensitive to high temperature. Silane deposits between 300 and 500 °C, dichlorosilane at around 900 °C, and TEOS between 650 and 750 °C, resulting in a layer of '''Low Temperature Oxide''' (LTO). However, silane produces a lower-quality oxide than the other methods (lower [[dielectric strength]], for instance), and it deposits non[[conformal film|conformal]]ly. Any of these reactions may be used in LPCVD, but the silane reaction is also done in APCVD. CVD oxide invariably has lower quality than [[thermal oxidation|thermal oxide]], but thermal oxidation can only be used in the earliest stages of IC manufacturing.
Oxide may also be grown with impurities ([[alloy]]ing or "[[doping (semiconductor)|doping]]"). This may have two purposes. During further process steps that occur at high temperature, the impurities may diffuse from the oxide into adjacent layers (most notably silicon) and dope them. Oxides containing 5% to 15% impurities by mass are often used for this purpose. In addition, silicon dioxide alloyed with [[phosphorus pentoxide]] ("P-glass") can be used to smooth out uneven surfaces. P-glass softens and reflows at temperatures above 1000 °C. This process requires a phosphorus concentration of at least 6%, but concentrations above 8% can corrode aluminium. Phosphorus is deposited from phosphine gas and oxygen:
:4PH<sub>3</sub> + 5O<sub>2</sub> → 2P<sub>2</sub>O<sub>5</sub> + 6H<sub>2</sub>
Glasses containing both boron and phosphorus (borophosphosilicate glass, BPSG) undergo viscous flow at lower temperatures; around 850 °C is achievable with glasses containing around 5 weight % of both constituents, but stability in air can be difficult to achieve. Phosphorus oxide in high concentrations interacts with ambient moisture to produce phosphoric acid. Crystals of BPO<sub>4</sub> can also precipitate from the flowing glass on cooling; these crystals are not readily etched in the standard reactive plasmas used to pattern oxides, and will result in circuit defects in integrated circuit manufacturing.
Besides these intentional impurities, CVD oxide may contain byproducts of the deposition process. TEOS produces a relatively pure oxide, whereas silane introduces hydrogen impurities, and dichlorosilane introduces [[chlorine]].
Lower temperature deposition of silicon dioxide and doped glasses from TEOS using ozone rather than oxygen has also been explored (350 to 500 °C). Ozone glasses have excellent conformality but tend to be hygroscopic -- that is, they absorb water from the air due to the incorporation of silanol (Si-OH) in the glass. Infrared spectroscopy and mechanical strain as a function of temperature are valuable diagnostic tools for diagnosing such problems.
===Silicon nitride===
[[Silicon nitride]] is often used as an insulator and chemical barrier in manufacturing ICs. The following two reactions deposit nitride from the gas phase:
:3SiH<sub>4</sub> + 4NH<sub>3</sub> → Si<sub>3</sub>N<sub>4</sub> + 12H<sub>2</sub>
:3SiCl<sub>2</sub>H<sub>2</sub> + 4NH<sub>3</sub> → Si<sub>3</sub>N<sub>4</sub> + 6HCl + 6H<sub>2</sub>
Silicon nitride deposited by LPCVD contains up to 8% hydrogen. It also experiences strong tensile [[stress (physics)]], which may crack films thicker than 200 nm. However, it has higher [[resistivity]] and dielectric strength than most insulators commonly available in microfabrication (10<sup>16</sup> [[ohm|Ω]]·[[centimetre|cm]] and 10 M[[volt|V]]/cm, respectively).
Another two reactions may be used in plasma to deposit SiNH:
:2SiH<sub>4</sub> + N<sub>2</sub> → 2SiNH + 3H<sub>2</sub>
:SiH<sub>4</sub> + NH<sub>3</sub> → SiNH + 3H<sub>2</sub>
These films have much less tensile stress, but worse electrical properties (resistivity 10<sup>6</sup> to 10<sup>15</sup> Ω·cm, and dielectric strength 1 to 5 MV/cm).
===Metals===
Some metals (notably aluminium and [[copper]]) are seldom or never deposited by CVD. [[As of 2002]], a commercially, cost effective, viable CVD process for copper did not exist- though many people have used Copper Formate, Copper(hfac)2, and other precursors(Cu(II) ethyl acetoacetate, etc...). Copper deposition of the metal has been done mostly [[electroplating]] due to cost. Aluminum can be deposited from tri-[[isobutyl]] aluminium (TIBAL), or Tri-ethyl Aluminum (TEA), but [[physical vapor deposition]] methods are usually preferred.
However, CVD processes for [[molybdenum]], [[tantalum]], [[titanium]], nickel, and tungsten are widely used. These metals can form useful [[silicide]]s when deposited onto silicon. Mo, Ta and Ti are deposited by LPCVD, from their pentachlorides. Nickel, Molybdenum, and Tungsten can be deposited at low temperatures from their carbonyl precursors. In general, for an arbitrary metal ''M'', the reaction is as follows:
:2MCl<sub>5</sub> + 5H<sub>2</sub> → 2M + 10HCl
The usual source for tungsten is [[tungsten hexafluoride]], which may be deposited in two ways:
:WF<sub>6</sub> → W + 3F<sub>2</sub>
:WF<sub>6</sub> + 3H<sub>2</sub> → W + 6HF
==See also==
* [[Atomic layer deposition]], a more precise and conformal coating technology
* [[Physical vapor deposition]], the deposition of materials from vapor without chemical reactions
* [[Ion plating]], a process that may use chemical vapor precursors
==References==
<references />
* {{cite book |last=Jaeger |first=Richard C. |title=Introduction to Microelectronic Fabrication |year=2002 |publisher=Prentice Hall |location=Upper Saddle River |id=ISBN 0-201-44494-7 |chapter=Film Deposition}}
* {{cite book |last=Smith |first=Donald |title=Thin-Film Deposition: Principles and Practice |year=1995 |publisher=MacGraw-Hill}}
* {{cite book |last=Dobkin and Zuraw |title=Principles of Chemical Vapor Deposition |year=2003 |publisher=Kluwer}}
* ISO 3529/1-1981 Vacuum Technology - Vocabulary - part 1: General terms As quoted by [http://www.npl.co.uk/pressure/faqs/glossary.html UK National Physical Laboratory]
==External links==
* [http://www.timedomaincvd.com/CVD_Fundamentals/Fundamentals_of_CVD.html Fundamental of Chemical Vapor Deposition], by TimeDomain CVD, Inc.
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[[Category:Plasma processing]]
[[Category:Semiconductor device fabrication]]
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