Chemical vapor deposition

For the scientific journal named Chemical Vapor Deposition, see Chemical Vapor Deposition (journal).
DC plasma (violet) enhances the growth of carbon nanotubes in laboratory-scale PECVD apparatus

Chemical vapor deposition (CVD) is a chemical process used to produce high quality, high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films. In typical CVD, the wafer (substrate) is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit. Frequently, volatile by-products 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 epitaxial. These materials include: silicon (SiO2, germanium, carbide, nitride, oxynitride), carbon (fiber, nanofibers, nanotubes, diamond and graphene), fluorocarbons, filaments, tungsten, titanium nitride and various high-k dielectrics.

Types

Hot-wall thermal CVD (batch operation type)
Plasma assisted CVD

CVD is practiced in a variety of formats. These processes generally differ in the means by which chemical reactions are initiated.

Most modern CVD is either LPCVD or UHVCVD.

Uses

CVD is commonly used to deposit conformal films and augment substrate surfaces in ways that more traditional surface modification techniques are not capable of. CVD is extremely useful in the process of atomic layer deposition at depositing extremely thin layers of material. A variety of applications for such films exist. Gallium arsenide is used in some integrated circuits (ICs) and photovoltaic devices. Amorphous polysilicon is used in photovoltaic devices. Certain carbides and nitrides confer wear-resistance.[7] Polymerization by CVD, perhaps the most versatile of all applications, allows for super-thin coatings which possess some very desirable qualities, such as lubricity, hydrophobicity and weather-resistance to name a few.[8] CVD of metal-organic frameworks, a class of crystalline nanoporous materials, has recently been demonstrated.[9] Applications for these films are anticipated in gas sensing and low-k dielectrics

Commercially important materials prepared by CVD

Polysilicon

Polycrystalline silicon is deposited from trichlorosilane (SiHCl3) or silane (SiH4), using the following reactions:[10]

SiH3Cl → Si + H2 + HCl
SiH4 → Si + 2 H2

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 °C and pressures between 25 and 150 Pa yield a growth rate between 10 and 20 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 (SiCl2H2) and nitrous oxide[11] (N2O), or tetraethylorthosilicate (TEOS; Si(OC2H5)4). The reactions are as follows :

SiH4 + O2 → SiO2 + 2 H2
SiCl2H2 + 2 N2O → SiO2 + 2 N2 + 2 HCl
Si(OC2H5)4 → SiO2 + 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 nonconformally. 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 oxide, but thermal oxidation can only be used in the earliest stages of IC manufacturing.

Oxide may also be grown with impurities (alloying or "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 5–15% 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:

4 PH3 + 5 O2 → 2 P2O5 + 6 H2

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 BPO4 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. 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 silicon nitride from the gas phase:

3 SiH4 + 4 NH3 → Si3N4 + 12 H2
3 SiCl2H2 + 4 NH3 → Si3N4 + 6 HCl + 6 H2

Silicon nitride deposited by LPCVD contains up to 8% hydrogen. It also experiences strong tensile stress, which may crack films thicker than 200 nm. However, it has higher resistivity and dielectric strength than most insulators commonly available in microfabrication (1016 Ω·cm and 10 MV/cm, respectively).

Another two reactions may be used in plasma to deposit SiNH:

2 SiH4 + N2 → 2 SiNH + 3 H2
SiH4 + NH3 → SiNH + 3 H2

These films have much less tensile stress, but worse electrical properties (resistivity 106 to 1015 Ω·cm, and dielectric strength 1 to 5 MV/cm).[12]

Metals

CVD for tungsten is achieved from tungsten hexafluoride (WF6), which may be deposited in two ways:

WF6 → W + 3 F2
WF6 + 3 H2 → W + 6 HF

Other metals, notably aluminium and copper, can be deposited by CVD. As of 2010, commercially cost-effective CVD for copper did not exist, although volatile sources exist, such as Cu(hfac)2. Copper is typically deposited by electroplating. Aluminum can be deposited from triisobutylaluminium (TIBAL) and related organoaluminium compounds.

CVD for molybdenum, tantalum, titanium, nickel is widely used. These metals can form useful silicides 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 chloride deposition reaction is as follows:

2 MCl5 + 5 H2 → 2 M + 10 HCl

whereas the carbonyl decomposition reaction can happen spontaneously under thermal treatment or acoustic cavitation and is as follows:

M(CO)n → M + n CO

the decomposition of metal carbonyls is often violently precipitated by moisture or air, where oxygen reacts with the metal precursor to form metal or metal oxide along with carbon dioxide.

Niobium(V) oxide layers can be produced by the thermal decomposition of niobium(V) ethoxide with the loss of diethyl ether[13][14] according to the equation:

2 Nb(OC2H5)5 → Nb2O5 + 5 C2H5OC2H5

Graphene

Many variations of CVD can be utilized to synthesize graphene. Although many advancements have been made, the processes listed below are not commercially viable yet.

The most popular carbon source used to produce graphene is methane gas. Less popular choices include petroleum asphalt, notable for being inexpensive but more difficult to work with.[15]

The use of catalyst is viable in changing the physical process of graphene production. Notable examples include iron nanoparticles, nickel foam, and gallium vapor. These catalysts can either be used in situ during graphene buildup,[15][16] or situated at some distance away at the deposition area.[17] Some catalysts require another step to remove them from the sample material.[16]

Physical conditions such as surrounding pressure, temperature, carrier gas, and chamber material play a big role in production of graphene.

Most systems use LPCVD with pressures ranging from 1 to 1500 Pa.[15][17][18][19] However, some still use APCVD.[16] Low pressures are used more commonly as they help prevent unwanted reactions and produce more uniform thickness of deposition on the substrate.

On the other hand, temperatures used range from 800-1050 °C.[15][16][17][18][19] High temperatures translate to an increase of the rate of reaction. Caution has to be exercised as high temperatures do pose higher danger levels in addition to greater energy costs.

Hydrogen gas and inert gases such as argon are flowed into the system.[15][16][17][18][19] These gases act as a carrier, enhancing surface reaction and improving reaction rate, thereby increasing deposition of graphene onto the substrate.

Standard quartz tubing and chambers are used in CVD of graphene.[15][16][17][18][19] Quartz is chosen because it has a very high melting point and is chemically inert. In other words, quartz does not interfere with any physical or chemical reactions regardless of the conditions.

Raman spectroscopy, X-ray spectroscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) are used to examine and characterize the graphene samples.[15][16][17][18][19]

Raman spectroscopy is used to characterize and identify the graphene particles; X-ray spectroscopy is used to characterize chemical states; TEM is used to provide fine details regarding the internal composition of graphene; SEM is used to examine the surface and topography.

Sometimes, atomic force microscopy (AFM) is used to measure local properties such as friction and magnetism.[18][19]

Diamond

A colorless faceted gem
Colorless gem cut from diamond grown by chemical vapor deposition
Main article: CVD diamond

Chemical vapor deposition (CVD) can be used to produce a synthetic diamond by creating the circumstances necessary for carbon atoms in a gas to settle on a substrate in crystalline form.

CVD production of diamonds has received a great deal of attention in the materials sciences because it allows many new applications of diamonds that had previously been considered too difficult to make economical. CVD diamond growth typically occurs under low pressure (1–27 kPa; 0.145–3.926 psi; 7.5-203 Torr) and involves feeding varying amounts of gases into a chamber, energizing them and providing conditions for diamond growth on the substrate. The gases always include a carbon source, and typically include hydrogen as well, though the amounts used vary greatly depending on the type of diamond being grown. Energy sources include hot filament, microwave power, and arc discharges, among others. The energy source is intended to generate a plasma in which the gases are broken down and more complex chemistries occur. The actual chemical process for diamond growth is still under study and is complicated by the very wide variety of diamond growth processes used.

Using CVD, films of diamond can be grown over large areas of substrate with control over the properties of the diamond produced. In the past, when high pressure high temperature (HPHT) techniques were used to produce a diamond, the result was typically very small free standing diamonds of varying sizes. With CVD diamond growth areas of greater than fifteen centimeters (six inches) diameter have been achieved and much larger areas are likely to be successfully coated with diamond in the future. Improving this process is key to enabling several important applications.

The growth of diamond directly on a substrate allows the addition of many of diamond's important qualities to other materials. Since diamond has the highest thermal conductivity of any bulk material, layering diamond onto high heat producing electronics (such as optics and transistors) allows the diamond to be used as a heat sink.[20][21] Diamond films are being grown on valve rings, cutting tools, and other objects that benefit from diamond's hardness and exceedingly low wear rate. In each case the diamond growth must be carefully done to achieve the necessary adhesion onto the substrate. Diamond's very high scratch resistance and thermal conductivity, combined with a lower coefficient of thermal expansion than Pyrex glass, a coefficient of friction close to that of Teflon (Polytetrafluoroethylene) and strong lipophilicity would make it a nearly ideal non-stick coating for cookware if large substrate areas could be coated economically.

CVD growth allows one to control the properties of the diamond produced. In the area of diamond growth, the word "diamond" is used as a description of any material primarily made up of sp3-bonded carbon, and there are many different types of diamond included in this. By regulating the processing parameters—especially the gases introduced, but also including the pressure the system is operated under, the temperature of the diamond, and the method of generating plasma—many different materials that can be considered diamond can be made. Single crystal diamond can be made containing various dopants.[22] Polycrystalline diamond consisting of grain sizes from several nanometers to several micrometers can be grown.[20][23] Some polycrystalline diamond grains are surrounded by thin, non-diamond carbon, while others are not. These different factors affect the diamond's hardness, smoothness, conductivity, optical properties and more.

Chalcogenides

Commercially, mercury cadmium telluride is of continuing interest for detection of infrared radiation. Consisting of an alloy of CdTe and HgTe, this material can be prepared from the dimethyl derivatives of the respective elements.

See also

References

  1. ↑ "Low Pressure Chemical Vapor Deposition – Technology and Equipment". Crystec Technology Trading GmbH.
  2. ↑ Crystec Technology Trading GmbH, Plasma Enhanced Chemical Vapor Deposition – Technology and Equipment
  3. ↑ Tavares, Jason; Swanson, E.J.; Coulombe, S. (2008). "Plasma Synthesis of Coated Metal Nanoparticles with Surface Properties Tailored for Dispersion". Plasma Processes and Polymers 5 (8): 759. doi:10.1002/ppap.200800074.
  4. ↑ Schropp, R.E.I.; B. Stannowski; A.M. Brockhoff; P.A.T.T. van Veenendaal; J.K. Rath. "Hot wire CVD of heterogeneous and polycrystalline silicon semiconducting thin films for application in thin film transistors and solar cells" (PDF). Materials Physics and Mechanics. pp. 73–82.
  5. ↑ Gleason, Karen K.; Kenneth K.S. Lau; Jeffrey A. Caulfield (2000). "Structure and Morphology of Fluorocarbon Films Grown by Hot Filament Chemical Vapor Deposition". Chemistry of Materials 12 (10): 3032. doi:10.1021/cm000499w.
  6. ↑ Dorval Dion, C.A.; Tavares, J.R. (2013). "Photo-Initiated Chemical Vapour Deposition as a Scalable Particle Functionalization Technology (A Practical Review)". Powder Technology. doi:10.1016/j.powtec.2013.02.024.
  7. ↑ Wahl, Georg et al. (2000) "Thin Films" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim. doi:10.1002/14356007.a26_681
  8. ↑ Gleason, Karen; Ayse Asatekin; Miles C. Barr; Samaan H. Baxamusa; Kenneth K.S. Lau; Wyatt Tenhaeff; Jingjing Xu (May 2010). "Designing polymer surfaces via vapor deposition". Materials Today 13 (5): 26–33. doi:10.1016/S1369-7021(10)70081-X.
  9. ↑ Stassen, I; Styles, M; Grenci, G; Van Gorp, H; Vanderlinden, W; De Feyter, S; Falcaro, P; De Vos, D; Vereecken, P; Ameloot, R (2015). "Chemical vapour deposition of zeolitic imidazolate framework thin films". Nature Materials. doi:10.1038/nmat4509.
  10. ↑ Simmler, W. (2005), "Silicon Compounds, Inorganic", Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH, doi:10.1002/14356007.a24_001
  11. ↑ Proceedings of the Third World Congress of Chemical Engineering, Tokyo, p. 290 (1986)
  12. ↑ Sze, S.M. (2008). Semiconductor devices: physics and technology. Wiley-India. p. 384. ISBN 81-265-1681-X.
  13. ↑ Maruyama, Toshiro (1994). "Electrochromic Properties of Niobium Oxide Thin Films Prepared by Chemical Vapor Deposition". Journal of the Electrochemical Society 141 (10): 2868. doi:10.1149/1.2059247.
  14. ↑ Rahtu, Antti (2002). Atomic Layer Deposition of High Permittivity Oxides: Film Growth and In Situ Studies (Thesis). University of Helsinki. ISBN 952-10-0646-3.
  15. 1 2 3 4 5 6 7 Liu, Zhuchen; Tu, Zhiqiang; Li, Yongfeng; Yang, Fan; Han, Shuang; Yang, Wang; Zhang, Liqiang; Wang, Gang; Xu, Chunming (2014-05-01). "Synthesis of three-dimensional graphene from petroleum asphalt by chemical vapor deposition". Materials Letters 122: 285–288. doi:10.1016/j.matlet.2014.02.077.
  16. 1 2 3 4 5 6 7 Patel, Rajen B.; Yu, Chi; Chou, Tsengming; Iqbal, Zafar (2014-01-01). "Novel synthesis route to graphene using iron nanoparticles". Journal of Materials Research 29 (14): 1522–1527. doi:10.1557/jmr.2014.165. ISSN 2044-5326.
  17. 1 2 3 4 5 6 Murakami, Katsuhisa; Tanaka, Shunsuke; Hirukawa, Ayaka; Hiyama, Takaki; Kuwajima, Tomoya; Kano, Emi; Takeguchi, Masaki; Fujita, Jun-ichi (2015-03-02). "Direct synthesis of large area graphene on insulating substrate by gallium vapor-assisted chemical vapor deposition". Applied Physics Letters 106 (9): 093112. doi:10.1063/1.4914114. ISSN 0003-6951.
  18. 1 2 3 4 5 6 Zhang, CanKun; Lin, WeiYi; Zhao, ZhiJuan; Zhuang, PingPing; Zhan, LinJie; Zhou, YingHui; Cai, WeiWei (2015-09-05). "CVD synthesis of nitrogen-doped graphene using urea". Science China Physics, Mechanics & Astronomy 58 (10): 1–6. doi:10.1007/s11433-015-5717-0. ISSN 1674-7348.
  19. 1 2 3 4 5 6 Kim, Sang-Min; Kim, Jae-Hyun; Kim, Kwang-Seop; Hwangbo, Yun; Yoon, Jong-Hyuk; Lee, Eun-Kyu; Ryu, Jaechul; Lee, Hak-Joo; Cho, Seungmin. "Synthesis of CVD-graphene on rapidly heated copper foils". Nanoscale 6 (9). doi:10.1039/c3nr06434d.
  20. 1 2 Costello, M; Tossell, D; Reece, D; Brierley, C; Savage, J (1994). "Diamond protective coatings for optical components". Diamond and Related Materials 3 (8): 1137–1141. Bibcode:1994DRM.....3.1137C. doi:10.1016/0925-9635(94)90108-2.
  21. ↑ Sun Lee, Woong; Yu, Jin (2005). "Comparative study of thermally conductive fillers in underfill for the electronic components". Diamond and Related Materials 14 (10): 1647–1653. Bibcode:2005DRM....14.1647S. doi:10.1016/j.diamond.2005.05.008.
  22. ↑ Isberg, J (2004). "Single crystal diamond for electronic applications". Diamond and Related Materials 13 (2): 320–324. Bibcode:2004DRM....13..320I. doi:10.1016/j.diamond.2003.10.017.
  23. ↑ Krauss, A (2001). "Ultrananocrystalline diamond thin films for MEMS and moving mechanical assembly devices". Diamond and Related Materials 10 (11): 1952–1961. Bibcode:2001DRM....10.1952K. doi:10.1016/S0925-9635(01)00385-5.

Further reading

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