Chemical vapor deposition

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DC plasma (violet) enhances the growth of carbon nanotubes in this laboratory-scale PECVD apparatus.
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DC plasma (violet) enhances the growth of carbon nanotubes in this laboratory-scale PECVD apparatus.

Chemical vapor deposition (CVD) is a chemical process often used in the semiconductor industry for the deposition of thin films of various materials. In a typical CVD process the 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 byproducts are also produced, which are removed by gas flow through the reaction chamber.

CVD is widely used in the semiconductor industry, as part of the semiconductor device fabrication process, to deposit various films including: polycrystalline, amorphous, and epitaxial silicon, carbon fiber, filaments, carbon nanotubes, SiO2, silicon-germanium, tungsten, silicon nitride, silicon oxynitride, titanium nitride, and various high-k dielectrics. The CVD process is also used to produce synthetic diamonds.

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[edit] 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.

  • Atmospheric pressure CVD (APCVD) - CVD processes at atmospheric pressure.
  • Atomic layer CVD (ALCVD) (also referred to as atomic layer epitaxy and atomic layer deposition (ALD)) - A CVD process in which two complementary precursors (e.g. Al(CH3)3 and H2O) are alternatively introduced into the reaction chamber. Typically, one of the precursors will adsorb onto the substrate surface, but cannot completely decompose without the second precursor. The precursor adsorbs until it saturates the surface and further growth cannot occur until the second precursor is introduced. Thus the film thickness is controlled by the number of precursor cycles rather than the deposition time as is the case for conventional CVD processes. In theory ALCVD allows for extremely precise control of film thickness and uniformity.
  • 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 (mainly metal-organic precursors are used : DLI -Metal-organic CVD)are injected in a vaporization chamber towards injectors (typically car injectors). Then the precursors vapours are transported to the substrtate as in classical CVD process. This technique is suitable for use on liquid or solid precursors. High growth rates can be reach using this technique.
  • Hot wire CVD (HWCVD) - Also known as catalytic CVD (Cat-CVD) or hot filament CVD (HFCVD)
  • 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.
  • Metal-organic CVD (MOCVD) - CVD processes based on metal-organic precursors, such as Tantalum Ethoxide, Ta(OC2H5)5, to create Ta2O5, Tetra Dimethyl amino Titanium (or TDMAT) to create TiN. MOCVD is also called as MOMBE when it is under ultra-high vacuum.
  • Microwave plasma-assisted CVD (MPCVD)
  • Plasma-Enhanced CVD (PECVD) - CVD processes that utilize a 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. See also Plasma processing.
  • Rapid thermal CVD (RTCVD) - CVD processes that use heating lamps or other methods to rapidly heat the wafer substrate. Heating only the substrate rather than the gas or chamber walls helps reduce unwanted gas phase reactions that can lead to particle formation.
  • 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.
  • Ultrahigh vacuum CVD (UHVCVD) - CVD processes at a very low pressure, typically below 10-6 Pa (~ 10-8 torr). Caution: in other fields, a lower division between high and ultra-high vacuum is common, often 10-7 Pa.
  • Polysilicon deposition
  • TEOS deposition - frequently deposited by thermal and PECVD techniques.

[edit] Silicon or silicon germanium epitaxy

Common use in industry is the growth of additional layers of doped silicon on the polished sides of prime silicon wafers, before they are processed into semiconductor devices. This is typical of the power devices, such as those used in pacemakers, vending machine controllers, automobile computers, etc.

Commonly, this is accomplished by either single or batch wafer processing using CVD in an epitaxial reactor, which heats the wafers, etches the exposed face with hydrogen chloride gas, and then grows the epitaxial layers by flowing a gas mixture that contains silicon and a dopant over the wafer which is so hot that it glows. The gaseous molecules deposit on the face, if done properly, and extend the crystalline structure.

Manufacturing issues include control of the amount and uniformity of the deposition's resistivity and thickness, the cleanliness and purity of the surface and the chamber atmosphere, the prevention of the typically much more highly doped substrate wafer's diffusion of dopant to the new layers, imperfections of the growth process, and protecting the surfaces during the manufacture and handling.

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