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What is Chemical Vapor Deposition?

What is Chemical Vapor Deposition?

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Chemical vapor deposition process

Chemical vapor deposition is a technique for depositing thin films of materials on wafers or other substrates. Source gases are introduced into a reaction chamber and energy is applied through heat, high frequency high voltage (RF Power), or other techniques that result in the decomposition of the source gas and reaction of the chemicals to form a film. 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 films. The semiconductor industry is the aggregate collection of companies engaged in the design and fabrication of semiconductor devices. A thin film is a layer of material ranging from fractions of a nanometer to several micrometers in thickness. In a typical CVD process, 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. A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the manufacture of integrated circuits and other micro devices. Frequently, volatile by-products are also produced, which are removed by gas flow through the reaction chamber.

 

What are the broad classifications of CVD?

A number of forms of CVD are in wide use and 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 sub-atmospheric pressures. Reduced pressures tend to reduce unwanted gas-phase reactions and improve film uniformity across the wafer. Most modern CVD processes are either LPCVD or UHVCVD.
    • Ultra-high vacuum CVD (UHVCVD):  CVD processes at a very low pressure, typically below 10-6 Pascal.
  • Classified by physical characteristics of vapor:

    • Aerosol assisted CVD (AACVD): A CVD process in which the precursors are carried to the substrate by means of a liquid/gas aerosol, which can be generated ultrasonically. This technique is suitable for use with non-volatile precursors.
    • Direct liquid injection CVD (DLICVD): DLICVD 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 precursor vapors 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 by using this technique.
  • Plasma methods:

    • Plasma-Enhanced CVD (PECVD): CVD processes that utilize plasma (plasma is a state of matter similar to gas) to boost 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 crystalline films.
  • Combustion Chemical Vapor Deposition (CCVD): Combustion chemical vapor deposition (CCVD) is a chemical process by which thin film coatings are deposited onto substrates in the open atmosphere.
  • Hot wire CVD (HWCVD): Also known as catalytic CVD (Cat-CVD) or hot filament CVD (HFCVD). It uses a hot filament to chemically decompose the source gases.

 

What are the Substances usually deposited for integrated circuits (ICs)?

  • PolySilicon: The chemical vapor deposition process is the synthesis of polycrystalline silicon from silane (SiH4), using this reaction:

SiH4 --> Si + 2H2

In the silane reaction, the medium would either be pure silane gas, or silane with 70-80% nitrogen. Using a temperature between 600 and 650 °C (1100 - 1200 °F), and pressure between 25 and 150 Pa — less than a thousandth of an atmosphere — pure silicon can be deposited at a rate of between 10 and 20 nm per minute, perfect for many circuit board components, whose thickness is measured in microns. Generally, temperatures inside a chemical vapor temperature deposition machine are high, while pressures are very low.

  • 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:

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.

  • Metals: CVD processes for molybdenum, tantalum, titanium, nickel, and tungsten are widely used. The usual source for tungsten is tungsten hexafluoride, which may be deposited in two ways:

WF6 --> W + 3 F2

WF6 + 3 H2 --> W + 6 HF

 

What are the various materials used in Chemical Vapor Deposition?

Materials used in chemical vapor deposition include silicon, carbon fiber, carbon nanofibers, filaments, carbon nanotubes, silicon dioxide, silicon-germanium, tungsten, silicon carbide, silicon nitride, silicon oxynitride, titanium nitride, and diamond.

 

What are the applications of Chemical Vapor Deposition?

  •  Chemical Vapor Deposition is often used in the semiconductor industry to produce thin films. The semiconductor industry is the aggregate collection of companies engaged in the design and fabrication of semiconductor devices. A thin film is a layer of material ranging from fractions of a nanometer to several micrometers in thickness.
  •  The Chemical Vapor Deposition process is also used to produce synthetic diamonds.
  •  Micro fabrication is the term that describes processes of fabrication of miniature structures, of micrometre sizes and smaller. Micro fabrication processes widely use Chemical Vapor Deposition to deposit materials.

 

What are the drawbacks of Chemical Vapor Deposition?

  • CVD can get very expensive due to the power requirements of the process, which partially accounts for the extremely high cost (hundreds of millions of dollars) of semiconductor factories.
  • Chemical vapor deposition reactions often leave byproducts, which must be removed by a continuous gas flow.
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