Chemical Vapour Deposition
Chemical vapor deposition (CVD) is a chemical process used to produce high quality, high-performance, solid materials. The process is often used in thesemiconductor industry to produce thin films. In typical Chemical Vapour Deposition, 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-productsare also produced, which are removed by gas flow through the reaction chamber.
Microfabrication processes widely use Chemical Vapour Deposition to deposit materials in various forms, including: monocrystalline, polycrystalline, amorphous, and epitaxial. These materials include: silicon, carbon fiber, carbon nanofibers, fluorocarbons, filaments,carbon nanotubes, SiO2, silicon-germanium, tungsten, silicon carbide, silicon nitride,silicon oxynitride, titanium nitride, and various high-k dielectrics. Chemical Vapour Deposition is also used to produce synthetic diamonds.
CVD is practiced in a variety of formats. These processes generally differ in the means by which chemical reactions are initiated.
- Classified by operating pressure:
- Atmospheric pressure CVD (APCVD) – CVD at atmospheric pressure.
- Low-pressure Chemical Vapour Deposition (LPCVD) – Chemical Vapour Deposition at sub-atmospheric pressures.Reduced pressures tend to reduce unwanted gas-phase reactions and improve film uniformity across the wafer.
- Ultrahigh vacuum CVD (UHVCVD) – CVD at very low pressure, typically below 10−6 Pa (~10−8 torr). Note that in other fields, a lower division between high and ultra-high vacuum is common, often 10−7 Pa.
Most modern Chemical Vapour Deposition is either LPCVD or UHVCVD.
- Classified by physical characteristics of vapor:
- Aerosol assisted Chemical Vapour Deposition (AACVD) – CVD 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 non-volatile precursors.
- Direct liquid injection CVD (DLICVD) – CVD 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). The precursor vapors are then transported to the substrate as in classical CVD. 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 CVD (PECVD) – CVD that utilizes 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. The lower temperatures also allow for the deposition of organic coatings, such as plasma polymers, that have been used for nanoparticle surface functionalization.
- 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. See Atomic layer epitaxy.
- Combustion Chemical Vapor Deposition (CCVD) – Combustion Chemical Vapor Deposition or flame pyrolysis is an open-atmosphere, flame-based technique for depositing high-quality thin films and nanomaterials.
- Hot filament Chemical Vapour Deposition (HFCVD) – also known as catalytic CVD (Cat-CVD) or more commonly, initiated Chemical Vapour Deposition (iCVD), this process uses a hot filament to chemically decompose the source gases. The filament temperature and substrate temperature thus are independently controlled, allowing colder temperatures for better adsorption rates at the substrate and higher temperatures necessary for decomposition of precursors to free radicals at the filament.
- Hybrid Physical-Chemical Vapor Deposition (HPCVD) – This process involves both chemical decomposition of precursor gas and vaporization of a solid source.
- Metalorganic chemical vapor deposition (MOCVD) – This Chemical Vapour Deposition process is based on metalorganic precursors.
- Rapid thermal CVD (RTCVD) – This Chemical Vapour Deposition process uses 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.
- Vapor-phase epitaxy (VPE)
- Photo-initiated CVD (PICVD) – This process uses UV light to stimulate chemical reactions. It is similar to plasma processing, given that plasmas are strong emitters of UV radiation. Under certain conditions, PICVD can be operated at or near atmospheric pressure.
Chemical Vapour Deposition 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 nitridesconfer wear-resistance. Polymerization by CVD Chemical Vapour Deposition, 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