Manufacturing: Surface Finishing

Chemical vapor deposition (CVD)

Chemical vapor deposition (CVD) is a chemical process for depositing 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 a very versatile process used in the production of coatings, powders, fibers and monolithic parts. With CVD, it is possible to produce almost any metallic or non-metallic element, including carbon and silicon, as well as compounds such as carbides, nitrides, borides, oxides, intermetallics and many others. And applications are boundless.

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, 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.

A key advantage of the CVD process lies in the fact that the reactants used are gases, thereby taking advantage of the many characteristics of gases. One result is that CVD is not a line-of-sight process as are most other plating/coating processes. In addition to being able to penetrate porous bodies, blind holes, large L/D tubes, etc., CVD offers many advantages over other deposition processes. These include:

• Versatile – can deposit any element or compound
• High Purity – typically 99.99-99.999%
• High Density – nearly 100% of theoretical
• Material Formation well below the melting point
• Coatings Deposited by CVD are conformal and near net shape
• Economical in production, since many parts can be coated at the same time

A number of forms of CVD are in wide use and are frequently referenced in the literature.

1. Atmospheric pressure CVD (APCVD) - CVD processes at atmospheric pressure.
2. Atomic layer CVD (ALCVD) (also referred to as Atomic Layer Epitaxy and Atomic layer deposition (ALD)) - A CVD process in which two complementary precursors (eg. 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.
3. 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.
   
4. Hot Wire CVD (HWCVD) - Also known as Catalytic CVD (Cat-CVD)or Hot Filament CVD (HFCVD)
   
5. 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.
   
6. 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.
   
7. Microwave plasma-assisted CVD (MPCVD)
   
8. 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.
   
9. 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.
   
10. 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.
    
11. Ultra-high vacuum CVD (UHVCVD) - CVD processes at very low pressures, typically in the range of a few to a hundred millitorrs (1 to 10 pascals.
    
12. Polysilicon deposition
    
13. TEOS deposition