Chemical Vapor Deposition (CVD) is the most widely used deposition method. CVD has historically been used mainly for silicon and dielectric deposition, primarily due to its good quality films and good step coverage.

Either a single gas is used that will decompose when heated to supply the necessary component or components for the film, or multiple gases are used which will react to produce the film.

In either case, the film-producing reaction should take place on the surface of the substrate as opposed to in the gas stream.

The films deposited during CVD are a result of the chemical reaction between the reactive gas(es) and between the reactive gases and the atoms of the substrate surface.

Two types of reactions can occur during the CVD process:

1. Homogeneous (gas phase) – Homogeneous reactions occur before the gas molecules reach the wafer surface and therefore the reaction rate at surface is reduced. The result is a low density and poor quality film.
2. Heterogeneous (surface phase) – Heterogeneous reactions occur on or near the substrate surface. They produce good quality films. Heterogeneous reactions are preferred over homogeneous reactions.

General steps in CVD process:

1. Diffusion of reactant gases to the substrate
2. Absorption of reactants on the surface
3. Chemical reaction (on the surface of film)
4. Desorption of gaseous byproducts
5. Out-diffusion of gaseous byproducts (away from the wafer surface and away from the reactor)