CVD - Chemical Vapor Deposition

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Chemical Vapor Deposition (CVD)

Chemical Vapor Deposition (CVD) refers to the formation of a non-volatile solid film on a substrate from the reaction of vapor phase chemical reactants containing the right constituents. A reaction chamber is used for this process, into which the reactant gases are introduced to decompose and react with the substrate to form the film.

Chemical vapor deposition is used in a multitude of semiconductor wafer fabrication processes, including the production of amorphous and polycrystalline thin films (such as polycrystalline silicon), deposition of SiO₂ (CVD SiO₂) and silicon nitride, and growing of single-crystal silicon epitaxial layers.

A basic CVD process consists of the following steps: 1) a predefined mix of reactant gases and diluent inert gases are introduced at a specified flow rate into the reaction chamber; 2) the gas species move to the substrate; 3) the reactants get adsorbed on the surface of the substrate; 4) the reactants undergo chemical reactions with the substrate to form the film; and 5) the gaseous by-products of the reactions are desorbed and evacuated from the reaction chamber.

During the process of chemical vapor deposition, the reactant gases not only react with the substrate material at the wafer surface (or very close to it), but also in gas phase in the reactor's atmosphere. Reactions that take place at the substrate surface are known as heterogeneous reactions, and are selectively occurring on the heated surface of the wafer where they create good-quality films.

Reactions that take place in the gas phase are known as homogeneous reactions. Homogeneous reactions form gas phase aggregates of the depositing material, which adhere to the surface poorly and at the same time form low-density films with lots of defects. In short, heterogeneous reactions are much more desirable than homogeneous reactions during chemical vapor deposition.

A typical CVD system consists of the following parts: 1) sources of and feed lines for gases; 2) mass flow controllers for metering the gases into the system; 3) a reaction chamber or reactor; 4) a system for heating up the wafer on which the film is to be deposited; and 5) temperature sensors.

Figure 1. Examples of CVD Systems

See Also: Epitaxy; Dielectric; Polysilicon; Thin Films;

PVD By Sputtering; PVD by Evaporation

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