CVD - Chemical Vapor Deposition

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

There are many ways of describing or classifying a CVD reactor. For instance, a reactor is said to be 'hot-wall' if it uses a heating system that heats up not only the wafer, but the walls of the reactor itself, an example of which is radiant heating from resistance-heated coils. 'Cold-wall' reactors use heating systems that minimize the heating up of the reactor walls while the wafer is being heated up, an example of which is heating via IR lamps inside the reactor. In hot-wall reactors, films are deposited on the walls in much the same way as they are deposited on wafers, so this type of reactor requires frequent wall cleaning.

Another way of classifying CVD reactors is by basing it on the range of their operating pressure. Atmospheric pressure CVD (APCVD) reactors operate at atmospheric pressure, and are therefore the simplest in design. Low-pressure CVD (LPCVD) reactors operate at medium vacuum (30-250 Pa) and higher temperature than APCVD reactors. Plasma Enhanced CVD (PECVD) reactors also operate under low pressure, but do not depend completely on thermal energy to accelerate the reaction processes. They also transfer energy to the reactant gases by using an RF-induced glow discharge.

The glow discharge used by a PECVD reactor is created by applying an RF field to a low-pressure gas, creating free electrons within the discharge region. The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them. Energetic species are then adsorbed on the film surface, where they are subjected to ion and electron bombardment, rearrangements, reactions with other species, new bond formation, and film formation and growth.

Table 1 compares the characteristics of typical APCVD, LPCVD, and PECVD reactors.

Table 1. APCVD, LPCVD, and PECVD Comparisons

CVD Process	Advantages	Disadvantages	Applications
APCVD	Simple, Fast Deposition, Low Temperature	Poor Step Coverage, Contamination	Low-temperature Oxides
LPCVD	Excellent Purity, Excellent Uniformity, Good Step Coverage, Large Wafer Capacity	High Temperature, Slow Deposition	High-temperature Oxides, Silicon Nitride, Poly-Si, W, WSi₂
PECVD	Low Temperature, Good Step Coverage	Chemical and Particle Contamination	Low-temperature Insulators over Metals, Nitride Passivation

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