What is Chemical Vapor Deposition (CVD)?

Introduction

Chemical vapor deposition is a coating method that is commonly used to produce thin films and coatings of very high quality. Gaseous reactants are usually used in this process. In chemical vapor deposition, you transport one or more volatile precursors to the reaction chamber. The volatile precursors usually decompose on a heated substrate surface in the reaction chamber. This process creates some chemical by-products which are emitted from the reaction chamber alongside the unreacted volatile precursors. A lot of materials are deposited via the chemical vapor deposition method, including silicides, metal oxides, sulfides, and arsenides.

Two Chemical Vapor Deposition Reactors

There are two types of chemical vapor deposition reactors. Each type is unique in its applications, advantages, and disadvantages.

Hot Wall Reactors

This is one of the two types of CVD reactors. In hot wall reactors, not only are the substrates heated but also the reactor walls. Hence, the name. Of the two types of chemical vapor deposition reactors, it is the least used.

Advantages

• Large batches of substrates can be produced at once.
• Hot wall reactors are easy to operate.
• There is a uniform substrate temperature. Hence, there is a uniform coating thickness.
• Hot wall reactors can operate at a wide range of temperatures and pressures.

Disadvantages

• It requires a very high temperature and energy.
• Coating occurs not only on the substrate but also on the reactor walls. This makes it difficult to clean the reactor.
• Gas-phase reactions are likely to occur.

Cold Wall Reactors

This is the most commonly used type of CVD reactor. In this type of reactor, while heating the substrate, the reactor wall is cooled. Therefore, the substrate is subjected to a higher temperature than the reactor walls. Compound semiconductor chemical vapor deposition processes typically take place in cold wall reactors.

Advantages

• There are fewer coatings on the walls, making cleaning easier.
• Cold wall reactors require less temperature and energy.
• Cold wall reactors support chemical vapor deposition processes that involve plasmas.
• Depositions occur at a faster rate.
• Gas-phase reactions are less likely to occur.

Disadvantages

• They are more difficult to operate.
• Less substrates can be worked on at once.
• The substrate temperature is not uniform. Hence coating thickness is usually not uniform.

Types of Chemical Vapor Deposition

Plasma-enhanced Chemical Vapor Deposition

This is one of the variants of chemical vapor deposition. In this variant, the reacting gasses are converted to plasma. The space between two electrodes is filled with the reacting gasses and radio frequency or direct current discharge is used to convert the gasses to plasma.

Thermal Chemical Vapor Deposition

In this type of chemical vapor deposition, coatings are easily deposited on the substrates in the open atmosphere. In this process, the precursor material is added to burning gas, making the precursor highly reactive.

Hot-filament Chemical Vapor Deposition

This is also called catalytic chemical vapor deposition. In this process, a hot filament is used to decompose the precursor gasses. The substrate usually has a lower temperature than the hot filament.

Metalorganic Chemical Vapor Deposition

This process is commonly used for single or polycrystalline thin films. One must not confuse this process with molecular beam epitaxy. While molecular beam epitaxy involves physical depositions to grow crystals, molecular chemical vapor deposition involves a chemical reaction. Commonly used precursors for this process include germane, phosphine, and ammonia

Laser Chemical Vapor Deposition

Laser Chemical Vapor Deposition is commonly used for spot coating in semiconductors. In this method, a laser beam is used to heat a part of the substrate. This causes deposition to occur more rapidly on the heated side of the substrate.

Benefits of Chemical Vapor Deposition

Chemical vapor deposition is a common method of thin-film deposition. The popularity of this method can be ascribed to some of its benefits. Some of these benefits are highlighted below.

• Chemical vapor deposition is a relatively affordable method of coating.
• It is a versatile method of coating. Several elements and compounds can be coated with this method.
• It has a high deposition rate with commendable adhesion.
• There is a uniform coating with the chemical vapor deposition method.
• Chemical vapor deposited products have high purity.
• It is a non-line of sight process.

Further Reading: Table Comparison: Physical Vapor Deposition Vs. Chemical Vapor Deposition

Applications of Chemical Vapor Deposition

Electronics

This is the most common application of chemical vapor deposition. Chemical vapor deposition is used to deposit a thin film on semiconductors used in electronics.

Cutting Tools

Chemical vapor deposition is used in coating cutting tools. This prevents corrosion and wears in the cutting tools. In addition, it improves the tool’s lubricity and provides a thermal barrier.

Solar Cells

The manufacture of thin-film solar cells usually involves chemical vapor deposition. One or more layers of photovoltaic materials are deposited on a substrate in thin-film solar cells.

Conclusion

Chemical vapor deposition is a highly welcome innovation in the coating industry. Research is still ongoing on how to maximize the potential of chemical vapor deposition. For more information about thin film deposition, please visit https://www.sputtertargets.net/.