Chemical vapor deposition (CVD) is a process in which a solid material, such as nanowire, particle or thin film, is deposited on a substrate by creating reactive species in the gaseous phase. These reactive species are produced when precursor gases travel over the heated substrate. Several types of CVD processes are employed in current applications and they include low pressure chemical vapor deposition (LPCVD), atmospheric pressure chemical vapor deposition (APCVD), and metal-organic chemical vapor deposition (MOCVD). In MOCVD, metal-organic species are utilized as precursors for making thin films of metals, metallic compounds, metal nitrides, metal oxides, etc.
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Figure 1. Typical CVD Reactor.
Atomic Layer Deposition (ALD)
Atomic layer deposition (ALD) is a special type of CVD process which makes it possible to control the atomic scale deposition, and as a result helps in creating smooth alternating layers of varied materials that are extremely thick, uniform and have minimum defects. To this end, both ALD and CVD processes offer practical options since they promote the growth of thin films that are even and have precise thickness control.
Some of the standard applications of CVD comprise the formation of protective coatings, such as coatings that are resistant to wear, corrosion, and extreme temperatures, as well as the development of thick structural parts, ceramic composites, optical fibers, and innovative powdered and fibrous materials. CVD is suitable for manufacturing optical storage media and is typically utilized for producing semiconductor devices.
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Figure 2. General MOCVD mechanism.
On the other hand, ALD provides better control during the formation of films and hence is increasingly being used for depositing thin films in a number of applications like ferroelectric memories, integrated circuits, microelectromechanical structures, switches, thin-film capacitors, radiation detectors, etc. ALD is also essential for improving electroluminescent device technology.
Significance of Precursor Selection in CVD and ALD
Proper selection of precursors is important to obtain the desired material. Originally, standard CVD precursors consisted of hydrides and metal halides, but in current applications a wide range of metal organic compounds, such as metal carbonyls, metal alkoxides, metal alkyls, metal amidinates, metal diketonates, etc., are being used.
When compared to halides, metal organic precursors offer a number of benefits. For instance, they help in developing customized systems for low-temperature deposition processes, and hence eliminate the difficulties associated with increased temperatures. Moreover, metal organic precursors also eliminate halogens, which tend to be corrosive during the deposition process.
Nonetheless, precursors should not decompose during vaporization and hence must be thermally stable but volatile. They should be able dissolve easily in inert solvents and must retain their liquid state at room temperature. Additionally, they must exhibit preferential as well as self-limiting reactivity towards the film surface and substrate.
Majority of precursors contribute only a single element to the deposited film, while the remaining molecule gets decomposed during the process. In contrast, some metal organic precursors can contribute to the inadvertent integration of oxygen and carbon into the thin films, and this factor should also be taken into account. Moreover, the prospect of precursors’ pre-reaction in the vapor phase should also be considered.
Metal Alkyl Compounds
Aluminum and zinc alkyls are the most popular metal alkyl compounds used in CVD process. These alkyl compounds are suitable for producing sulfide and oxide films. In case of aluminum, alkyls of higher molecular weight alkyls are generally used since less amount of unwanted carbon is embedded into the films. Typically, metal alkyls react with oxygen, water, sulfur, and other basic reagents to create the desired film composition. Upon reaction with alkylphosphates or alkoxysilanols, metal phosphates or silicates, respectively, can be deposited.
Use of Metal Alkyl Precursors
MOCVD helps in producing better conformal coverage of aluminum in contrast to those deposited through sputtering or evaporation technique. As a result, MOCVD is being widely adopted for manufacturing semiconductor devices. In fact, MOCVD aluminum is utilized for coating carbon fibers utilized in composites, protective coatings for steel, and for the metallization of semiconductor devices.
Generally, dialkyl zinc compounds are utilized for producing zinc-containing materials, such as ZnO, ZnTe, ZnS and ZnSe, through CVD. These materials are used in solar cells and LEDs. Moreover, as an alternative to traditional electroplating, decomposition of dimethyl cadmium allows the deposition of cadmium through CVD.
Conclusion
CVD/ALD processes present an attractive option as they allow the formation of thin films that are smooth, uniform and exhibit an accurate thickness control.
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This information has been sourced, reviewed and adapted from materials provided by Strem Chemicals.
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