Deposition of a solid material such as a particle, thin film, nanowire or nanotubes on a substrate by generating reactive species in the gaseous phase is referred to as chemical vapor deposition. When precursor gases come in contact or pass over the heated substrate reactive species are generated. There are a number of CVD forms such as atmospheric pressure chemical vapor deposition (APCVD), low pressure chemical vapor deposition (LPCVD), and metal-organic chemical vapor deposition (MOCVD).
Metal-organic species are used as the precursors for the formation of thin films of metals, metal oxides, metal nitrides, and other types of metallic compounds in the case of MOCVD.
Atomic layer deposition (ALD) is a specialized form of CVD in which atomic scale deposition control is possible. Different precursors are alternatively fed into the reaction chamber one at a time and undergo self-limiting surface reactions such that the same amount of material is deposited during each reaction cycle. The result is the formation of uniformly thick, very smooth, alternating layers of different materials that are highly dense and have few defects.
Figure 1. Typical CVD reactor.
CVD/ALD processes are attractive as they allow the growth of thin films of high uniformity and conformity with a precise thickness control. Basic CVD applications include manufacturing temperature, wear and corrosion resistant protective coatings and the formation of dense optical fibers, structural parts, ceramic composites and new powdered/fibrous materials. CVD is ideal for fabricating optical storage media and is used for the production of semiconductor devices.
Figure 2. General MOCVD mechanism.
However, with its excellent film formation control ALD is highly attractive for depositing thin films in applications in microelectronic devices such as ferroelectric memories, switches, radiation detectors, thin-film capacitors, micro-electromechanical structures (MEMS), and possibly as new high-k gate dielectrics to substitute silica in coming generations of metal oxide semiconductor field effect transistors. They also are important for the advancement of electroluminescent device technology.
Importance of Precursor Selection in CVD/ALD
It is important to select the right precursors in order to obtain the desired material. The first typical CVD precursors included metal hydrides and halides, however today a large array of metal organic compounds are used including metal alkyls, metal alkoxides, metal diketonates, metal amidinates, metal carbonyls, and others.
Figure 3. Process of precursor selection.
There is also a chance to develop customized systems for low temperature deposition processes and avoid complexities related to high temperatures such as interlayer atomic diffusion, reduced adhesion of mismatching overlayers, changes in the crystallinity and morphology. Furthermore eliminating halogens, which can be corrosive during the deposition process and even in the formed film if incorporated, is beneficial.
However precursors must be volatile but thermally stable so that decomposition does not occur during vaporization and are preferably liquid at room temperature or soluble in an inert solvent. Also, they must have preferential reactivity towards the substrate and the growing film. ALD precursors must also have self-limiting reactivity with the film surface and the substrate.
Figure 4. ALD Precursors.
Only one element is contributed to the deposited film with remaining molecule vaporized during the process. Certain compounds can contribute more than one element and bring down the number of reactants needed for a particular process.
Also certain metal organic precursors can contribute to the inclusion of carbon and oxygen into the thin films, and this factor must also be considered. Furthermore, the potential for the undesired pre-reaction of precursors in the vapor phase must also be evaluated.
Metal Halide Precursors
The first kind of precursor used in CVD processes are metal halides. They are relatively inexpensive and readily available. The halogen byproducts can be corrosive to the deposited film, both during the CVD process and afterward if trapped in the film. Also halides of many metals are not highly volatile. Hence they have in a number of processes been replaced with organometallic derivatives. However, there are still a number of cases where metal halides continue to be used for CVD processes.
Applications of Metal Halide Precursors
Mo, Ta, and Ti are often deposited through LPCVD using their pentachlorides. Titanium obtained in this manner is used to produce metal foils and shapes, corrosion resistant coatings for steel and other metals, and diffusion barriers for semiconductors.
Metal halides are also used along with metal alkoxides, such as the reaction of TiCl4 and titanium tetraisopropoxide (Ti[(OC3H7)4], TTIP), where the alkoxide serves as the oxygen source in place of water, O2 plasma, O3, etc.
Metal Halide and Other Precursors from Strem
Selected examples of metal halide and other precursors from Strem include:
- Titanium (IV) chloride
- Carbon tetrabromide
- Vanadium (V) trichloride oxide
- Boron bromide
About Strem Chemicals
Strem Chemicals, Inc. established in 1964, is a privately-held company that manufactures and markets specialty chemicals of high purity. Its clients include academic, industrial and government research and development laboratories as well as commercial scale businesses in the pharmaceutical, microelectronic and chemical / petrochemical industries. Strem Chemicals also provides custom synthesis and cGMP manufacturing services.
Strem Chemicals' corporate headquarters is located in Newburyport 38 miles (60km) north of Boston. In 1999 this site was expanded to 30,000 sq. ft (2,800 m2). Its cGMP facility is FDA inspected. The European headquarters and warehouse are located in Bischheim, France.
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This information has been sourced, reviewed and adapted from materials provided by Strem Chemicals.
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