Chemical vapor deposition (CVD) involves the generation of reactive species in the gaseous phase in order to deposit a solid material, such as nanotubes, nanowires, particle, thin film, and much more, on a substrate. The reactive species are formed when precursor gases come in contact with or pass over the heated substrate. There are many different forms of CVD, including metal-organic chemical vapor deposition (MOCVD), low pressure chemical vapor deposition (LPCVD), and atmospheric pressure chemical vapor deposition (APCVD). Metal-organic species are utilized as precursors in the MOCVD process to form thin films of metals and metallic compounds such as metal nitrides and metal oxides.
A specialized form of CVD is the atomic layer deposition (ALD) process, which enables atomic scale deposition control. Various precursors are introduced into the reaction chamber one by one and involve in self-limiting surface reactions in order to deposit the same amount of material during each reaction cycle. This leads to the formation of very smooth, homogenously thick, alternating layers of high dense materials with fewer defects.
Figure 1. Typical CVD Reactor.
Applications of CVD/ALD Processes
CVD/ALD processes gain interest as they can grow thin films of high homogeneity and conformality with a precise control over thickness. The following are the applications of the CVD process:
- Formation of ceramic composites, optical fibers and dense structural components
- Production of protective coatings, including high temperature-, corrosion-, and wear-resistance coatings
- Synthesis of exotic new powdered/fibrous materials
- Production of semiconductor devices
- Fabrication of optical storage media
Nevertheless, the ALD process is gaining more attraction thanks to its even greater control over film formation. The following are the applications of the ALD process:
- Fabrication of microelectronic devices, including thin-film capacitors, radiation detectors, switches, ferroelectric memories, integrated circuits, and microelectromechanical structures (MEMS)
- Production of new high-k gate dielectrics that hold potential to serve as an alternative to silica in next generation of metal oxide semiconductor field effect transistors
- Development of electroluminescent device technology
Significance of Selection of Appropriate Precursors for CVD/ALD Processes
As the properties of the materials produced by the CVD/ALD processes are greatly affected by process conditions, appropriate precursors need to be selected in order to obtain the desired material. Metal halides and hydrides were the CVD precursors initially used. Nowadays, there are many different metal organic compounds available, including metal carbonyls, metal amidinates, metal diketonates, metal alkoxides, metal alkyls, and much more.
Figure 2. General MOCVD Mechanism.
Only one element is contributed to the deposited film by most precursors and all other molecules are vaporized during the process. However, certain compounds can contribute over one element, thus lowering the number of reactants needed for a specific process. Nevertheless, in certain cases, carbon and oxygen may be unintentionally incorporated into the thin films by some metal organic precursors. It is necessary to take into account this factor. Moreover, it is essential to assess the possibility for the undesired pre-reaction of precursors in the vapor phase.
Appropriate precursors hold potential to develop customized systems for lower-temperature deposition processes, thus avoiding the complexities involved in higher temperatures, including changes in the morphology and crystallinity, reduced adhesion of mismatching overlayers, and interlayer atomic diffusion. Moreover, they can eliminate halogens, which can cause corrosion during the deposition process and even after the formation of the film.
Precursors need to be volatile in nature, but also have thermal stability in order to prevent decomposition during vaporization. They are preferably soluble in an inert solvent or liquid at room temperature. Moreover, they must be preferentially reactive with the growing film and substrate. Especially, ALD precursors must exhibit self-limiting reactivity with the film surface and the substrate.
Figure 3. Precursor selection.
Metal Amidinate Precursors
Propionamidinates, acetamidinates, and formamidinates are examples of metal amidinates. Metal amidinates are ideal for the CVD/ALD processes because of the reactivity of the M-N bonds with hydrogen, ammonia, water and others increases the thermal stability of the chelate structure of the ligand. For instance, the stability of zirconium amidinates is much higher than zirconium amides. Moreover, it is possible to tune the properties by selecting different alkyl substituents.
Figure 4. Amidinate precursors.
Amidinates of many different metals, including La, Ag, Ni, cu, Co, Fe, Mn, V, and Ti, have been created and analyzed as CVD/ALD precursors. They exhibit high thermal stability, high volatility and self-limiting reactivity with water vapor to create metal oxide films and with hydrogen to create metal films.
Applications of Metal Amidinate Precursors
The use of alternating doses of copper(I) NN'-diispropylacetamidinate vapor and hydrogen gas can form highly pure, conductive, conformal copper metal films with better adhesion to various substrates, while cobalt metal films have been formed from alternating doses of cobalt(II) bis(N,N'-diispropylacetamidinate) vapor and hydrogen gas. By using water vapor and ammonia in place of hydrogen, oxides and nitrides of these metals can be formed. The following are the potential applications of these films:
- Magnetoresistant layers in magnetic information storage devices
- Electrical interconnects in microelectronics
Metal Amidinate Precursors from Strem Chemicals
Strem Chemicals, a supplier of high-purity specialty chemicals, provides a wide range of metal organic precursors, including metal alkyl, halide, cycopentadienyl, carbonyl, beta-diketonate, amidinate, alkoxide, alkyl amide, and other derivatives of nearly 60 metals. The following are the selected examples of metal amidinate precursors offered by Strem Chemicals:
- Bis(N,N'-di-i-propylacetamidinato)cobalt (II)
This information has been sourced, reviewed and adapted from materials provided by Strem Chemicals.
For more information on this source, please visit Strem Chemicals.