Chemical vapor deposition (CVD) is a process wherein precursor gases are allowed to come in contact with or pass over a heated substrate in order to generate reactive species in the gaseous phase, which in turn, facilitates the deposition of a solid material, such as nanotubes, nanowires, particle, thin film, and much more, onto the substrate. There are many different types of CVD processes available, including low pressure chemical vapor deposition (LPCVD), atmospheric pressure chemical vapor deposition (APCVD), and metal-organic chemical vapor deposition (MOCVD). The MOCVD process uses metal-organic species as precursors to create thin films of metals and metallic compounds such as metal nitrides and metal oxides.
Figure 1. Typical CVD Reactor.
Atomic layer deposition (ALD) is a unique CVD process that controls deposition on the atomic scale. In this process, the reaction chamber is alternatingly fed with various precursors one by one. The introduced precursors then experience self-limiting surface reactions, which result in the deposition of the same amount of material during each reaction cycle. This leads to the generation of homogeniously thick, highly smooth, alternating layers of different high dense materials with fewer defects.
Figure 2. General MOCVD Mechanism.
Applications of CVD/ALD
CVD/ALD processes are becoming popular thanks to their ability to grow thin films of high conformality and homogeneity with a precisely controlled thickness. The CVD process is used in the following applications:
- Synthesis of exotic new powdered/fibrous materials
- Production of semiconductor devices
- Production of protective coatings, including high temperature-, corrosion-, and wear-resistance coatings
- Fabrication of optical storage media
- Formation of ceramic composites, optical fibers and dense structural components
Especially, the ALD process is gaining more attraction due to its ability to provide even better control over film formation. This process is useful in the following applications:
- Fabrication of microelectronic devices, including thin-film capacitors, radiation detectors, switches, ferroelectric memories, integrated circuits, and microelectromechanical structures (MEMS)
- Development of electroluminescent device technology
- 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.
Importance of Selecting Proper Precursors for CVD/ALD
Since process conditions significantly affect the properties of materials that are formed by CVD/ALD, it is critical to select precursors suitable for producing the desired material. Metal hydrides and halides were initially utilized as typical CVD precursors. At present, there are many different metal organic compounds available, including metal carbonyls, metal amidinates, metal diketonates, metal alkoxides, metal alkyls, and much more.
Figure 3. Process of precursor selection.
The preferred form of precursors is liquid at room temperature and they must exhibit solubility in an inert solvent. They must to be volatile in nature but have to exhibit thermal stability so as to eliminate decomposition during vaporization. Moreover, they must be preferentially reactive with the substrate and the film being grown. Especially, ALD precursors must have self-limiting reactivity with the film surface as well as the substrate.
Most precursors supply only one element to the film being deposited, with all other molecules vaporized during the process. Nonetheless, some compounds can contribute over one element, thus reducing the number of reactants needed for a specific process. Moreover, it is necessary to consider the factor that some metal organic precursors are capable of contributing to the accidental incorporation of oxygen and carbon into the thin films. It is also essential to evaluate the possibility of the undesired pre-reaction of precursors in the vapor phase.
Using proper precursors, customized systems can also be developed for lower-temperature deposition processes in order to eliminate the complexities related to higher temperatures, including changes in the morphology and crystallinity, reduced adhesion of mismatching overlayers, and interlayer atomic diffusion, to name a few. Moreover, their ability to eliminate halogens is another advantage as halogens can cause corrosion during the deposition stage and even after the formation of the film.
Metal β-Diketonate Precursors
Extensive research is ongoing to explore the role of acetylacetonate (acac) complexes and their derivatives as precursors for depositing oxide films using CVD/ALD. Hexafluoroacetylacetone(hfacac), trifluoroacetylacetone (tfacac), dipivaloylmethane (dpm), and acac complexes of copper, nickel, iron, hafnium, and zirconium have been utilized at temperatures down to 450 °C. For noble metals, b-diketonates bearing trifluoro groups demonstrate the thermal stability and volatility ideal for CVD/ALD processes.
For the lanthanides, the precursors with high volatility and low melting point can be prepared using the β-diketonate ligands. With its low melting point, β-diketonate precursors can also be used for www.azonano.com/ads/abmc.aspx?b=6298 of gold.
Applications of Metal β-Diketonate Precursors
Complexes of β-diketonates hold potential to deposit high temperature superconductors using CVD/ALD. Alkaline earth β-diketonates are utilized as precursors to deposit semiconducting alkaline earth sulfide and thiogallate films, which can be served as the matrix in the emissive layers of electroluminescent devices.
Gold films formed from β-diketonates are utilized for contact metallization in semiconductors, while copper films deposited can serve as conductive coatings. Moreover, β-diketonates of lanthanides and a few other metals, including potassium, sodium, and manganese, show promise to be used as precursors for producing the dopants and codopants required to form the desired colors.
Metal β-Diketonate Precursors from Strem Chemicals
Strem Chemicals, a high-purity specialty chemicals supplier, offers a variety of metal organic precursors, such as metal alkyl, cycopentadienyl, halide, carbonyl, β-diketonate, amidinate, alkoxide, alkyl amide and other derivatives of roughly 60 metals. The following are the selected examples of metal β-diketonate precursors offered by Strem Chemicals:
This information has been sourced, reviewed and adapted from materials provided by Strem Chemicals.
For more information on this source, please visit Strem Chemicals.