CVD involves thermal dissociation and/or chemical reaction (thermal decomposition (pyrolysis) oxidation, reduction, hydrolysis, nitridation, etc.) of the gaseous reactants on or near the vicinity of a heated surface to form stable solid products . The deposition involves homogeneous and/or heterogeneous chemical reactions leading to the formation of films or powders. During the CVD film deposition, the active gaseous species is generated and transported into the reactor chamber where the gaseous precursor undergoes gas phase reactions forming an intermediate phase. The species of this phase are then absorbed onto the heated substrate, where heterogeneous reactions take place producing the deposit and byproducts. The latter are diffused away from the substrates and transported out from the deposition chamber. The deposit is diffused along the heated substrate surface forming embryos for subsequent growth of the film.
Photo-assisted chemical vapor deposition (PCVD) is a variant of the CVD process that relies on absorption of laser radiation in the substrate to raise its temperature and cause thermal decomposition/chemical reaction of the gaseous precursor to form the required products. Plasma-enhanced CVD (PECVD) uses electron energy (plasma) as the activation method. By supplying electrical power at sufficiently high voltage to a gas at reduced pressure (<10 mbar), breakdown of the gas occurs and generates a glow discharge plasma consisting of electrons, ions, and electronically excited species. The plasma ionizes and decomposes the reactant gases at a low temperature. Chemical vapor deposition can also be classified according to the type of precursor used. The use of a metalorganic precursor, for example, has led to the development of metalorganic chemical vapor deposition (MOCVD). Atomic layer epitaxy (ALE) is a variant of the CVD process that involves surface deposition for the controlled growth of epitaxial films and fabrication of tailored molecular structures on solid surfaces . "Monoatomic" layers can be grown in sequence by sequentially saturating surface reactions.
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