Biomedical Engineering Reference
In-Depth Information
ultrahigh-vacuum CVD are named after the
typical chamber pressure at which the reactions
take place. Depending on the characteristics of
the plasma, the following forms can be found:
microwave plasma-assisted CVD, plasma-
enhanced CVD, magneto-microwave plasma
CVD, and remote plasma-enhanced CVD. If the
characteristics of the vapor used are considered,
the following two forms are commonly found:
aerosol-assisted CVD and direct liquid-injection
CVD. Metal-organic CVD uses metal-organic
precursors, whereas in rapid thermal CVD the
substrate is heated. Catalytic CVD is based on
the catalytic decomposition of precursors using
a resistively heated filament. This technique is
also known as hot-wire CVD or hot-filament
CVD. In laser-assisted CVD, a laser heats a
localized spot and no other heating source is
present [25] .
In conventional thermally activated CVD,
resistive heating of the hot-wall reactor provides
sufficiently high temperatures for dissociation
of the various gaseous species. This leads to the
entire heating of the substrate to a high tempera-
ture before the desired reaction is achieved. It
precludes the use of substrates having melting
points much lower than the reaction tempera-
ture. Alternately, one could heat the reacting
gases in the vicinity of the substrate by placing
a hot filament of tungsten inside the chamber.
Plasma-enhanced CVD is known to exhibit a
distinct advantage over thermally activated CVD
owing to its lower deposition temperature. Vari-
ous types of energy resources--e.g., DC, RF,
microwave, and electron-cyclotron-resonance
microwave (ECR-MW)--are used for plasma
generation in CVD. In a DC-activated process, the
reacting gases are ionized and dissociated by an
electrical discharge, thereby generating a plasma
consisting of electrons and ions. Microwave
plasma is an attractive option because the micro-
wave frequency (2.45 GHz) can oscillate electrons;
thus, high ionization fractions are generated as
electrons collide with gas atoms and molecules.
Laser-assisted CVD is associated with the
deposition of chemical vapors using a laser beam
generated from CO 2 , Nd:YAG, or excimer lasers.
Laser-assisted CVD differs from conventional
CVD in that the area of growth can be limited to
that of where the laser beam passes. Neverthe-
less, laser-assisted CVD can be used for a large
variety of target materials and substrates [25] .
Although CVD is a complex chemical process,
it has several advantages [24] . Highly dense,
very pure, uniform thin films are produced with
good reproducibility and adhesion at reasonably
high deposition rates. Control of crystal struc-
ture, surface morphology, and orientation of the
CVD products is easily possible by controlling
the CVD process parameters. The deposition
rate can be adjusted readily. Low deposition
rates are preferred for the growth of epitaxial
thin films for microelectronic applications. How-
ever, for the deposition of thick protective coat-
ings, a high deposition rate is preferred, and it
can be greater than tens of mm per hour. High
deposition rates lower the production costs.
CVD also exhibits the flexibility of using a
wide range of chemical reagents such as halides,
hydrides, and organometallics that enable the
deposition of a large spectrum of materials,
including metals, carbides, nitrides, oxides,
sulfides, III-V materials, and II-VI materials.
Relatively low deposition temperatures are
employed in CVD, and the desired materials can
be deposited in situ at low energies through
vapor phase reactions, followed by nucleation
and growth on the substrate. This enables the
deposition of refractory materials at a small frac-
tion of their melting points. For example, refrac-
tory materials such as SiC (melting point:
2,700 °C) can be deposited at 1,000 °C. Finally,
CVD can be used to uniformly and conformally
coat substrates with complex surfaces.
Like any deposition technique, CVD has
several drawbacks as well [24] . Foremost are the
chemical and safety hazards caused by the use
of toxic, corrosive, flammable, and/or explosive
reagent gases. However, these drawbacks have
been minimized using variants of CVD, such as
electrostatic spray-assisted CVD and combus-
tion CVD, that use environmentally friendly
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