Chemical Vapor Deposition (CVD)
the formation of a non-volatile solid film on a substrate from the
reaction of vapor phase chemical reactants containing the right
constituents. A reaction chamber is used for this process, into
which the reactant gases are introduced to decompose and react with the
substrate to form the film.
Chemical vapor deposition is
used in a multitude of semiconductor wafer fabrication processes, including the production of
amorphous and polycrystalline thin films (such as polycrystalline
silicon), deposition of SiO2 (CVD SiO2)
and silicon nitride, and growing of single-crystal
silicon epitaxial layers.
A basic CVD process consists
of the following
steps: 1) a predefined mix of reactant gases and diluent inert gases are introduced at a specified flow rate into the
reaction chamber; 2) the gas species move to the substrate;
3) the reactants get adsorbed on the surface of the substrate; 4) the
reactants undergo chemical reactions with the substrate to form the
film; and 5) the gaseous by-products of the reactions are desorbed and
evacuated from the reaction chamber.
process of chemical vapor deposition, the reactant gases not only react
with the substrate material at the wafer surface (or very close to it),
but also in gas phase in the reactor's atmosphere. Reactions that
take place at the substrate surface are known as
reactions, and are selectively occurring on the heated surface of the
wafer where they create good-quality films.
that take place in the gas phase are known as
reactions. Homogeneous reactions form gas phase aggregates of the
depositing material, which adhere to the surface poorly and at the same
time form low-density films with lots of defects. In short,
heterogeneous reactions are much more desirable than homogeneous
reactions during chemical vapor deposition.
consists of the following parts: 1) sources of and feed
lines for gases; 2) mass flow controllers for metering the gases into
the system; 3) a reaction chamber or reactor; 4) a system for heating up
the wafer on which the film is to be deposited; and 5) temperature
Examples of CVD Systems
many ways of describing or classifying a CVD reactor. For
instance, a reactor is said to be
uses a heating system that heats up not only the wafer, but the walls of
the reactor itself, an example of which is radiant heating from
resistance-heated coils. 'Cold-wall'
reactors use heating systems that minimize the heating up of the reactor
walls while the wafer is being heated up, an example of which is heating
via IR lamps inside the reactor. In hot-wall reactors, films are
deposited on the walls in much the same way as they are deposited on
wafers, so this type of reactor requires frequent wall cleaning.
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of classifying CVD reactors is by basing it on the range of their
reactors operate at atmospheric pressure, and are therefore the simplest
reactors operate at medium vacuum (30-250 Pa) and higher temperature
than APCVD reactors.
(PECVD) reactors also operate under low pressure, but do not depend
completely on thermal energy to accelerate the reaction processes. They
also transfer energy to the reactant gases by using an RF-induced glow
discharge used by a PECVD reactor is created by applying an RF field to
a low-pressure gas, creating free electrons within the discharge region.
The electrons are sufficiently energized by the electric field that
gas-phase dissociation and ionization of the reactant gases occur when
the free electrons collide with them. Energetic species are then
adsorbed on the film surface, where they are subjected to ion and
electron bombardment, rearrangements, reactions with other species, new
bond formation, and film formation and growth.
compares the characteristics of typical APCVD, LPCVD, and PECVD
APCVD, LPCVD, and PECVD Comparisons
High-temperature Oxides, Silicon Nitride, Poly-Si, W, WSi2
and Particle Contamination
Low-temperature Insulators over Metals, Nitride Passivation
By Sputtering; PVD