-
A number of forms of CVD are in wide use
and are frequently referenced
in
the
literature.
These
processes
differ
in
the
means
by
which
chemical
reactions are
initiated (e.g., activation process) and process
conditions.
Classified by
operating pressure:
o
Atmospheric pressure CVD
(APCVD)
–
CVD processes at
atmospheric pressure.
o
Low-pressure
CVD
(LPCVD)
–
CVD
processes
at
subatmospheric
pressures.
[1]
Reduced pressures tend to reduce unwanted
gas-phase reactions and improve film
uniformity across the
wafer.
Most
modern
CVD
processes
are
either
LPCVD
or
UHVCVD.
o
Ultrahigh
vacuum
CVD
(UHVCVD)
–
CVD
processes
at
a
very
low
pressure, typically below
10
?
6
Pa
(~10
?
8
torr
). Note that in
other fields, a lower division between
high and
ultra-high
vacuum
is common, often
10
?
7
Pa.
?
Classified by
physical characteristics of vapor:
o
Aerosol
assisted CVD
(AACVD)
–
A CVD process in which the
precursors are transported to the
substrate by means of a
liquid/gas
aerosol, which can be generated ultrasonically.
This technique is suitable for use with
non-volatile
precursors.
o
Direct
liquid
injection
CVD
(DLICVD)
–
A
CVD
process
in
which
the
precursors
are
in
liquid
form
(liquid
or
solid
dissolved
in a convenient
solvent). Liquid solutions are injected in
a vaporization chamber towards
injectors (typically car
injectors).
Then
the
precursor
vapors
are
transported
to
the
substrate
as in classical CVD process. This technique is
suitable for use on liquid or solid
precursors. High growth
rates can be
reached using this technique.
?
Plasma methods
(see also
Plasma
processing
):
o
Microwave plasma-assisted
CVD
(MPCVD)
o
Plasma-Enhanced CVD
(PECVD)
–
CVD processes that utilize
plasma
to
enhance
chemical
reaction
rates
of
the
precursors.
[2]
PECVD processing allows deposition at
lower temperatures,
which
is
often
critical
in
the
manufacture
of
semiconductors.
o
Remote plasma-
enhanced CVD
(RPECVD)
–
Similar to PECVD
except
that
the
wafer
substrate
is
not
directly
in
the
plasma
discharge region. Removing the wafer
from the plasma region
allows
processing temperatures down to room temperature.
?
Atomic
layer
CVD
(
ALCVD
)
–
Deposits
successive
layers
of
different
substances
to
produce
layered,
crystalline
films.
See
Atomic
layer
epitaxy
.
?
?
Combustion Chemical Vapor
Deposition
(CCVD)
–
nGimat's
proprietary Combustion Chemical Vapor
Deposition process is an
open-
atmosphere, flame-based technique for depositing
high-quality thin films and
nanomaterials.
Hot wire CVD
(HWCVD)
–
also known as
catalytic CVD (Cat-CVD) or
hot filament
CVD (HFCVD). Uses a hot filament to chemically
decompose the source
gases.
[3]
Metalorganic chemical vapor
deposition
(MOCVD)
–
CVD processes
based on
metalorganic
precursors.
Hybrid Physical-Chemical Vapor
Deposition
(HPCVD)
–
Vapor
deposition processes that involve both
chemical decomposition of
precursor gas
and
vaporization
of a solid
source.
Rapid thermal CVD
(RTCVD)
–
CVD processes that
use heating lamps
or
other
methods
to
rapidly
heat
the
wafer
substrate.
Heating
only
the
substrate rather than the gas or chamber walls
helps reduce
unwanted gas phase
reactions that can lead to
particle
formation.
Vapor phase epitaxy
(VPE)
?
?
?
?
?
Uses
Integrated circuits
Various
CVD
processes
are
used
for
integrated
circuits
(ICs).
Particular
materials are
deposited best under particular conditions.
Polysilicon
Polycrystalline
silicon is deposited from
silane
(SiH
4
), using the
following reaction:
SiH
4
→
Si + 2 H
2
This
reaction is usually performed in LPCVD systems,
with either pure
silane feedstock, or a
solution of silane with
70
–
80%
nitrogen
.
Temperatures between 600 and
650 °C and p
ressures between
25 and 150
Pa
yield
a
growth
rate
between
10
and
20
nm
per
minute.
An
alternative
process
uses
a
hydrogen
-based
solution.
The
hydrogen
reduces
the
growth
rate,
but
the temperature is
raised to 850 or even 1050 °C to
compensate.
Polysilicon
may
be
grown
directly
with
doping,
if
gases
such
as
phosphine
,
arsine
or
diborane
are added to the
CVD chamber. Diborane increases the
growth rate, but arsine and phosphine
decrease it.
Silicon dioxide
Silicon dioxide (usually called simply
industry)
may
be
deposited
by
several
different
processes.
Common
source
gases include
silane
and
oxygen
,
dichlorosilane
(SiCl
< br>2
H
2
) and
nitrous
oxide
(N
2
O),
or
tetraethylorthosilicate
(TEOS;
Si(OC
2
H
5
)
4
).
The
reactions
are as
follows
[
citation
needed
]
:
SiH
4
+
O
2
→
SiO
2
+ 2
H
2
SiCl
2
H
2
+ 2
N
2
O →
SiO
2
+ 2
N
2
+ 2 HCl
Si(OC<
/p>
2
H
5
)
4
→
SiO
2
+ byproducts
The
choice
of
source
gas
depends
on
the
thermal
stability
of
the
substrate;
for
instance,
aluminium
is
sensitive
to
high
temperature.
Silane
deposits
between
300
and
500
°C,
dichlorosilane
at
around
900
°C,
and
TEOS
between
650 and 750 °C,
resulting in a layer of
low-
temperature oxide
(LTO).
However, silane produces a lower-
quality oxide than the other methods
(lower
dielectric
strength
,
for
instance),
and
it
deposits
non
conformally
.
Any of these reactions may be used in
LPCVD, but the silane reaction is
also
done in APCVD. CVD oxide invariably has lower
quality than
thermal
oxide
, but thermal oxidation
can only be used in the earliest stages of
IC manufacturing.
Oxide
may
also
be
grown
with
impurities
(
alloying
or
doping
< br>
This
may
have
two purposes. During further process steps that
occur at high
temperature, the
impurities may
diffuse
from
the oxide into adjacent
layers (most
notably silicon) and dope them. Oxides containing
5
–
15%
impurities
by
mass
are
often
used
for
this
purpose.
In
addition,
silicon
dioxide alloyed with
phosphorus pentoxide
(
smooth out uneven surfaces. P-glass
softens and reflows at temperatures
above 1000 °C. This process requires a
phosphorus concentra
tion of at
least 6%, but concentrations above 8%
can corrode aluminium. Phosphorus
is
deposited from phosphine gas and oxygen:
4 PH
3
+ 5
O
2
→ 2
P
2
O
5
+
6 H
2
Glasses
containing
both
boron
and
phosphorus
(borophosphosilicate
glass,
BPSG) undergo viscous flow at lower
temperatures; around 850 °C is
achievable
with
glasses
containing
around
5
weight %
of
both
constituents,
but
stability
in
air
can
be
difficult
to
achieve.
Phosphorus
oxide
in
high
concentrations
interacts
with
ambient
moisture
to
produce
phosphoric
acid.
Crystals of
BPO
4
can also precipitate
from the flowing glass on cooling;
these crystals are not readily etched
in the standard reactive plasmas
used
to
pattern
oxides,
and
will
result
in
circuit
defects
in
integrated
circuit
manufacturing.