Biomedical Engineering Reference
In-Depth Information
vacuum chamber wherein the substrate to be
coated is exposed to the two reactive precursors.
Once those react, a film starts to grow. There
are, however, some very significant differences
between ALD and CVD, which are as follows.
The two precursors for CVD are usually
injected into the vacuum chamber simultane-
ously. One has certainly to consider various fac-
tors such as the process temperature, the vapor
pressures of the precursors, and the doses of the
precursors. Properly adjusted, the CVD process
will result in a formation of a film with thickness
as a function of processing time. Since both pre-
cursors are usually highly reactive with each
other and present in the chamber at the same
time, the lifetimes of the reacting species are lim-
ited and thus the CVD process is quite rapid. The
precision of the coating dimension has some
limitations, however. These limitations include
the step coverage of coatings on trenched struc-
tures with high aspect ratios: The bottom of pores
or grooves in the substrate may not be coated
with the same thickness as the upper surface of
the substrate. Another aspect of the simultaneous
injection of the precursors is the formation of
particles in the vapor phase of the reactor and
their precipitation on the substrate, which often
leads to nonconformal coatings, pinholes, rough-
ness of the coating's surface, and so on.
The principle of ALD is based on similar
chemistry to that of CVD, but technologically
the process shows one significant difference,
which eventually results in better control
[4-7]
.
The process is chemically split into two half-
reactions (
Figure 16.1
). In the first stage, only
one precursor is introduced to the substrate
and a saturative chemisorption mechanism is
used, allowing the precursor to form a layer of
chemisorbed but still reactive species on the
surface of the substrate. In other words, the
reactive precursor attaches to every accessible
surface site, provided that chemical anchor
groups are present, in this way ensuring that
the forthcoming step will not lead to a line-of-
sight deposition. After the precursor has been
TABLE 16.1
List of abbreviations used in this chapter.
AAO
anodic aluminum oxide
ALD
atomic layer deposition
ALE
atomic layer epitaxy
CNT
carbon nanotube
CVD
chemical vapor deposition
DEZ
diethylzinc
DNA
deoxyribonucleic acid
DSSC
dye-sensitized solar cell
EDX
energy-dispersive X-ray spectroscopy
HA
hydroxyapatite
hADSC
human adipose-derived adult stem cells
MLD
molecular layer deposition
MMO
mixed metal-oxide framework
MPI
multiple pulsed vapor phase infiltration
NMR
nuclear magnetic resonance
ODTS
octadecyltrichlorosilane
SOD
superoxide dismutase
TEM
transmission electron microscopy
TFEL
thin-film electroluminescent display
TMA
trimethylaluminum
TMV
tobacco mosaic virus
XRD
X-ray diffraction
epitaxy
(ALE) was used. The fact that most of the
films deposited with this method do not grow
epitaxially led to a change in the terminology
from ALE to ALD, which is now in common use
(see
Table 16.1
).
16.1.1 ALD Technology
The ALD process can, to a certain extent, be com-
pared with chemical vapor deposition (CVD)
[3]
.
In both processes, the deposition of an inorganic
film occurs when two reactive chemical species
(precursors) encounter each other and form a
more stable compound through an exothermic
reaction. Usually, the process takes place in a
