Biomedical Engineering Reference
In-Depth Information
reagents. Furthermore, it is difficult to deposit
multicomponent materials with well-controlled
stoichiometry using multiple reagents because
different reagents have different vaporization
rates.
Finally, the use of sophisticated CVD
variants--such as low-pressure or ultrahigh-
vacuum CVD, plasma-assisted CVD, and photo-
assisted CVD--tends to increase the cost of
fabrication. If production costs need to be
reduced, however, simpler variants of CVD,
such as aerosol-assisted CVD and flame-assisted
CVD, may be employed.
Low-temperature CVD has been used for bio-
replication. Controlled vapor-phase oxidation of
silanes on the surface of biological structures
produces exact, inorganic oxide replicas of
several biological structures, including a wing
of a butterfly, a wing of a housefly, and a leaf of
Colocasia esculenta
(a self-cleaning plant)
[26]
.
Thus, CVD was used to replicate intricate and
hierarchical structures on several length scales.
Likewise, multifunctional zinc oxide interfaces
were fabricated by the use of metal-organic
CVD, with the compound eyes of butterflies
serving as biotemplates
[27]
.
Moreover, a combination of the FIB
technique
[28]
and CVD has been used to
fabricate artificial structures inspired by the
scales of the
Morpho
wings
[29]
. The original
and artificial scales show comparable optical
characteristics, as discussed by Dushkina and
Lakhtakia in Chapter 11. The overall reflectance
spectrums of both structures for various
incidence angles of light are quite similar and
contain reflectance peaks at around 440-nm
wavelength.
self-limiting growth mechanism that imparts
to it several attractive characteristics: accurate
and easy control of film thickness, production
of sharp interfaces, uniformity over large areas,
excellent conformality with the substrate, good
reproducibility, multilayer processing capabil-
ity, and desirable qualities in thin films made at
relatively low temperatures
[31]
. For nanotech-
nologists, the two most important characteris-
tics of ALD are excellent conformality and the
possibility of subnanometer-level control of film
thickness.
ALD relies on alternate pulsing of the
precursor gases and vapors onto the substrate
surface--in a vacuum chamber--and subsequent
chemisorption or surface reaction of the
precursors. The vacuum chamber is purged with
an inert gas between the precursor pulses. The
ALD process is schematically depicted in
Figure 16.1. With a proper adjustment of the
experimental conditions, the process proceeds
via saturative steps, i.e., the precursors exposed
on the surface chemisorb on it (or react with the
surface groups), saturatively forming a tightly
bound monolayer on the surface. The subsequent
purging step removes all the excess molecules
from the vacuum chamber. When the next
precursor is sent in to the chamber, it encounters
only the surface monolayer with which it reacts,
producing the desired solid product and gaseous
byproducts. Under such conditions, the growth
of the thin film is self-limiting, since the amount
of solid deposited during one cycle is dictated
by the amount of precursor molecules present in
the saturatively formed surface monolayer.
Therefore, the growth is stable and the thickness
increase is constant in each deposition cycle. The
self-limiting growth mechanism facilitates the
growth of conformal thin films with accurate
thickness on large areas. This technique also
allows the growth of multilayer structures.
However, a major limitation of ALD is its very
low deposition rate.
The distinctive sequencing feature in ALD
makes it an attractive method for the precise
growth of crystalline compound layers, complex
15.4
ATOMIC LAYER DEPOSIT
ION
Atomic layer deposition
(ALD) is a surface-
controlled and self-limiting method for deposi-
ting thin films from gaseous precursors
[30]
.
Although ALD can be considered a modifica-
tion of CVD, ALD's distinctive feature is the
