Biomedical Engineering Reference
In-Depth Information
10
reflective loss from glass is not as severe
as that from semiconductors, a
∼
4% opti-
cal reflection from each air-glass inter-
face could still degrade the performance
of optical devices, especially when mul-
tiple components are involved
[2, 33]
.
To generate transparent ARCs on glass sub-
strates, various bottom-up self-assembly tech-
niques have been exploited
[1, 3, 53, 60, 61,
137-142]
. For instance, layer-by-layer assembly
of polyelectrolyte or polyelectrolyte-colloid
multilayers has been demonstrated as an effi-
cient means of creating ARCs on glass
[2, 60,
138, 140]
. Unfortunately, traditional bottom-up
techniques suffer from low throughput and
incompatibility with standard microfabrica-
tion, limiting the mass production of practical
coatings. By contrast, the spin-coating techno-
logical platform enables scalable production
of transparent moth-eye ARCs with tunable
structural parameters on glass substrates.
As-fabricated
200
o
C for 6 h
8
6
4
2
0
400
600
800 1000 1200 1400 1600
Wavelength (nm)
FIGURE 12.16
Comparison of the normal-incidence
specular reflectance between an as-fabricated GaSb moth-
eye grating and the same sample after annealing at 200°C for
6 h. Reprinted with permission from
Appl Phys Lett
92
(2008),
141109. Copyright 2008, American Institute of Physics.
concentrating solar cells and thermophotovoltaic
cells
[104, 108]
. The surface temperature of these
cells is usually higher than that of a conventional
cell
[134, 135]
. Therefore, ARCs with high ther-
mal stability are highly preferred. Fortunately,
the templated moth-eye ARCs exhibit excellent
thermal stability because the resulting coatings
are directly patterned on the wafer surface and
no foreign materials, as in conventional quarter-
wavelength design, need to be deposited on the
substrates.
Figure 12.16
compares the normal-
incidence specular reflectance spectra of a tem-
plated GaSb moth-eye ARC prior to and after
annealing at 200°C for 6 h. The change in reflec-
tance is very small. This is in sharp contrast to
the conventional quarter-wavelength ARCs that
exhibit significant antireflection degradation,
even at temperature as low as 100°C
[136]
.
12.4.1 Templated Polymer Moth-Eye
Antireflection Coatings
A schematic outline of the templating procedures
for fabricating polymer moth-eye ARCs on glass
is shown in
Figure 12.17
[100, 107]
. The polymer
matrix of spin-coated colloidal crystal-polymer
nanocomposites can be plasma-etched (40 mTorr
oxygen pressure, 40 sccm flow rate, and 100 W)
to adjust the height of the protruded portions of
silica spheres. The long-range periodic surface
protrusions of the exposed silica spheres can be
easily transferred to a poly(dimethylsiloxane)
(PDMS) mold. The solidified PDMS mold can
then be peeled off and put on top of ETPTA
monomer supported by a glass slide with spac-
ers in between. After polymerization of ETPTA
and peeling off PDMS mold, polymer moth-eye
nipple arrays with tunable depth can be easily
generated. The flexible PDMS mold enables the
creation of polymer moth-eye ARCs on both pla-
nar and curved surfaces.
12.4 TEMPLATED TRANSPARENT
MOTH-EYE ANTIREFLECTION
COATINGS
Transparent substrates, such as glass, are
widely used in our daily life. Although the
