Biomedical Engineering Reference
In-Depth Information
Colloidal silica
Si
Spin-coated nanocomposite
Reactive ion etch
Moth-eye antireflection coating
PDMS casting
Polymerization
Replication
PDMS
PDMS
Monomer
Glass
FIGURE 12.17
Schematic of the templating procedures for making polymer moth-eye ARCs on glass. Reprinted with
permission from
Appl Phys Lett
91
(2007), 101108. Copyright 2007, American Institute of Physics.
Figures 12.18
a and
12.18
b show atomic force
microscope (AFM) images of two polymer moth-
eye nipple arrays replicated from the same
nanocomposite sample consisting of 360-nm
silica spheres after 20 s and 45 s oxygen-plasma
etching, respectively. The shape of nipples in the
latter sample is close to hemispherical, as
revealed by the AFM depth profile.
Figure 12.18
c
compares the specular reflectance spectra
obtained from a flat polymer control sample and
two polymer moth-eye arrays with different nip-
ple depths. It is apparent that the hemispherical
nipple array shows significantly lower reflec-
tance than that of the flat control sample and the
moth-eye grating with shallower spherical caps
for the whole visible spectrum.
The antireflection performance of polymer
moth-eye ARCs can be further improved by
using fluoropolymers
[107]
. A few perfluoroether
acrylate monomers have been synthesized and
used in creating moth-eye gratings using the
same templating procedures as described
previously. The utilization of fluoropolymers as
the materials for templating moth-eye ARCs has
at least three advantages over nonfluorinated
polymers. First, the low refractive indices of
fluoropolymers, coupled with the templated moth-
eye microstructures, improve the antireflective
performance. The normal-incidence reflectance
from a perfluoroether acrylate hemispherical
nipple array is below 0.5% for most of the visible
spectrum (400-700 nm). Second, the elastomeric
properties of fluoropolymers with low glass
temperature enable the creation of high-
performance ARCs on curved surfaces. Third,
fluoropolymers are intrinsically hydrophobic,
facilitating the realization of self-cleaning ARCs.
The described soft-lithography-like templat-
ing technology enables wafer-scale fabrication
of polymer moth-eye ARCs with periodic arrays
of unitary hemispherical nipples. However, the
height of the resulting nipples is at most the
radius of the templating spheres, limiting the
optical depth and the final antireflection perfor-
mance of the unitary ARCs.
The spin-coating technological platform has
therefore been extended one step further by
demonstrating the production of binary dim-
ple-nipple moth-eye ARCs on glass substrates
[103]
. The optical depth of the resulting binary
