Biomimetic Antireflection Surfaces - Engineered Biomimicry - page 312

Interestingly, arrays of micro-candles , consist-
ing of silicon columns as candle bodies and poly-
mer dots as candle wicks, are clearly evident
after removing the templating silica spheres
( Figure 12.11 c). The polymer dots are unetched
residues of the thin polymer wetting layer ( ∼ 100
nm thick) between the spin-coated colloidal
monolayer and the silicon substrate. These dots
can be easily removed by brief oxygen RIE to
generate clean silicon pillar arrays ( Figure 12.11 d).
The resulting silicon moth-eye ARCs with
high aspect ratio show excellent broadband
antireflection properties, as illustrated by the
specular reflectance spectra shown in Figure
12.12 . Very low reflectance (<2.5%) over a wide
range of wavelengths is obtained. The experi-
mental reflectance measurements are comple-
mented by theoretical calculations using the
RCWA. The experimental spectra match reason-
ably well with the simulated spectra.
Besides optical depth, the crystal structure of
the moth-eye ARCs also affects their antireflection

performance. As shown in Figure 12.7 , the spin-
coating technology enables wafer-scale assembly
of non-close-packed colloidal crystals with meta-
stable square ordering. This allows us to create
moth-eye ARCs with square arrays using the
same templating technique described previously
and then compare their antireflection perfor-
mance with the nature-inspired hexagonal arrays
[110] . Figures 12.13 a and 12.13 b show side-view
SEM images of a square and a hexagonal moth-
eye array fabricated using the same templating
conditions. The antireflection performance of the
square array is apparently better than that of the
hexagonal array ( Figure 12.13 c).
The pillar pitch of moth-eye ARCs also affects
the final antireflection performance. Figure
12.14 a shows a moth-eye ARC fabricated using
70-nm silica spheres (see Figure 12.5 a) as tem-
plate. Figure 12.15 b compares the specular reflec-
tion from a commercial crystalline silicon solar
cell with PECVD-deposited SiN x ARC, and the
templated nanopillar array [106] . It is apparent

60

Experiment Simulation

Si wafer
Si pillar array

50

40

30

5.0

2.5

0.0

400 600 800 1000 1200 1400 1600

Wavelength (nm)

FIGURE 12.12 Experimental (solid) and RCWA-simulated (dotted) specular reflection at normal incidence from a flat
silicon wafer and a 60-min Cl 2 -RIE-processed silicon pillar array. Adapted from Ref. 105 .