Fabrication and tolerances of moth-eye structures for perfect ...

transmittance [%]
110
100
90
80
no gap space
gap space
No. 1
No. 2
70
65
10 20 30 40 50 60 70
wavelength [µm]
Figure 7. The result of simulation for flared models with and without gap space. The solid line (red) corresponds to the
case without gap and the longest dashed line (green) with gap. The second longest (blue) represents the measurement of
No. 1 and the shortest (magenta) represents the measurement of No.2.
The best sample has transmitted more than 98 % at the maximum and more than 96 % in the wide range of
20-50 µm. To investigate what made the spectra different, and why the transmittance did not reach more than
99 % in the wide range, we have carried out a numerical analysis with RCWA. Consequently, we have acquired
a few tolerances for fabricating moth-eye structures:
1. The shape of the protuberances must be straight from the tops to the bottoms.
2. The tops may be flat if the filling factor is a few percent.
3. Gap space at the bottom must be as little as possible, that is, the porosity is less than 15 %.
If the moth-eye structure can be made within the allowed tolerances, the transmittance of more than 99 % will
be achieved.
In the near future, moth-eye structures will be applied to both sides of silicon substrates and finally to silicon
lens for mid-infrared instruments.
ACKNOWLEDGMENTS
We are grateful to NTT Advanced Technology Corporation for fabricating the moth-eye structure samples which
meet our requirements. This work was supported by Grant-in-Aid for Young Scientists (A) 21740131 and Grant
for development of on-board instruments from JAXA.
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