Anti-Reflecting Nanostructures - Biomimetic Architectures by Plasma Processing - page 51

the implementation of nature's technology into manufacturing
technologies are pursued.

2.4.1 Fabrication of AR Nanostructures

Fabrication routes adopted for the AR nanostructures can generally
be classified into bottom-up and top-down modes. The bottom-up
technique involves direct deposition of film or nanomaterial growth
routes such as solution processing, physical vapour deposition (PVD)
and chemical vapour deposition (CVD). On the other hand, top-down
approach relies on dry- or wet-etching techniques performed with
or without a mask (Fig. 2.8).

Figure 2.8 Classification scheme for the fabrication of AR nanostructures.

2.4.1.1 Bottom-up technology

After Fraunhofer [13], the sol-gel technique has been used widely
to achieve porous surfaces. Polymer materials with low RIs can be
coated on transparent substrates such as glass or plastic maintaining
similar transmission levels; however, when the porosity is imparted
on the coating, the reflection does go down as discussed before
(Fig. 2.2b). The porosity of coatings and, consequently, their RI can
be tuned by maximising the pore-volume ratio, but keeping the sub-
wavelength pore size by incorporation of colloids that are removed
upon thermal treatment [45] or by addition of heterometal alkoxides
[46]. A nanoporous polymethyl-methacrylate (PMMA) film, with
polystyrene (PS) co-polymer, can achieve transmission of 99.7%
averaged over 400-680 nm wavelength, with calculated RIs between
1.255 and 1.285 [31]. AR porous coatings based on microcrystalline
alumina are already applied to Canon camera lenses [47].
Porous films can also be fabricated by glancing angle deposition
(GLAD), a PVD technique. GLAD principle relies on the incidence of