Antireflective grassy surface on glass substrates with self-masked dry etching
© Song et al.; licensee Springer. 2013
Received: 17 October 2013
Accepted: 18 November 2013
Published: 1 December 2013
Although recently developed bio-inspired nanostructures exhibit superior optic performance, their practical applications are limited due to cost issues. We present highly transparent glasses with grassy surface fabricated with self-masked dry etch process. Simultaneously generated nanoclusters during reactive ion etch process with simple gas mixture (i.e., CF4/O2) enables lithography-free, one-step nanostructure fabrication. The resulting grassy surfaces, composed of tapered subwavelength structures, exhibit antireflective (AR) properties in 300 to 1,800-nm wavelength ranges as well as improved hydrophilicity for antifogging. Rigorous coupled-wave analysis calculation provides design guidelines for AR surface on glass substrates.
Antireflective (AR) coatings/structures are needed for most of existing optical components and optoelectronic devices, ranging from glasses, polymers, and fibers to solar cells, photodetectors, light-emitting diodes, and laser diodes, to remove undesired optical loss and improve optical performance[1–3]. For advanced AR properties compared to the conventional AR coatings (i.e., very low reflection at broad wavelength ranges and large incident angles), subwavelength structures (SWSs) with tapered profile, which is inspired by insect's eye, have been developed[4–6]. Because the SWSs have only zeroth diffraction order, it is possible to control the effective refractive index by changing the curvature of SWSs. From the theoretical understanding of SWSs and precise control of geometries (i.e., period, height, shape and packing density), improved AR performances of various materials and their device applications have been recently reported[7–9].
There are a variety of fabrication processes for AR SWSs, such as electron-beam or laser interference lithography, nanoimprint lithography, nanosphere or colloid formation, metal nanoparticles, and Langmuir-Blodgett assembly[5, 6, 8–15]. However, these techniques are still expensive, time consuming, and sophisticated, which block the penetration of commercial market. In case of transparent glasses, although the importance of AR structures for improvement of optical efficiency, the cost issues have hindered the use of AR structures in applications such as photovoltaics and optoelectronics. In this letter, we present a simple, fast, and cost-effective method for fabricating AR grassy surfaces composed of tapered SWSs on glass substrates. Reactive ion etch (RIE) process of glasses with gas mixture of CF4 and O2 generates nanoclusters that can be used as an etch mask. Control of etch conditions provides optimal AR performance in the visible wavelength ranges.
Design and fabrication
Results and discussion
The reflectance difference between the glasses with flat and grassy surface is revealed visually in Figure 5B. An intense light reflection from the flat glass is observed and as a result, reflections occurring at both sides of the glass make the words difficult to read. The grassy surface showed improved readability due to the reduced reflection. In addition to the AR property, the wetting property is also affected by both the structured surface and the oxygen plasma treatment. To confirm the antifogging performance, the SWS-integrated glass and the bare glass were exposed to steam at the same time. Figure 5C shows the antifogging behavior of the glasses with flat and grassy surface. The water droplets beaded up on the flat surface of the bare glass substrate and the bead-like water droplets caused light scattering, which degrades the readability of the words. However, the water droplets on the roughened surface of the SWS-integrated glass evenly spread over the whole surface, and the hydrophilic glass still remained transparent, and the words below it were clearly readable. Water contact angle measurement results also support this hydrophilic effect. The contact angles of glass with and without grassy surface were 12.5° and 71.5°, respectively. The surface energy of structured glass was 87.8 mN/m, which is a higher value than that of bare glass (39.0 mN/m).
In summary, we demonstrated the subwavelength scale grassy surfaces on the glass substrate by using simple one-step dry etch process without any lithography. The resulting grassy surface showed very high transmittance in very wide spectral ranges as well as antifogging effects. Optimization of self-masked dry etching for improving the optical/material properties remains as a future work. We expect that this low-cost, high-performance optical materials are applicable in various optical and optoelectronic devices.
This work was partially supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (no. 2011–0017606) and by the ‘Systems Biology Infrastructure Establishment Grant’ provided by Gwangju Institute of Science and Technology in 2013.
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