Session: Research Posters

Paper Number: 120061

120061 - Multifunctional Sapphire Nanostructures Fabricated by Low Rf Power Icp-Rie 

Biological surfaces in nature possess different functionalities, such as anti-reflection, self-cleaning, and water harvesting, to adapt to various environments and situations. Furthermore, many biological surfaces can serve multiple purposes rather than meeting a single requirement. Sapphire finds extensive applications in photonics and optoelectronics due to its high mechanical hardness, thermal tolerance, chemical stability, and excellent optical transmittance in the infrared range. However, when used as a window material, the higher refractive index of sapphire, compared to other transparent materials like fused silica and glass, results in a larger refractive index mismatch at the interface and consequently leads to higher Fresnel reflection losses. One way to address this issue is by applying bio-mimic taper sapphire nanostructures on the surface, which gradually match the refractive indices of the two media across the interface, mitigating Fresnel reflection losses over a broad range of wavelengths and incident angles. Additionally, these sapphire taper nanostructures not only enhance the transmittance of sapphire substrates but also exhibit remarkable self-cleaning and antifogging abilities, thereby providing multiple benefits to sapphire as a window material.

 

However, the fabrication of large-area multifunctional sapphire nanostructures is challenging due to the high chemical stability of single-crystal sapphire. In this study, we employ inductively coupled plasma reactive ion etching (ICP-RIE) with low RF power to create high aspect ratio (HAR) polysilicon pillars as an etch mask, enabling the realization of multifunctional sapphire nanostructures. First, a thick polysilicon layer is deposited on the sapphire substrate using low-pressure chemical vapor deposition. Subsequently, a photoresist and an antireflection coating are spin-coated on the silicon substrate. The photoresist is exposed using Lloyd's mirror interference lithography to form 2D periodic pillar arrays. Oxygen ICP-RIE is employed to transfer the photoresist pattern into the antireflection coating, and HBr ICP-RIE with low RF power enhances the etch selectivity, enabling further etching into the polysilicon layer and the formation of HAR polysilicon pillars as an etch mask for the sapphire etching process. Finally, the BCl3/HBr ICP-RIE is employed to etch the pattern into the sapphire substrate.

 

Preliminary results demonstrate that the transmission can be enhanced from 86% to 96% by applying multifunctional sapphire nanostructures on both sides of sapphire substrates. Moreover, when hot water is placed under both planar and nanotextured sapphires, the nanotextured sapphire surface exhibits superior antifogging ability compared to the planar sapphire surface. This is attributed to the significant increase in hydrophilicity achieved by introducing the surface roughness, leading to the formation of a thin water film instead of multiple tiny water droplets, thereby eliminating light scattering caused by water droplets.

 

In conclusion, we have successfully demonstrated that sapphire nanostructure surfaces possess exceptional antireflection and antifogging capabilities. We plan to quantify the self-cleaning ability in future experiments by conducting anti-dust tests.

Presenting Author: Kun-Chieh Chien The University of Texas at Austin

Presenting Author Biography: Kun-Chieh Chien is a Ph.D. candidate in the Walker Department of Mechanical Engineering at the University of Texas at Austin. His research focuses on nanometrology, nanofabrication, process monitoring, and data analysis.

Authors:

Kun-Chieh Chien The University of Texas at Austin
Chih-Hao Chang The University of Texas at Austin