Young Hak Cho

Facile fabrication of superhydrophobic poly(methyl methacrylate) substrates using ultrasonic imprinting

Superhydrophobic surfaces (SHSs) have received increasing attention in the last decade, and have been generally developed from hydrophobic materials. In this study, a facile fabrication method based on ultrasonic imprinting is proposed to develop SHSs from a hydrophilic polymer, poly(methyl methacrylate) (PMMA). To fabricate SHSs on PMMA substrates, micro electrical discharge machining, micromachining and ultrasonic imprinting were sequentially used. The ultrasonic imprinting was performed for various channel designs and imprinting conditions, and the resulting water contact angles were measured for the replicated samples. As a result, superhydrophobic characteristics could be obtained on a hydrophilic PMMA replica without any chemical treatments. The effects of nanoscale roughness on the replicated channel as well as composition change are discussed with respect to the analyses using an atomic force microscope, x-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy.

Effects of Grooved Surface with Nano-Ridges on Amplification of Hydrophobic Property

This paper shows how much the size of groove and ridge, which were fabricated on the silicon wafer, affected on the hydrophobicity of the grooved surface without chemical treatments and with chemical treatment such as self-assembled monolayers (SAMs) of organic silane. Furthermore, the contact angles (CAs) of water droplet on specimens were measured and compared with theoretical CAs of droplet in Cassie-Baxter state. The experimental results indicated that the specimen with the extensive groove width had super-hydrophobicity as compared to the specimen with relatively narrow groove width. The highest CAs of chemically non-treated specimens and chemically treated specimens were 171.9˚ and 173.1˚. This work explains that the grooved surface with nano-ridges and SAM coating contributes to improve hydrophobicity of surface.

Surface Polishing of Polymer Microlens with Solvent Vapor

Today, there are lots of progresses in the field of lens researches, especially in the microlens fabrication. Unlike normal lenses, microlens has been widely used as a role of improving the performance of photonic devices which increase the optical precision, and also used in the fields of the display. In this paper, polymer microlenses with diameter were replicated through hot-embossing from nickel mold which was fabricated by micro-EDM. After hot-embossing process, the polymer microlenses have a rough surface due to the crater formed by micro-EDM process, which is projected onto the surface of the lenses. The surface of polymer microlenses was polished using solvent vapor to improve the surface roughness of the microlenses without changing their shape. In the experiment, the surface roughness was improved with the processing time and vapor temperature. Also, the roughness improvement was greatly affected by the solubility difference between polymer and solvent.

Thermal characterisation of high-aspect-ratio nanoheaters using IR thermography

Recently, micro/nanoheater have been used and studied in various fields such as sensors. In particular, they have become important in biology and medicine. In this paper, we propose a novel measurement method which can indirectly measure the temperature distribution of a nanoheater’s surface from side temperature measurements using infrared microscopes. Two different types of nanoheaters are fabricated and their thermal behaviours are analysed and characterised. They are fabricated in a top-down approach using silicon-on-insulator wafers and basic MEMS processes. Nanoheaters with a high-aspect-ratio are fabricated: a width of 200 nm, length of 100 μm and height of 30 μm. The nanoheater’s surface is doped with boron by ion implantation for joule heating. Their thermal profile depending on their heat sink type was investigated. In conclusions, it is expected that a nanoheater with exact temperature control and thermally efficient structural design can be applied for bio-molecule manipulation.

Micro/nano-hybrid lens for enhancing light extraction using micro-milling and anodic aluminium oxide (AAO)

In the recent past there has been much research towards increasing the transmission of light in optical systems by reducing the Fresnel reflection of radiation, as the reflection of light from surfaces seriously decreases the performance of an optical device. These drawbacks have been overcome by mainly two methods, which are anti-reflective coating and anti-reflective nanostructure formation. In this study, we developed a simple fabrication process for Al micro/nano hybrid lens (MNHL) moulds for efficient light extraction using micro-milling and anodic aluminum oxide (AAO). From these moulds, two different types of polymer MNHL were fabricated using hot-embossing; one was a polymer MNHL that was covered with nanostructures over the entire surface, and the other was one for which only the microlens surface was covered with nanostructures. Two different types of polymer MNHLs were evaluated and compared with each other concerning the light extraction performance. The MNHL with nanostructures only on the microlens surface exhibited a higher light extraction performance than the other by 20.7%. It is expected that the fabricated MNHL can be used for the amplification of small signals when observing the presence of bio-molecules dyed with a fluorescent material.

Fabrication and Characterization of Polymer Microlens using Solvent-vapor-assisted Reflow

In this paper, we propose a simple and low-cost fabrication method of polymer microlens using solvent-vapor-assisted reflow (SVAR). Metal molds for replication of polymer were fabricated using micro milling and the cylindrical shape of polymer was imprinted using hot-embossing process. The cylindrical shape of polymer was changed to hemispherical lens shape by SVAR. The characteristics of fabricated microlens were evaluated according to the condition of SVAR such as temperature and time. The focal length of polymer microlens could be controlled more easily in low-temperature and long-time condition than in high-temperature and short-time condition. That is, the level of concentrated light to focal point could be improved through the control of temperature and time. Also, we confirmed that toluene was more appropriate solvent than acetone in fabrication of PMMA polymer microlens using SVAR.