Kwang-Cheol Lee

A simple method for microlens fabrication by the modified LIGA process

Microlenses and microlens arrays were fabricated using a novel fabrication technology based on the exposure of a resist (usually PMMA) to deep x-rays and subsequent thermal treatment. The fabrication technology is very simple and produces microlenses and microlens arrays with good surface roughness (less than 1 nm). The molecular weight and glass transition temperature of PMMA is reduced when it is irradiated with deep x-rays. The microlenses were produced through the effects of volume change, surface tension, and reflow during thermal treatment of irradiated PMMA. The geometry of the microlens was determined by parameters such as the x-ray dose applied to the PMMA, the diameter of the microlens, along with the heating temperature, heating time and cooling rate in the thermal treatment. Microlenses were produced with diameters ranging from 30 to 1500 μm. The modified LIGA process was used to construct not only hemispherical microlenses, but also structures that were rectangular-shaped, star-shaped, etc.

Carbon nanotube film piezoresistors embedded in polymer membranes

We present carbon nanotube film (CNF) piezoresistors embedded in polymer membranes. CNFs by vacuum filtration are patterned on an Au-deposited Si-wafer and transferred onto the poly-dimethylsiloxane (PDMS) using the weak adhesion property between Au-layer and Si-wafer. Transmittance and I-V characteristic are measured to confirm transferred CNFs as transparent electrodes. The pressure sensor consists of CNF piezoresistors embedded in 130 μm thick circular PDMS membranes. The gauge factor of CNFs at different thickness is obtained around 10–20 when the resistance increases from 2.7 to 5.6 kΩ with applied pressure, which shows that CNFs can be used as transparent piezoresistors in polymer-based microelectromechanical systems.

Fabrication and characterization of freestanding 3D carbon microstructures using multi-exposures and resist pyrolysis

We present a fabrication method for freestanding complex 3D carbon microstructures utilizing a lithogaphy step and a heating step. We developed two fabrication methods for multi-level 3D SU-8 microstructures, which were used as polymer precursors in a carbonization process. In one method, multiple SU-8 layers were successively coated and cross-linked. In the other method, aligned partial exposures were used to control the thickness of the freestanding SU-8 layer. Freestyle, freestanding carbon microstructures were fabricated by heating 3D SU-8 microstructures below 1000 °C in a nitrogen atmosphere. Characterization of the pyrolysis process, through measurements such as dimensional changes, roughness, hardness, elastic modulus and resistivity, was performed for positive resists AZ5214 and AZ9260 as well as SU-8. 3D carbon microstructures fabricated using our methods can be utilized for various applications such as low cost resonating microsensors and microfluidics.

Design of the KRISS watt balance

We report the design of the KRISS watt balance, which includes a magnet, a guiding stage and a coil position measurement system. The KRISS watt balance incorporates a closed-type cylindrical permanent magnet and a motion guiding stage. For the magnet, a flux shunt is used to reduce flux changes due to temperature variations. A piston gauge is used to achieve linearity in the motion guiding stage. In the weighing mode, the residual force between the weight of the test mass and the Lorentz force generated in a coil is measured by a commercial weighing cell. In the dynamic mode, a linear motor in the motion guiding stage vertically translates the coil and the weighing cell. The in-plane motion of the coil is measured by position sensors, and the out-of-plane motion is measured by single-pass homodyne interferometers.

Biosensor utilizing resist-derived carbon nanostructures

The authors present a biosensor using pyrolyzed electron beam resist nanostructures as an active conducting channel. Versatile, arbitrarily shaped nanostructures such as nanowires, nanodots, and suspended nanobridges are fabricated by a facile electron beam resist thermal decomposition method. The nanostructures typically show 15–21nm thickness, 100–200nm width, 0.6nm roughness, and p-type majority conduction with tailored resistivity of 5.2–0.75Ωcm. Streptavidin-biotin binding and pH dependent conductance modulation are demonstrated using pyrolyzed resist based devices.

Fabrication of an electrothermally actuated electrostatic microgripper

A novel electrostatic microgripper, which can handle micro objects with the electrostatic force, is designed and fabricated. It consists of gripping part with one inner and two outer electrodes, and releasing part with an electrothermal actuator. It grips micro objects using electric field generated between the electrodes, and releases them using the backward motion of inner electrode with electrothermal actuation. The backward motion of inner electrode breaks the electric field and rapidly reduces the remaining charges of objects. The graspless work shows the safe object handling method and also suggests the solution for the releasing problem. The microgrippers are fabricated with 20 /spl mu/m and 40 /spl mu/m thick SOI wafers, and they successfully grip glass beads with 170 /spl mu/m diameter at 250 V and 100 V, respectively.

Micro heat flux sensor using copper electroplating in SU-8 microstructures

A micro heat flux sensor which can measure the thermal energy transfer per unit area has been designed, fabricated, and calibrated in a convective environment. The sensor which is based on a circular foil gage is composed of thermal paths and a thermopile. The thermal path is made in a LIGA-like process of SU-8 high aspect ratio microstructures and electroplated copper layers. The thermopile, a series of thermocouples, is used to amplify the output signal as a thermometer. When the sensor is placed on a high-temperature wall, heat flux from the wall flows through thermal paths and drains out to the environment, producing a temperature difference along its paths. Heat flux is obtained by calibrating this temperature difference in the thermopile of Ni-Cr or Al-Chromel pairs. The sensitivity of the heat flux sensor of Ni-Cr and Al-Chromel pairs is in the range of 0.1-2.0 and 0.4-2.0 µV mW-1 cm-2, respectively, in the heat flux range of 0-180 mW cm-2.

Microlens fabrication by the modified LIGA process

A microlens or microlenses array has been fabricated by a novel fabrication technology which is based upon a deep X-ray exposure and a thermal treatment of a resist, usually PMMA. The fabrication technology is very simple and produces the microlenses array as well as the microlens which has good surface roughness less than I nm. Molecular weight and glass transition temperature of PMMA is reduced when it is exposed to the deep X-ray. The microlens is produced through the effect of surface tension and reflow by applying the thermal treatment on the irradiated PMMA. A configuration of the microlens is determined by parameters such as absorbed X-ray dose on PMMA, heating temperature, and heating time in the thermal treatment. Diameters of the produced microlens range from 30 /spl mu/m to 1500 /spl mu/m and their heights vary between 10 /spl mu/m and 25 /spl mu/m.

The influence of phosphor sedimentation on the white LEDs with different structure chip

The influence of phosphor sedimentation on the white light-emitting diode with different structure chip was investigated. The phosphor sedimentation phenomenon occurred seriously as encapsulant viscosity lowers. The influence of phosphor sedimentation on the white light-emitting diode with the vertical structure chip whose one side only emits is larger than that of lateral structure chip whose all sides emit. Hence, Difference in luminous efficacy by the phosphor sedimentation reached about 20 % in the case with the vertical structure chip due to optical loss stemmed from the phosphor sediment layer.

3D fabrication using deep X-ray mask with integrated micro-actuator

This paper describes a novel 3D fabrication method with single X-ray process utilizing a deep X-ray mask in which a micro-actuator is integrated. An X-ray absorber is electroplated on the shuttle mass driven by the integrated micro-actuator during deep X-ray exposures. 3D X-ray resist microstructures are revealed according to the in-depth dose distribution modulated by the shuttle mass motion and the resist development kinetics. Fabrication of two kinds of the X-ray exposure devices, electrostatically driven and electrothermally driven devices, and discussions on X-ray experiments are presented. S-shaped and conical shaped PMMA microstructures are fabricated with the devices.