Hyung Choi

Magnetic MEMS sensor device

A microelectromechanical system (MEMS) sensor and method for measuring the motion of an intermediate member and a method for making the MEMS sensor. The MEMS sensor includes a substrate, a lower magnetic member disposed on the substrate, a layer disposed over the substrate, an upper magnetic member disposed at a side of the layer facing the lower magnetic member, an intermediate magnetic member magnetically levitated between the lower magnetic member and upper magnetic member; and a component measuring at least one of motion, forces acting on, and a displacement of the intermediate magnetic member.

Fabrication of 3D MEMS toroidal microinductor for high temperature application

Based on Microelectromechanical systems (MEMS) technique and thick photoresist lithography technology, a new toroidal-type inductor for high temperature application has been successfully developed. In the fabrication process, heat-resistant materials are used, alumina as insulator and supporting materials instead of polyimide, heat resistant glass for underlay instead of normal glass, and copper for coil. The maximum inductance is 87nH at 0.826GHz and maximum of quality factor (Q-factor) is 4.63 at 0.786GHz, at room temperature. With simulation of thermal deformation, it shows that the developed toroidal inductor can be suitable for high temperature application, from 300 to 700^oC.

One dimensional conversion of color temperature in perceived illumination

This paper proposes a one-dimensional conversion method of color temperature in perceived illumination. It also presents the design and implementation of the method. The proposed method is verified using the Xilinx Virtex FPGA XCV 2000-6BG560. This method is demonstrated experimentally for color temperatures in the range of 3000 K to 25000 K. This can be applied into special effect for multimedia applications.

Impact of transparent electrode on photoresponse of ZnO-based phototransistor

ZnO-based photo-thin film transistors with enhanced photoresponse were developed using transparent conductive oxide contacts. Changing the electrode from opaque Mo to transparent In-Zn-O increases the photocurrent by five orders of magnitude. By changing the opacity of each source and drain electrode, we could observe how the photoresponse is affected. We deduce that the photocurrent generation mechanism is based on an energy band change due to the photon irradiation. More importantly, we reveal that the photocurrent is determined by the energy barrier of injected electrons at the interface between the source electrode and the active layer.

A low voltage actuated micromirror with an extra vertical electrode for 90? rotation

We have proposed a new type of electrostatic actuator for optical switching which has trench structure to make low voltage driving possible. Its lower or fixed electrode consists of a horizontal electrode and an extra vertical one which acts as a stopper for the exact 90° rotation. To fabricate the proposed actuator, a new planarization approach, laminating an organic film on a wafer, has been introduced to make a planar sacrificial layer over the trench. Comparison of theoretical analysis with experimental result has been made for the actuator. We have found that the actuation voltage (47 V) could be reduced up to 50% of that of the conventional one. The rising and falling times were measured to be 5 and 15 ms, respectively. The planarization process can be very effective and powerful for structures with deep trenches or pits.

A robust digital timing recovery with asymmetry compensator for high speed optical drive systems

This paper presents a new four-sampled zero crossing asymmetry compensation algorithm for high-speed DVD/CD applications. Simulations show 34% improvement of jitter performances, 50% reduction of compensation time and 2.0 dB gain of bit error rate (BER) compared with other algorithms. Simultaneously, a new digital timing recovery scheme for the optical drive system is presented. By comparative simulations using DVD patterns with marginal input conditions, the proposed timing recovery algorithm shows enhanced performances in jitter variance and SNR margin by 4 times and 3 dB respectively.

A novel digital lock detector for QPSK receiver

A new lock detection algorithm for digital quadrature phase-shift keying (QPSK) receiver is proposed. Analysis of the detectors output characteristics is given and is verified by using computer simulation. Performance degradation due to carrier jitter is also considered. Analytic and simulation results show that the proposed algorithm is very useful as a lock detector in digital receivers because it has a good detection performance and simple structure.

An optical switch with newly designed electrostatic actuators for optical cross connects

We have developed an optical switch having low voltage driven electrostatic actuators and measured its optical performance. The simple structure of the actuator makes the fabrication process easy, helping to yield a low-cost device. In addition, a low-voltage actuating mechanism also provides good compatibility with IC and a simple driving circuit.

Performance of coherent DS-SS/QPSK for mobile communications in fast-fading multipath and high-frequency offset

We derive a closed-form bit error rate (BER) solution for equal- and nonequal-strength L-path channels considering imperfect channel estimation. The channel model assumes independent paths with Rayleigh fading statistics in a single-cell downlink environment. By using a simple maximum likelihood (ML) estimator, the effects of the channel estimation error due to Doppler shift, residual carrier frequency offset, interference, and additive white Gaussian noise are analyzed. In addition, we present the tradeoff between the noise compression capability and phase tracking capability of the ML estimator with observation length as a parameter. The results indicate that even with channel estimation, the high carrier frequency offset makes an uncoded BER unacceptably high. Also, we present two kinds of modulation techniques such as EC-QPSK and NC-QPSK. Through analysis, we show the performance comparison between these modulation techniques. Finally, we verify the derived BER by using Monte Carlo computer simulation.

Fabrication of vertical moving micro-optical switch for display applications

In this paper, we present a new concept and fabrication of micro optical switch of which application is transmissive display devices. The micro optical switch consists of two parallel plate electrodes and is driven by electrostatic force. The first electrode is patterned on the glass substrate and the second electrode is disposed spaced apart from the first electrode. Each electrode has holes and the holes in each electrode do not overlap with one another. Light passes through the holes in the second electrode via the holes in the first electrode. All dimensions and fabrication process of the micro optical switch were designed to be compatible with Liquid Crystal Display (LCD) fabrication process. The size of the fabricated micro optical switch was 254μm × 254 μm. The micro optical switch was fabricated by surface micromachining. Aluminum was used as electrodes and patterned by plasma etching process. Photoresist was used as a sacrificial layer, which defined the gap between the two electrodes. Plasma ahsing was used to remove the sacrificial layer. Finally, anti-stiction layer was coated by Molecular Vapor Deposition (MVD) process. When voltage was applied, the second electrode moved down and contacted the first electrode. When voltage applying stopped, the second electrode returned to its original position. The voltage required to pull in the second electrode was below than 15 V. The sum of transition time, from on to off-state and from off to on-state, was below than 100 ㎲ and operating frequency was more than the 10 kHz.