Papers by Author: C.H. Lee

Abstract: We present a study of the photoresist (PR) etching and the low-k materials damage using a ferrite-core inductively coupled plasma (ICP) etcher, in order to develop an etching process for the low-k dielectric devices. We reveal that the N2/O2 flow ratio and bias power affected the PR etching rate. By Fourier transform infrared spectroscopy and HF dipping test, we investigated the effect of the gas flow ratio and bias power on the amount of etching damage to the low-k material.

Abstract: Experiments under a cold rotary swaging process have been conducted to investigate the dimensional characteristics of the swaged tube and solid bar for obtaining proper product of desirable quality. Dimensional characteristics are expressed in terms of dimensional deviation and surface roughness of swaged product through the rotary swaging process. The process variables such as forming speed and percent reduction of outer diameter of the product are considered and selected because of playing a key role in the rotary swaging process. Furthermore the developed rotary swaging machine with four-split dies, named as KRSM25, is used in the swaged experiment. Based on the experimental results, it is observed that the process variables affect the quality of swaged product such as dimensional precision, surface roughness of the product. Defect could be found to occur at a value of more than 2.0 mm/rev in forming speed. The dimensional precision of swaged product depends on the percentage reduction of outer diameter and forming speed. The work presented in this paper might be used for available information in the design of the optimum rotary swaging process.

Abstract: This study deals with the dimensional accuracy of outer diameter and geometrical workability in rotary swaged product for various process parameters such as percent reduction in outer diameter and the ratio of thickness to outer diameter of a tube. It is generally known that greater cold strengthening is achieved by rotary swaging process rather than by conventional process such as rolling with respect to the same reduction of cross-sectional area. Percent reduction in the diameter and the ratio are considered and selected as important process parameters because of playing a key role during rotary swaging process. In case of tube under rotary swaging process the ratios have influence on geometrically proper workability without defect for different percent reductions in the diameter. In addition the change of metal flow of workpiece under the swaging process is microscopically and globally observed to analyze the reason why dimensional accuracy of the outer diameter of final product is improved after the rotary swaging process. This work might provide available information for the optimum rotary swaging process.

Abstract: The microstructure and thermoelectrical properties of the 4wt% Te doped p-type Bi0.5Sb1.5Te3 compounds, fabricated by using spark plasma sintering in the temperature ranging from 250°C to 350°C, were characterized. The density of the sintered compounds was increased to 99.2% of theoretical density by carrying out the consolidation at 350oC for 2 min. The Seebeck coefficient, thermal conductivity and electrical resistivity were dependent on hydrogen reduction process and sintering temperature. The Seebeck coefficient increased with reduction process while the electrical resisitivity significantly decreased. Also, the electrical resistivity decreased and thermal conductivity increased with sintering temperature. The results suggest that the carrier density and mobility vary with reduction process and sintering temperature. The highest figure of merit of 3.5×10-3/K was obtained for the compounds spark plasma sintered at 350°C for 2 min by using the hydrogen-reduced powders.

Abstract: In this study, we examined brazeability of Al5052 alloys using a Ag-28Cu insert metal in vacuum condition. A high frequency induction-heating vacuum hot press was used for the brazing. Under a static pressure of 0.5 MPa and a vacuum of 5×10-5 Torr, the temperature of hot press was raised up to brazing temperature (480-590°C) at a speed of 10°C/min and held at the brazing temperature for various times. Interface behaviors of the brazed joints were observed after brazing at 480-590°C by optical microscopy and Electron Probe Micro Analyzer (EPMA). Also, bonding strength of the brazed joints was examined. During vacuum brazing of Al5052 alloys with Ag-28Cu insert metal, eutectic melts were formed by eutectic reaction between Al5052 and Ag-28Cu insert metal at about 490°C. It was found that reaction layers of two phases are formed at the interface between Al5052 and Ag-28Cu insert. EPMA analysis revealed that two phases in the reaction layers consist of Ag-rich phase and Cu-rich phase. The bonding strength of the joints increased with brazing temperature and time. The highest bonding strength was 141 MPa at 580°C, 10min.

Abstract: In automotive industries, components have replaced steel with aluminum alloys since the automotive parts that used aluminum alloys have the ability to improve the environment by lower weights and recycling of waste materials. In this study, experiments were carried out to investigate the characteristics of the developed aluminum alloys according to the forming temperature and speed. The microstructure of forged product in hot forging process was also investigate. The results revealed that the grain size of aluminum alloys was grown according to the forming temperature and speed. Three-dimensional finite element analyses were also carried out to investigate the forming load, deformed shape, and the stress distribution of the workpiece in the hot forging process. The deformed shape of the workpiece was consistent with the trend of experimental results.

Abstract: The rotary swaging process for producing solid bars, tubes or wires is often used to obtain the chipless forming process compared with the conventional process such as the machining and welding operation. Furthermore, machined chip, dust particles, harmful gas, and surface contamination to be commonly generated at hot working during the conventional process could be eliminated as the swaging process is applied and conducted at room temperature. Experiments for swaged product were performed to analyze the influence of the swaging process on the mechanical properties and dimensional precision in terms of surface roughness, surface hardness, and deviation of formed dimension. The changes of dimensional precision in swaged product are also checked out and considered for various process parameters such as the forming speed and the reduction in the outer diameter. Based on the experimental results, the surface roughness is improved under the swaging process. The weight of rod shift is reduced by about 15 % from 223.5 g to 190.2 g. Thus the number of operations to produce the final product under the conventional process is larger than that under the swaging process to be able to omit the machining and welding operation, etc. This paper demonstrates that a rotary swaging process turns out to be an environment-friendly forming process or chipless forming process without the machined chip.

Abstract: In this study, a diffusion bonding of aluminium alloy A6061 was preformed using an Ag-28Cu filler metal in order to conduct eutectic brazing. Melting mechanism was surveyed. Interface behaviors of the brazed joints were observed after brazing and T6 solution treatment. Also, tensile property of the brazed joints was examined. During diffusion bonding of Al6061 alloys with Ag-28Cu filler metal, eutectic melts were formed by eutectic reaction between Al6061 and Ag-28Cu filler metal. It was found that the reaction layer consist of two phases formed at the interface between AA6061 and Ag-28Cu filler metal. EPMA analysis revealed that two phases in the reaction layers consist of Ag-rich phase and Cu-rich phase. Tensile strength was 300 MPa after ageing treatment at 175°C.

Abstract: P-type Bi0.5Sb1.5Te3 compounds doped with 3wt% Te were fabricated by spark plasma sintering and their mechanical and thermoelectric properties were investigated. The sintered compounds with the bending strength of more than 50MPa and the figure-of-merit 2.9×10-3/K were obtained by controlling the mixing ratio of large powders (PL) and small powders (PS). Compared with the conventionally prepared single crystal thermoelectric materials, the bending strength was increased up to more than three times and the figure-of-merit Z was similar those of single crystals. It is expected that the mechanical properties could be improved by using hybrid powders without degradation of thermoelectric properties.

Abstract: Efficient development of method for performance evaluation of machine tools has been regarded as the most important issue to enhance accuracy and quality for users and manufacturers. Recently, a method of accuracy evaluation for machine tools has been studied as interests on precision machine tools have rapidly increased. From this point of view, the circular interpolation test of machine tools is recognized as the most useful method to identify the dynamic accuracy of NC machine tools by ISO, ANSI/ASME and others. In this paper, we studied and developed a formal measurement system with a grid encoder to analyze the final accuracy of NC machine tools. We analyzed the servo system error and the geometric error of NC machine tools through measuring the dynamic error signals by using this system. We verified the experimental result and enhanced the reliability by means of comparing the characteristics of the developed system with the kinematical ball-bar system.