Papers by Author: Hak Joo Lee

Abstract: To apply nano-structured materials in micro/nano system, understanding of the mechanical properties of nano-structured materials is required. In order to perform the mechanical test of nano-structured materials, the mechanical testing system was installed in a scanning electron microscope (SEM). The nano-manipulator was set up in the SEM, and the force sensor formed as a cantilever was mounted on the nano-manipulator. Then, the force sensor can be controlled by using the nano-manipulator. The nano-structured materials were dispersed on the transmission electron microscope (TEM) grid, and both end of the nano-structured materials were welded on the TEM grid and the tip of force sensor by exposing E-beam of the SEM. The tensile tests for carbon nanotubes, ZnO nanorods and ZnS nanowires were carried out in the SEM, respectively. The load response during the mechanical test was obtained by force sensor. The dimension of nano-structured materials was obtained by determining the configuration measured from the TEM. And, strain-stress curve was obtained after mechanical test. The elastic modulus of the nano-structured materials after the tensile tests were calculated and compared. The elastic modulus for multi-walled carbon nanotubes, ZnO nanorod and ZnS nanowire were ~0.98 TPa, ~59 and ~39 GPa, respectively.

Abstract: In this study, we propose an interesting scheme for analyzing micro-pillar compression test results based on finite element method. It uses only load and displacement data measured by micro-pillar compression test, a framework of conventional incremental metal plasticity, and an iterative scheme for updating the material parameters. This is an inverse approach to determine the constitutive relation of materials based on experimentally measured load and displacement. As a demonstrative example, Ni-Co micro-pillars with a diameter of 10 ㎛ and a height of 20 ㎛ were fabricated by micro-machining process, and their load-displacement data were measured by a micro-pillar compression tester. Using the proposed scheme and the measured load-displacement data, the stress-strain curves for the Ni-Co micro-pillars were estimated.

Abstract: Mechanical behavior of small size materials has been explored due to many industry applications such as MEMs and semiconductors. The accurate measurements for mechanical properties of thin films are very challenge due to several technical difficulties. The proposed solution is the Visual Image Tracing (VIT) strain measurement system coupled with a micro tensile testing unit, which consists of a piezoelectric actuator, load cell, microscope and CCD cameras. This system has shown advantages of real time strain monitoring during the test and ability to measure the Young’s modulus, yiled strength and Poisson’s ratio of the material. Free standing Au films 0.5, 1 and 2 μm thick with average grain sizes of 104, 148 and 219 nm prepared by sputtering were studied using VIT system. The yield stresses of the films are dependent on film thickness and grain size.

Abstract: The reliable reproducibility of nano patterns or other nano structures is one of many issues in the nano-imprint lithography process. An important prerequisite for reproducibility is suitable adhesion properties of adhesion promoters or anti-sticking layer. In this study, rhombus shaped symmetrical probe with a flat tip was developed and fabricated using MEMS fabrication technique. For the experimental setup of the adhesion test using a UV curable PAK01 resin coated AFM tip with several adhesion promoters, the flat tip is covered by PAK01 resist using micromanipulator. Anti-sticking layers of silane agents were prepared on the tip by vapor deposition method. Adhesion force between various adhesion promoters (GPTS, APMDS, APTS, DUV30J, O2 planairzation) and PAK01 resist and the force between anti-sticking layer (FOTS, DDMS) and PAK01 resist were evaluated using the force-distance mode of AFM. Adhesion force of GPTS and FOTS are about 7180 nN and 1660 nN, respectively.

Abstract: Vibration in a driveline is presented in this paper. In the experiment, the rear subframe and propeller shafts and axle were composed and mounted with rubber each other. For applying the vibration input instead of the torsional vibration effect of an engine, the shaker was taken. In particular, torsional vibration due to fluctuating forced vibration excitation across the joint between driveline and rear subframe was carefully examined. Accordingly, the joint response was checked from experiments and the FE-simulation using FRF (frequency response function) analysis was performed. All test results were signal processed and validated against numerical simulations. In present study, the new test bench for measuring the vibration signal and simulating the vehicle chassis system was proposed. The modal value and the mode shape of components were analyzed using the model to identify the important components affecting driveline noise and vibration. It could be reached that the simplified test bench could be well established and be used for design guide and development of the vehicle chassis components.

Abstract: We have developed a novel atomic force microscope (AFM) probe as a highly sensitive sensor and an application of the probe into various mechanical tests for characterizing micro/nanostructures. Using MEMS fabrication technique, we have designed and fabricated rhombus-shaped symmetric AFM probe. Adhesion forces between silicon tip and artificial nano-hair structures of cyclic olefin copolymer (COC) and polypropylene (PP) were measured using the probe with a flat tip. The results exhibited the usual characteristics of force-displacement curves of COC and PP nano-hair structures, in which a pull-off force was detected at the point of unloading. The average adhesion forces of the COC and PP hair structures are about 9.48 μN and 10.67 μN, respectively.

Abstract: In the semiconductor inspection, number of pads at unit area is increasing and pad array have become irregular 2-dimensional. So we develop a bellows type micro contact probe. Our micro contact probe is a vertical-type micro contact probe for area arrays and narrow pitch electrode pads. We choose bellows type micro contact probe to prevent stress concentration. Our design targets are 120 um pad pitch, 20-50 um OD(over drive) and over 5 mN reaction force. In this research, micro contact probe is made by electroplating(Ni-Co). structural analysis is accomplished by FEM. And characterization of micro contact probe is done by our own mechanical tester. So material property is measured for structural analysis and structural analysis result will be compared with experimental result.

Abstract: Flexible printed circuit board (PCB), which is used for folder and slide type cellular phones, consists of flexible copper clad laminate (FCCL) and cover layer. Through it an electric current is applied to liquid crystal display (LCD) from the main board of cellular phone. In thin Cu foils of flexible PCB fatigue cracks due to repeated bending motion generate and propagate, and they cause a short circuit. Fatigue behavior of thin Cu foils being used for flexible PCB must be evaluated and confirmed to resolve this problem. It is based on findings by several researches that the mechanical properties of thin film materials differ from those of their bulk counterparts. Thin film properties have been investigated over the last years; however fatigue behavior of thin films has not yet been studied as thoroughly as monotonic behavior. In this study fatigue properties of thin Cu foils for the application in flexible PCB are obtained. Fatigue testing was performed for two kinds of Cu foils that were made by rolling and electrochemical procedures respectively. Differences of fabrications in fatigue behavior of thin foils were distinguished. Especially for rolled Cu foils, effects of rolling directions in fatigue properties were evaluated.

Abstract: It is quite difficult to accurately measure the mechanical properties of thin films. Currently, there are several methods (or application) available for measuring mechanical properties of thin films. Their properties, however, have been determined by indirect methods such as cantilever beam test and diaphragm bulge test. This paper reports the efforts to develop a direct strain measurement system for micro/nano scale thin film materials. The proposed solution is the Visual Image Tracing (VIT) strain measurement system coupled with a micro tensile testing unit, which consists of a piezoelectric actuator, load cell, microscope and CCD cameras. The advantage of this system is the ability to monitor the real time images of specimen during the test in order to determine its Young’s modulus and Poisson’s ratio at the same time. Stress-strain curve, Young’s modulus, yield strength and Poisson’s ratio of copper thin film measured using VIT system are presented.

Abstract: Nanoimprint lithography is a promising technology to produce sub-100 nm scale features on silicon chips. One of key issues in the nanoimprint lithography is how to make uniform contact between the stamp and the substrate on a large area. In this study a rubber membrane unit under substrate is introduced to resolve this problem. Two layers of membrane were designed to consider air flow in the middle of resist on a silicon wafer. The geometry design for accomplishing uniform contact was carried out using finite element analysis. The material modeling of hyperelastic properties of rubber is characterized by the Mooney-Rivlin strain energy functions. Material constants in the strain energy functions are able to be determined via the curve fitting of experimental stress-strain data. Simple tension and equi-biaxial tests were performed to determine the material constants. To evaluate the effects of a rubber membrane unit, nanoimprint lithography process with it was executed. We could confirm that a distinct improvement of uniform contact was shown and air flow problem was solved during the process.