Papers by Author: Hak Joo Lee

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Authors: Sang Joo Lee, Seung Min Hyun, Seung Woo Han, Hak Joo Lee, Jang Hyun Kim, Young Il Kim
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.
Authors: Ki Ho Cho, Hak Joo Lee, Jae Hyun Kim, Jong Man Kim, Yong Kweon Kim, Chang Wook Baek
Abstract: We have designed and fabricated diamond-shaped AFM cantilevers capable of performing multi-functioning tasks by using single crystal silicon (SCS) micromachining techniques. Structural improvement of the cantilever has clearly solved the crucial problems resulted from using conventional simple beam-AFM cantilever for mechanical testing. After forcecalibration of the cantilever, indentation tests are performed to determine the mechanical behaviors in micro/nano-scale as well as topographic imaging. A diamond Berkovich tip of which radius at the apex is approximately 20 nm is attached on the cantilever for the indentation test and 3D topography measurement. The indentation load-depth curves of nano-scale polymeric pattern (PAK01-UV curable blended resin) are measured and surface topography right after indenting is also obtained. Development of this novel cantilever will extend the AFM functionality into the highly sensitive mechanical testing devices in nano/pico scale.
Authors: Hee Jung Lee, Seung Min Hyun, Hak Joo Lee, Dae Geun Choi, Dong Il Lee, Eung Sug Lee
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.
Authors: Hak Joo Lee, Jae Hyun Kim, Ki Ho Cho, Seung Min Hyun, Jung Yup Kim, Young Eun Yoo, Wan Doo Kim
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.
Authors: Jae Hyun Kim, Hyun Ju Choi, Bong Kyun Jang, Hak Joo Lee, Byung Ik Choi
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.
Authors: Am Kee Kim, Md Anwarul Hasan, Hak Joo Lee, Seong Seock Cho
Abstract: Nanoindentation test has been performed to characterize the mechanical properties of aluminium alloy foam cell wall. Two of the mechanical properties: hardness and Young’s modulus of cell wall material were evaluated using the stiffness of contact during both loading and unloading. Properties obtained from unloading stiffness were in better agreement with the conventional test result than those obtained from loading stiffness. The finite element analysis using nonlinear finite element code ABAQUS was performed to characterize the yield strength and the stress-strain curve of the cell wall material of the foam. Properties of foam cell wall material were found to be substantially different from the properties of the material before foaming. The methodology used in this paper can be effectively used to characterize the mechanical properties of cell wall of any cellular material.
Authors: Seung Woo Han, Ki Jeong Seo, Jae Jong Lee, Seung Woo Lee, Hak Joo Lee, Jung Yup Kim
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.
Authors: Jung Yup Kim, Hyun Ju Choi, Sang Joo Lee, Hak Joo Lee
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.
Authors: Chung Seog Oh, Sung Hoon Choa, Chang Seung Lee, Hak Joo Lee
Abstract: The accurate characterization of linear coefficient of thermal expansion (CTE) of thin films is vital for predicting the thermal stress, which often results in warpage and failure of a MEMS structure. In this paper, special emphasis is placed on the development of novel test method to extend an ISDG (Interferometric Strain/Displacement Gage) technique to the direct and accurate CTE measurement of MEMS materials, AlN and Au. The freestanding AlN and Au films are 1 μm thick and 5 mm wide. Strain is directly measured by a brand-new digital type ISDG with two Cr lines deposited on the specimen while heating a specimen in a furnace. The whole test system is verified first by measuring the CTE for the NIST’s SRM (Standard Reference Material) 736 (Cu) block. The measured CTE is 17.3 με/oC up to 167 oC, which agrees well with the NIST’s certified value. The CTE of Au is 25.4 ± 1.15 με/oC and that of AlN film is 3.77 ± 0.12 με/oC. The in-plane displacement resolution is about 5 nm at the best circumstances.
Authors: Seung Woo Han, Ki Jeong Seo, Wan Doo Kim, Hak Joo Lee, Hyun Woo Lee, Jae Ho Shin, Jae Joon Lee
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.
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