Papers by Author: Jae Hyun Kim

Abstract: In this study, we propose a simplification scheme for modeling a complex bellows structure. Using 3-dimensional finite element analysis, vibration modes and natural frequencies are analyzed. The analysis results are compared with those measured by telemetry system of acceleration. It is found that bending mode of vibration can be activated even a low operation frequency and this leads to uneven distribution of stress. The uneven distribution of stress can be a possible cause for the early failure of a bellows with a large diameter.

Abstract: In this study, an analytical model is developed to analyze non-uniform deformation of a pantograph. The analytical model is verified by comparing predicted deformations with experimentally measured deformations of a pantograph. The effects of total displacement and gaps on non-uniformity are numerically simulated to provide the physical insight into the deformation mechanism of a pantograph. Design guidelines are suggested to minimize non-uniform deformation of a pantograph, and hence also of an associated bellows.

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: 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: NIL(Nano Imprint Lithography) is one of the most promising lithography techniques. There are many variants of NIL, and two major techniques of them are thermal NIL and UV NIL. Here, we focus ourselves on the thermal NIL. During the thermal NIL, the polymeric patterns experience large mechanical strain and high temperature, and this often leads to malformation of polymeric patterns. So it is needed to improve the pattern fidelity and contrast, and these are believed to be closely related to the process condition and mechanical properties. In thermal NIL, PMMA is widely used and chosen as target polymer. Generally, mechanical properties in nano scale are really hard to acquire. In this study, we estimate the mechanical properties of PMMA by molecular dynamic simulation. These properties will be used as input of continuum simulation. We will estimate stress-strain relationship of PMMA. This stress-strain relationship depends on strain rate and temperature. So we will study about strain rate and temperature effect.

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: 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.

Abstract: In this study, we use the strip-bending test to measure the residual stress of a thin film structure. The principle of the strip bending test and the test procedures are described and the analysis of the strip deformation is presented. The explicit formula for estimating the residual stress is given, which requires the initial stress as an input. As an example, the E-beam evaporated Au thin film is chosen, and the residual stress is measured by the present method. The Au thin film structure has a tensile or compressive residual stress depending on the film thickness. The tensile and the compressive residual stresses of Au thin film are successfully measured by the present method.

Abstract: The GaAs wafer bonding process is investigated to reduce the mechanical failures of GaAs wafer based on strength design concept. Three-point bending experiment is performed to measure the fracture strength of GaAs wafer, of which cleavage takes place on (110) plane. We propose a simple method for minimizing the thermal residual stress in a three-layer structure, of which the basic idea is to use an appropriate steady-state temperature gradient to the wafer bonding process. The optimum bonding condition of GaAs/wax/sapphire structure is determined based on the proposed method. The effect of material anisotropy on the thermal residual stress is also analyzed by finite element method.

Abstract: Atomic force microscope (AFM) is a powerful tool for exploring a nano-scale world. It can measure a nano-scale surface topography with very high resolution and detect a very small force. In this paper, we propose a novel AFM cantilever and its calibration scheme to utilize AFM as a mechanical testing machine. We call this AFM with a new cantilever as a force-calibrated AFM. The feasibility of the AFM cantilever is validated through measurement of mechanical properties of freestanding Au thin films.