In-Ha Sung

Micro/nano-tribological characteristics of self-assembled monolayer and its application in nano-structure fabrication

The fundamental tribological characteristics of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS), octadecyltrichlorosilane (OTS), and single chain alkanethiol self-assembled monolayers (SAMs) with various chain lengths were investigated in order to identify the mechanical scribing condition for micro-machining applications. The concept of the novel surface micro-machining explored in this work is to mechanical scribe away the SAM resist coated on the workpiece surface where, pattern formation by subsequent chemical etching is desired. From the experimental results, it was found that the FDTS surface was damaged about 20% more rapidly than the OTS surface due to higher friction, even though the surface energy of FDTS was lower than that of OTS. Also, it was found that thiol on a copper surface could be removed even under a few nN normal load. The nano-tribological characteristics of alkanethiol SAM on various metals were largely dependent on the native oxide layer of metals. Based on these findings, FDTS and 1-hexadecanethiol (HDT) were chosen as the resists for silicon and metal surfaces, respectively. By using the mechano-chemical process with a diamond-coated tip, nano-patterns with sub-micrometer width and depth on surfaces of Au, Ag, Cu and Si could be fabricated.

Design of Surface Micro-structures for Friction Control in Micro-systems Applications

Abstract In this work, the effectiveness of controlling the frictional force at the micro-scale using well structured surface micro-grooves is demonstrated. Such micro-structured surfaces are designed to eliminate wear particles or contaminants from the sliding interface, thereby, minimizing the effect of surface plowing. Also, the surfaces are designed to minimize the effective area of contact, which is found to have a profound effect on the magnitude of the surface force for micro-systems. Experimental evidence of stiction/friction reduction even at high relative humidity by using such micro-structured surfaces are provided in this work. The results of this work can be utilized in optimization of tribological surface design for micro-systems applications.

Analytical model development for the prediction of the frictional resistance of a capsule endoscope inside an intestine

Abstract For the purpose of optimizing the design of the locomotion mechanism as well as the body shape of a self-propelled capsule endoscope, an analytical model for the prediction of frictional resistance of the capsule moving inside the small intestine was first developed. The model was developed by considering the contact geometry and viscoelasticity of the intestine, based on the experimental investigations on the material properties of the intestine and the friction of the capsule inside the small intestine. In order to verify the model and to investigate the distributions of various stress components applied to the capsule, finite element (FE) analyses were carried out. The comparison of the frictional resistance between the predicted and the experimental values suggested that the proposed model could predict the frictional force of the capsule with reasonable accuracy. Also, the FE analysis results of various stress components revealed the stress relaxation of the intestine and explained that such stress relaxation characteristics of the intestine resulted in lower frictional force as the speed of the capsule decreased. These results suggested that the frontal shape of the capsule was critical to the design of the capsule with desired frictional performance. It was shown that the proposed model can provide quantitative estimation of the frictional resistance of the capsule under various moving conditions inside the intestine. The model is expected to be useful in the design optimization of the capsule locomotion inside the intestine.

Prediction of asperity contact condition using FFT-based analysis for micro-grooved surface design in tribological applications

In this paper, the frictional behaviours of single and multi-balls slid against micro-grooved silicon surfaces were investigated by using a micro-tribotester built inside a scanning electron microscope. Various micro-grooves were fabricated on the silicon surface to investigate the frictional behaviour with respect to the contact geometry between the surface asperities. Particularly, fast Fourier transform (FFT) analyses of the friction signals were performed with the motivation to assess the contact conditions. The primary objective of this paper was to understand better the asperity interaction condition at the sliding interface by analysing the frictional force signal using FFT. The experimental and numerical simulation results showed that the relative geometric ratio and the distribution of contact asperities on the surfaces could be predicted by the power and frequency spectra of the FFT analysis of the friction signal. Also, the frictional behaviour for multi-asperities was found to be the result of superposition of the frictional interaction of each asperity contact. It is expected that these observations will be utilized for the design of micro-structured surface with optimum geometry for better tribological performance.

Effects of material pair properties on the frictional behavior of metals

The objective of this work was to investigate the validity of the adhesion theory and the mechanical interaction view point in describing the frictional behavior of metals. Specifically, experimental work was conducted to investigate the relative importance of the material compatibility and hardness ratio on the friction and wear behavior of various metals in dry sliding condition in ordinary laboratory environment. Initial and steady state friction coefficients were measured using both pin-on-disk and pin-on-reciprocating testers for various metal pairs, which were strategically selected based on their compatibility and hardness ratio. The wear characteristics were also observed following the experiments. The experimental results showed that material compatibility had no significant correlation with either initial or steady state friction coefficient. As for the hardness ratio, material pairs with very large differences in initial hardness values resulted in high initial friction coefficient. Overall, it was observed that factors such as sliding motion, whether unidirectional or bi-directional, and wear particle dynamics were found to be more critical on frictional interaction than either material compatibility or initial hardness ratio. Furthermore, among several material properties analyzed, initial friction coefficient had the highest correlation with shear modulus while the steady state friction coefficient had the highest correlation with hardness.

Surface Smoothing of Blasted Glass Micro-Channels Using Abrasive Waterjet

Powder blasting, which is an efficient micromachining method for glass, silicon, and ceramics, has a critical disadvantage in that the surface finish is poor owing to the brittle fracture of materials. Low-pressure waterjet machining can be applied to smoothen the rough surface inside the blasted structure. In this study, the surface roughness and sectional dimension of micro-channels are observed during the repetitive application of a waterjet to blasted micro-channels. The asperities and subsurface cracks created by blasting are removed by waterjet machining. Along with the surface roughness, it is found that the sectional dimension increases and the edges of the finished micro- channel become slightly round. Finally, a microfluidic chip is machined by the blasting-waterjet process and a transparent microfluidic channel is obtained efficiently.

Fluid-Structure Interaction Modeling and Simulation of CMP Process for Semiconductor Manufacturing

Chemical mechanical planarization is one of the core processes in fabrication of semiconductors, which are increasingly used for information storage devices like solid state drives. For higher data capacity in storage devices, CMP process is required to show ultimate precision and accuracy. In this work, 2-dimensional finite element models were developed to investigate the effects of the slurry particle impact on microscratch generation and the phenomena generated at pad-particle-wafer contact interface. The results revealed that no plastic deformation and corresponding material removal could be generated by simple impact of slurry particles under real CMP conditions. From the results of finite element simulations, it could be concluded that the pad-particle mixture formed in CMP process would be one of major factors leading to microscratch generation.

Experimental Investigation of the Frictional Behaviors at Particle-Surface Interfaces in CMP Application Using an Atomic Force Microscope

In order to obtain a fundamental understanding of the tribological characteristics at chemical-mechanical polishing (CMP) process for wafer planarization, an experimental investigation of the frictional behavior between nano/microscale tips and various surfaces were performed using atomic force microscope (AFM) cantilevers with different stiffnesses and tips. In this paper, frictional behaviors according to load/pressure and materials were obtained by using various semiconductor material surfaces. Contact stiffnesses were observed in various tip-surface contact situations. Based on the experimental results, the relationship between the frictional behaviors observed, contact stiffness, and particle-surface material was carefully investigated.