Papers by Author: Chul Hee Lee

Abstract: The polishing is one of the important methods in manufacturing of silicon wafer and in thinning of completed device wafer. Generally, getting a flat surface such as a mirror is the purpose of the process. The wafer surface roughness is affected by many variables such as the characteristics of the carrier head unit, operation, speed, the pad and slurry temperature. Optimum process conditions for experimental temperature, down-force, slurry ratio are investigated, time is used as a fixed factor. This study will report the evaluation on surface of wafer by dependent of varying platen, chuck rpm, temperature variation, and oscillation which affect it has on the surface roughness. In this experiment, it is determined the optimum condition for polishing silicon wafers. By using optimum condition, it helps to achieve an ultra precision mirror like surface.

Abstract: In this study, Magneto-rheological (MR) fluid was modified by mixing and adding the additives. The researches on MR fluid have been active progress all around the worlds, especially for the application to many devices on the various fields. For the success of application, tribological characterisitcals are important to be satisfied. There are many progress mainly aimed to the methods to improve the stability and performance of MR fluid in application. In this study, the experiments on friction and wear with pin-on-disk tester as well as SRV tester are conducted in different operation and magnetic. From the results, tribological performance of modified MR fluid is analyzed.

Abstract: Electropolishing, the anodic dissolution process without contact with tools, is a surface Treatment method to make a surface planarization using an electrochemical reaction with low current density. Stainless steel can be put various applications which require purity and high precision surface of products. The aim of this study is to investigate the characteristic of electropolishing effect for stainless steel workpieces. In order to analyze the characteristics of electropolishing effect, surface roughness and micro-burr size were measured in terms of machining conditions such as current density, machining time and electrode gap. The tendencies about improvement of surface roughness by electropolishing for stainless steel workpieces were determined.

Abstract: In this paper, the effect of particle concentration and magnetic field on the tribological behavior of magneto-rheological (MR) fluid is investigated using a pin-on-disc tribometer. The wear loss and friction coefficient are measured to study the friction and wear properties of MR fluid. The morphology of the worn pin is also observed by scanning electron microscope (SEM) in order to analyze the wear mechanism. The results obtained in this work show that the wear loss and friction coefficient decrease with increasing particle concentration under the magnetic field. Furthermore, it is demonstrated that the magnetic field has a significant effect on improving tribological properties of MR fluid, especially the one with high particle concentration. The predominant wear mechanism of the MR fluid has been identified as abrasive wear.

Abstract: This experimental investigation presents wear characteristics of magnetorheoloigcal (MR) fluid under boundary lubrication contact condition. Three different specimens; aluminum, copper and steel pins are prepared and wear test is performed under consideration of several operational factors such as normal load, sliding distance, and sliding speed by using pin-on-disk test equipment. Wear rate and friction coefficients of the MR fluid are experimentally evaluated with respect to the sliding ranges. Microscopic surface and roughness changes of the worn surface of pin specimens are also analyzed by using the scanning electron microscope (SEM). In addition, Energy Dispersive X-ray Spectroscopy (EDS) analysis is conducted and chemical changes are investigated.

Abstract: In this paper, a smart structure for the micro position control is proposed using the piezo stack actuator. The smart structure is comprised with PZT based stack actuator, mechanical displacement amplifier and positioning devices. Based on the bridge-type flexural hinge mechanism, a displacement amplifier is designed and integrated with a piezo stack actuator to produce a desirable positioning stroke of the device. In order to achieve the high precision control performance in a positioning device, a stick-slip phenomenon should be suppressed in contacting surfaces of the device, which is generally indispensable in the mechanically connected systems and particularly obvious for the micro-scale system. Therefore, the stick-slip model is enhanced by theoretically calculating the static friction based on the statistical rough surface contact model. Then, a PID feedback control algorithm with the developed stick-slip model is formulated for achieving accurate positioning of the device. Using the proposed smart structure, simulations of precise position control under the representative operating condition of positioning are conducted to demonstrate the stick-slip suppressing and micro positioning performance.

Abstract: This paper presents a new type of jetting dispenser driven by a piezostack. Via a flexible beam mechanism, the amplitude of a needle motion is amplified to such a value that can make a dispensing of medium and high viscosity adhesive. By designing the flexible beam with high resonant frequency, the dispenser can operate at a frequency much higher than that of conventional dispensers. Therefore, it is expected that the dispenser can provide very small dispensing dot size at high dispensing flow rate, which is imperatively required in modern semiconductor packaging processes. Furthermore, the dispensing flow rate and dot size can be effectively controlled by driving voltage applied to the piezostack. After describing the mechanism and operational principle of the dispenser, a mathematical model of the system is derived by considering dynamic behaviors of structural parts such as the piezostack, the flexible beam and the needle structure, and the adhesive fluid dynamics. In the modeling, a lumped parameter method is applied to model the adhesive dynamics and the governing equation of the whole dispenser is then derived by integrating the structural model with the fluid model. Based on the proposed model, the dispenser is designed and manufactured. Subsequently, dispensing performances such as a dot size and dispensing flow rate are experimentally evaluated.

Abstract: In this paper, a stick-slip compensation for the micro-positioning is presented using the statistical rough surface contact model. As for the micro-positioning structure, PZT (lead(Pb) zirconia(Zr) Titanate(Ti)) actuator is used to drive the load for precise positioning with its high resolution incorporating with the PID (Proportional Integral Derivative) control algorithm. Since the stick-slip characteristics for the micro structures are highly nonlinear and complicated, it is necessary to incorporate more detailed stick-slip model for the applications involving the high precision motion control. Thus, the elastic-plastic static friction model is used for the stick-slip compensation considering the elastic-plastic asperity contact in the rough surfaces statistically. Mathematical model of the system for the positioning apparatus was derived from the dynamic behaviors of structural parts. Since the conventional piezoelectric actuator generates the short stroke, a bridge-type flexural hinge mechanism is introduced to amplify the linear motion range. Using the proposed smart structure, simulations under the representative positioning motion were conducted to demonstrate the micro-positioning under the stick-slip friction.

Abstract: This paper presents a solution of the vibration reduction in driving automotive shafts. Generally, vibration modes in automotive driveshafts are divided into the bending and torsional vibrations. However, the bending vibration is more dominant factor when it excites with the resonance frequencies in automotive driveshafts. In this paper, the vibration damping structure of automotive driveshaft is introduced by incorporating piezofiber composite structure, which is also called as MFC (Macro Fiber Composite). The MFC is an innovative actuator that offers high performance and flexibility than other piezo-materials, so it is the best candidate of actuator to apply to the curved surface of shaft. In order to simulate the bending vibration reduction in the automotive shaft, analytical model based on cylindrical shell theory was developed. Moreover, Finite Element Analysis (FEA) using the piezoelectric-thermal analogy technique was conducted to confirm the analytical results and demonstrate the vibration reduction performance. The effect by the polarity of MFC on the vibration damping is also studied to find the best combination of MFC activation. Thus, the results showcase the optimal vibration damping capabilities using MFC in automotive driveshafts, and provide an outlook for the active damping control using the multi-mode resonance controllers.