HyungTae Kim

An Optical Mixer and RGB Control for Fine Images using Grey Scale Distribution

We considered the issue of color mixing from the viewpoint of gray scale images and investigated the effect of the mixed light. The goal of this study was to improve the quality of the grey scale images by using mixed RGB illumination. The mixing mechanism and image quality were formulized and applied to a machine vision system with an RGB mixer. The RGB mixer was made of RGB LEDs, a power AMP, a mixing chamber, and a bundle of optical fibers. The intensity of the RGB light was adjusted by varying the 12 bit DAC voltage. The optical characteristics of the RGB LEDs were measured by a spectrometer. The color variation of the mixed light was monitored by a chromatic meter. Grey scale images were captured and analyzed under various lighting conditions provided by the mixer. The optimal illumination was determined by the distribution of the grey scale in the captured image. The chromatic diagram showed that various colors can be generated, and the image histogram showed that the mixed light can increase the quality of the grey scale image.

Mixed-color Illumination and Quick Optimum Search for Machine Vision

Quick search for optimal illumination using mixed color is proposed, and the possibility for use in industrial vision systems is considered. Multidimensional formulation of the optimum search methods is reviewed to use multiple inputs for various color sources and one output of image sharpness. The voltage inputs are used to adjust the intensity of each color source and the optimal illumination is found by the maximum of the sharpness. The optimum search methods reveal that the number of iterations for the optimal condition in mixed color illumination was under 0.1% the number required by equal section search. The searching paths and convergence were considered in both of the input coordinate and the color coordinate. The results show that the optimum search methods make it possible to find the optimal lighting conditions quickly and accurately.

Mixing and Simplex Search for Optimal Illumination in Machine Vision

Mixed-color illumination affects the quality of images in industrial vision system and it is important to optimize color and intensity for image acquisition. This study used simplex search to find the optimal illumination in a short amount of time. A typical color mixer synthesized various color of lights by changing the inputs of RGB power LEDs and passing the lights through an optical system. The image quality under mixed-color illumination was calculated according to the sharpness. For the purpose of optimal illumination using simplex search, a probe network was organized with N + 1probing points for N inputs. The shape of the probe network, simplex, was varied through procedures of extension, contraction, and shrinkage. The inputs of the color mixer were changed until the size of the simplex became smaller than a threshold. The simplex search was tested for commercial semiconductor patterns, and was useful for finding the optimal illumination.

A novel triple-actuating mechanism of an active air mount for vibration control of precision manufacturing machines: experimental work

With the goal of vibration control and isolation in a clean room, we propose a new type of air mount which consists of pneumatic, electromagnetic (EM), and magnetorheological (MR) actuators. The air mount is installed below a semiconductor manufacturing machine to reduce the adverse effects caused by unwanted vibration. The proposed mechanism integrates the forces in a parallel connection of the three actuators. The MR part is designed to operate in an air spring in which the EM part is installed. The control logic is developed with a classical method and a switching mode to avoid operational mismatch among the forces developed. Based on extended microprocessors, a portable, embedded controller is installed to execute both nonlinear logic and digital communication with the peripherals. The pneumatic forces constantly support the heavy weight of an upper structure and maintain the level of the air mount. The MR damper handles the transient response, while the EM controller reduces the resonance response, which is switched mutually with a threshold. Vibration is detected by laser displacement sensors which have submicron resolution. The impact test results of three tons load weight demonstrate practical feasibility by showing that the proposed triple-actuating mechanism can reduce the transient response as well as the resonance in the air mount, resulting in accurate motion of the semiconductor manufacturing machine.

A review of light intensity control and quick optimum search in machine vision

Optimum searching methods were applied to find optimal illumination of a single color to enhance the quality of a monochrome image and reduce searching time. The problem was defined as 1D searching optimization between light intensity and image sharpness. The image sharpness is the degree of distinctness and is a non-linear function of the input voltage, which can be used to adjust the light intensity. We considered conventional optimum search methods, such as steepest descent, conjugate gradient, Newtons method, bisection, and golden section. These derivative methods and direct searches were tested for a sample pattern using single color lights under coaxial illumination. The iteration for the optimal condition was 6.7% of full scanning in average. The test results show that the steepest descent and golden search are recommended. We checked the possibility of applying these search methods to automatic lighting in machine vision.

Effects of Current Modulation Conditions on the Chromaticity of Phosphor Converted (PC) White LEDs

For two well-known modulation methods, stepwise current modulation (SCM) and pulse width modulation (PWM), the effects of driving current modulation conditions on chromaticity were experimentally investigated in a white LED lighting system. For the experimental implementation of both SCM and PWM, a white LED lighting was fabricated using phosphor converted (PC) white light emitting diodes (LEDs) and a driving circuit module was developed. By using them, the variations of illuminance, color coordinates, and spectrum were evaluated under various forward current conditions. Through the analysis in color coordinates, yellow shift in SCM and blue shift in PWM were observed on chromaticity diagrams with increasing average driving current. In addition, in order to analyze color deviation quantitatively, color distance before and after current increase, and the correlated color temperature (CCT) were calculated. As a result, for the white LED lighting in both modulation conditions, the maximum difference in the calculated CCT was obtained close to 1000 K. It means that careful consideration is required to be taken in the design of illumination systems to avoid serious problems such industrial accidents.

An electro-magneto-pneumatic spring for vibration control in semiconductor manufacturing

An active vibration isolator was developed for semiconductor manufacturing machines using electro-magnetic(EM) levitation force. The spring is composed of a pneumatic elastic chamber and a magnetic actuator. The actuator has a coil and a permanent magnetic plate and is installed inside of the chamber. The mechanical and electronic parts are designed to operate under a weight of 2.0 tons. An air mount is constructed for the experiment with a stone surface plate, 4 active air springs, 4 gap sensors, a DSP controller, a linear power amp and vibration measurement systems. The impulse response of the air mount was monitored before and after the active control. The time duration was reduced by 77% in the impulse response. The maximum peak in frequency domain was reduced by 62%. We also found that the active system can avoid the resonance caused by the natural frequency of the passive system.

The Wafer Alignment Algorithm Regardless of Rotational Center

Wafer alignment is a sequence of translational and rotational movement, so typical alignment algorithms require a rotational center. The method proposed in this paper simplifies the alignment algorithm regardless of the placement of a rotational center. The original algorithm was derived from rigid body transformation, so it had the terms for the rotational center. These terms can be omitted if we make some assumptions on the characteristics of wafer alignment. The derived equation had a form similar to that of root finding in numerical analysis. The algorithm was experimented on the dicing machine and its results were compared with the exact solutions. The result showed that the simplified form was effective enough to be applied for manufacturing. And the iteration speed can be varied by the convergence constant. The derived equation was so simple that the alignment algorithm can be applied to PLC-based systems

A dual side electroluminescence measurement system for LED wafer manufacturing

The development of a system that evaluates the performances of an LED (light-emitting diode) wafer using a non-destructive test method after crystal growth and before chip processing is presented. The system measures electroluminescent characteristics of two sides of an epi-wafer. When a probe makes contact with an epi-wafer, a source meter drives an electric current, then emits LED light. Optical characteristics were measured by two spectrometers and two pico-am meters. The measured data were peak wavelength, FWHM (full width at half maximum), forward current, forward voltage and reverse current. The measuring devices are installed on the front and rear of the wafer. The probe and measuring devices were transferred by a 3-axis stage for the EL contact. Mapping images of the epi-wafer were obtained using these characteristics. The correlation between wafer state and chip state, and repeatability for an epi-wafer, were considered in the test.

A precise inspection technique for wafer pre-sawing lines using Affine transformation

This study proposes a simple and fast technique for detecting wafer cutting lines using the Affine transformation before sawing. This technique was developed for the purpose of wafer alignment when alignment marks are unavailable. The edges of the pre-sawing lines are complex and there are bright shapes in the sawing lines, so it is difficult to apply the conventional edge finding method. The sawing lines are represented by rectangular windows which are deformed by an Affine transformation. The amount of deformation is determined when the grey level in the window is a global minimum in the image. The global minimum is found by the golden section search. The algorithm was tested using 13 sample images and repeated 10 times for each image. The results showed that the processing time was below 1200 m sec and the deviation was in the sub-pixel level. We found that the proposed method can be applied to align wafers in the dicing process.