Jo Oct 16, 2023
A lot of convex corner compensation methods of V-grooves structure thin membranes of a semiconductor pressure sensor have been introduced, but they still make it difficult to make thin membranes with square mass. In order to make silicon thin membranes in different thicknesses with compensated stress, thin corner compensation method for preserving the square shape have to be developed.
Yu Nam Chol, a researcher at the Science Engineering Institute, has designed a convex corner compensation pattern to make V-grooves structures thin membranes and proved the superiority of this method by means of the final analytical results.
His method ensured the membrane thickness of mass structure close to a right-angled pattern by combining several compensation patterns from 10㎛ to 20㎛. He made a membrane with mass whose a/h is less than 3.72 and b/h is less than 3.33, by applying corner compensation patterns.
You can find the details of this in his paper “A Compensating method of Convex Corner in Etching of (110) Si” in “Microsystem Technology” (SCI).
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Jo Oct 14, 2023
In recent years, carbon nanotubes (CNTs) are widely used as a high-sensitivity sensor material. The fine structure of the surface morphology largely affects the characteristics. Therefore, delicate observation of the microstructures within the range of nanometer is vitally important for achieving better CNT surface properties.
On the basis of theoretical analysis on the observation of the microstructure of CNT surface, Yu Nam Chol, a researcher at the Science Engineering Institute, has observed the image of CNT surface microstructure by Quanta 200 SEM to select a proper accelerating voltage.
The theoretical analysis of the effects of energy of incident electrons on the range of the electron-CNTs interaction and the resolution of SEM indicated that the most proper accelerating voltage for microstructure observation of CNTs surface by using an SEM (Quanta200) is within 5~10kV. Through the experiments based on it, he found that the accelerating voltage of 7.5kV provides the sharpest image of the microstructure of CNT surface.
You can find more information about this in his paper “Study on Accelerating Voltage of SEM in Observation of Carbon Nanotube Surface” in “Nanoscience and Nanometrology”.
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Jo Oct 12, 2023
Pak Se Yang, a researcher at the Science Engineering Institute, has proposed a new control technique to realize the maximum torque per ampere (MTPA) in direct torque control (DTC) of a permanent magnetic synchronous motor (PMSM).
Because MTPA condition is provided by the relation between d-axis and q-axis components of the stator current, rotor position has to be known for frame conversion. Therefore, he chose to employ the estimation method of rotor position for realization of MTPA.
For the sake of rotor position estimation, he formulated the recursive least square problem for online estimation of back electro-magnetic force by using a motor model based on the rotational reference frame, and then determined the rotor position. In order to minimize the torque ripple of conventional DTC system, he synthesized the 3 phase output voltages by using the space vector pulse width modulation (PWM) technique.
His paper was presented in the “10th International Conference on Advanced Technologies”.
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Jo Oct 9, 2023
Generally, rotary angular sensors can be classified into two types ― sensors based on changes in an electric field and a magnetic field, and optical sensors ― according to the physical principle used for measurement. Among them, optical sensors that operate on visible light or infrared light offer advantages of contactless measurement and insensitivity to electric and magnetic fields.
Yu Nam Chol, a researcher at the Science Engineering Institute, has proposed an absolute rotary angular sensor with a nonlinear transparent disc between light source and light dependent resistors (LDR).
In this sensor, the absolute rotary angle is measured by the output resistance of double LDRs which has a linear change by nonlinear transparency of the disc in a range of 0~360° according to the characteristics of LDR’s resistance via irradiance.
This sensor’s advantage is that it is immune to shock and vibration as it has a nonlinear transparent disc instead of a binary coded disc and it has a large gap between the disc and the optical sensor. It means that it is easy to make a sensor. The disc is made of fiber glass and covered up by a self-adhesive tape whose transparency changes nonlinearly in a range of 0~360°.
Another advantage of this sensor is that its resolution is determined by an AD converter in the signal processing circuit as the output signal of LDRs is analogue. Therefore, this rotary angular sensor is provided with a high resolution in a range of 0~360°.
For more information, please refer to his paper “Improving of Characteristics of Rotary Angular Sensor Using Nonlinear Transparent Disc” in “International Journal of Sensors and Sensor Networks”.
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Jo Oct 5, 2023
The Shockley-Read-Hall (SRH) lifetime is a very important parameter in silicon bipolar power devices that determine switching characteristics.
As for carrier lifetime control, recombination center levels in impurities-doped silicon are dependent on the properties of impurities and silicon materials, but not on the amount of impurities.
Preceding researchers studied diffusion of impurities and radiation action for recombination center levels by means of Au, Pt, electron irradiation, proton irradiation, etc.
Yu Nam Chol, a researcher at the Science Engineering Institute, has determined new recombination center levels in n-type silicon with gold and platinum, and investigated the effectiveness of the levels for low-level and high-level injection lifetime control.
At injection levels below 3×1013cm-3, reverse recovery technique was used, while open-circuit carrier decay (OCCD) technique was employed at levels above 1015cm-3. He performed lifetime measurements in the temperature interval of 303~403K. He studied the deep levels introduced by diffusion using DLTS, and calculated the corresponding SRH lifetime, and finally compared it with the measured values. The comparison suggests that low-level lifetime is mainly controlled by defect B located at 0.54eV below the conduction band, and high-level lifetime by defect A located at 0.25 eV.
The result shows that the recombination center levels estimated from DLTS measurement are dominant levels in n-type silicon doped gold and platinum.
If further information is needed, please refer to his paper “Analysis of recombination center levels in gold and platinum doped n-type silicon” in “Journal of Power electronics and devices”.
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Jo Sep 28, 2023
High-intensity focused ultrasound (HIFU) is being widely used to treat tumors in prostate, liver, kidney, pancreas, bone, breast, and uterine fibroids.
Several numerical simulation methods have been proposed for ultrasound propagation. They include Rayleigh–Sommerfelt diffraction integral, Khokhlov–Zabolotskaya–Kuznetsov (KZK) method, FDTD method, etc. In most methods, FDTD algorithm was applied to compute the temperature field caused by ultrasound exposure. In some methods, HIFU heating model by the finite element method was proposed to solve the linear Westervelt equation and the Pennes bio-heat transfer equation.
Kim Sang Jin, a researcher at the Faculty of Physics Engineering, has proposed a new numerical simulation method to predict the temperature distribution in human tissues during ultrasound tumour therapy. First, in order to calculate the pressure distribution in human tissue, he solved the nonlinear Westervelt equation using finite difference time domain (FDTD) algorithm. Second, to calculate the temperature distribution in human tissue, he solved the Pennes bio-heat transfer equation using lattice Boltzmann method (LBM). To verify the theoretical analysis, he compared the results with experimental measurements of previous works.
The results showed that the FDTD-LBM provides a high degree of accuracy in the simulation on the sound pressure field and temperature field in human tissue during high-intensity focused ultrasound therapy, and that the temperature value at focal point clearly increases and the focal length gets smaller with the increase in the ultrasound frequency.
For more information, please refer to his paper “Numerical Simulation Method for Prediction of HIFU Induced Lesions in Human Tissue: FDTD-LBM” in “Physics of Wave Phenomena”.
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