Earticle

본 논문에서는 웨어러블 디바이스용 SVGA급(800×600 pixel) Flip-chip 기반의 마이크로 LED 디스플레 이 광원 모듈 연구를 통해 마이크로LED 제작 공정을 개선하고, 시제품의 성능을 측정하여, 제품의 상용화 가능성 을 분석하였다. 먼저, 800×600 Flip-chip 기반 마이크로LED 어레이와 PM 구동 평가를 위해 32×32 어레이를 설 계 및 제작하여, 그 성능 및 제품화의 가능성을 확인하였으며, 제작 공정 기술에서 Self align 전극 형성 기술과 고집적화 어레이의 Mesa 형성을 위한 드라이 에칭 기술을 적용하여 생산성을 향상하였다. 본 연구를 기반으로 추 후 마이크로LED가 제품화시 향후 플렉서블 디스플레이 뿐만이 아닌 다양한 분야에서 적용 가능함으로 관련 산업 으로의 파급효과가 클 것으로 예상된다.

In this paper, we try to analyze the commercial potential of products by improving the micro-LED manufacturing process and measuring the performance of prototypes through research on the SVGA-class (800×600 pixel) flip-chip-based micro LED display light module for wearable devices. First, the 800×600 flip-chip based microLED array and the 32×32 array were designed and manufactured for PM driving evaluation, and their performance and potential for commercialization were confirmed. Productivity was improved by applying dry etching technology to form Mesa. Based on this study, when microLED is commercialized in the future, it is expected to have a large ripple effect on related industries as it can be applied to various fields as well as flexible displays in the future.

LED-to-LED visible light communication (VLC) is a technology that LEDs are used as both transmitters and receivers. In this paper, we studied a micro-LED-to-micro-LED VLC since micro LEDs have faster modulation speed and are used in various applications recently. The frequency responses and SNRs were measured for all possible combinations of four LED colors (red, green, blue, and white) in the transmitter and the receiver. Through this, the theoretical maximum channel capacity was estimated for each combination case. Finally, we conducted an experiment with the optimal transmitter and receiver LED set and investigated its performance.

현재 마이크로 LED (Light Emitting Diode)는 디스플레이와 함께 조명 분야에서 관심을 받고 있으며, 높은 휘도, 빠른 동작 속도, 에너지 효율, 오랜 시간 동작 등의 장점을 가지고 있다. 스마트폰, 텔레비젼, 웨어러블 전 자소자 등에서 새로운 혁신을 불러올 것으로 예측된다. 이러한 마이크로 디스플레이는 마이크로미터 크기의 LED 소 자들로 이루어진 픽셀로 구현되며, 자체적으로 빛을 내는 자체발광형 디스플레이다. 주요 제조 공정으로는 결정성장, 패터닝과 식각, 칩 분리와 전사, 본딩과 배선, 패널 조립과 봉지, 검사와 품질 관리 등으로 구분할 수 있다. 최근 몇 년 동안 이 기술은 빠른 속도로 발전했으며, 기업들이 이러한 분야에의 투자를 확대하고 있다. 최신 시장 조사 결과 에 의하면, 마이크로 LED 디스플레이 시장은 지속적으로 성장할 것으로 관측되고 있으며, 주요 발전 방향은 제조 공정 개선, 소재 혁신, 구동 기술 발전 등으로 요약할 수 있다. 이러한 연구를 통해 상용화가 가속화되어 높은 성능 과 다양한 응용 가능성으로 디스플레이 산업의 혁신을 이끌 것이라 판단된다.

Currently, micro LEDs (Light Emitting Diode) are attracting attention in the lighting field along with next-generation displays and have advantages such as high luminance, operating speed, energy efficiency, and long-term driving. It is predicted to bring new innovations in smartphones, televisions, and wearable electronic devices. These micro displays are self-luminous displays that emit light by themselves by being implemented as pixels composed of micrometer-sized LED devices. The main manufacturing processes can be divided into crystal growth, patterning and etching, chip separation and transfer, bonding and wiring, panel assembly and encapsulation, inspection, and quality management. Recently, this technology has developed at a rapid pace, and companies are expanding their investments in these fields. According to recent market research results, the micro LED display market is expected to continue to grow, and the main development direction of development can be summarized as manufacturing process improvement, material innovation, and driving technology development. It is believed that commercialization will accelerate through these studies and lead to innovation in the display industry with high performance and various application possibilities.

본 논문은 10~100㎛ 수준의 마이크로 LED가 수십 개를 배열하였을 때 광학계를 적용하여 광원의 모양이 조사 면에 똑같이 생기도록 시뮬레이션 하였다. 이때 광학계를 통과한 빛이 근접거리의 조사면에서 발생하는 색수차를 보완 하고자 렌즈의 표면에 DOE 계수를 적용하였다. 렌즈 표면에 DOE 계수값이 적용되지 않은 렌즈와, 적용된 렌즈의 시뮬 레이션 결과를 통해, 개선된 점을 확인하고 응용 분야에 대해 가능성을 볼 수 있었다. 본 연구를 통해서 각 산업에서 도입기에 머물고 있는 마이크로 LED를 조명의 광학분야에서 응용될 수 있는 방안과 단점을 보완할 수 있는 방법을 고 안하였다.

In this paper, it simulated the shape of the light source evenly on the irradiation surface by applying the optical system when the micro LEDs of 10 to 100 ㎛ level arranged several chess. At this time, in order to complement the chromatic aberration generated at the irradiated surface of the light passing through the optical system, the doe coefficient was applied to the surface of the lens. Through the simulation results of the DOE coefficient value applied to the lens surface and the applied lens it was possible to see the improved points and see the possibility of the applied field. Through this research, we devised a method to be able to complement a scheme and a disadvantage that can apply micro LEDs in the optics field of lighting, which are only in the introduction period in each industry.

본 논문에서는 스텝핑 모터와 연동하여 방향 제어되는 LED 빔 포밍 통신 시스템을 HW로 구현하였다. ATMega1284 MCU를 이용하여 제어 보드를 구성하였으며, PC와 연결되는 RS232 IO와 스텝핑 모터 구동 보드 제어용 포트(Port)를 이용하여 외부 인터페이스를구성하였다. 스텝핑 모터는 200 PWM으로 360도 회전하고 동시에 반경이 커지는 아르키메디스의 소용돌이(Archimedian Spiral) 방식을 적용하여 목표하는 지점을 선택적으로 조명할 수 있도록 하였다. 이 빔 포밍은 RS232와 연결된 PC 툴(tool)에 의해서 제어되며,ATMega1284 MCU 보드가 호스트(host) 역할을 하였다. 빔 포밍의 동작은 실제 구현된 HW/SW 기반으로 그 유용성을 검증하였다.

In this paper, we implemented LED beam forming communication system controlled by stepping motor. ATMega1284 was used as a MCU of main control board which has two main external IO, one is RS232 for connection with PC, the other is PORT for connection with motor driving board. Stepping motor rotated 360 degree when provided 160 clock and its rotation radius was increased by Archimedian Spiral. So LED can provide its light anywhere in the space and its beam forming was controlled by PC connected with RS232 of main control board. The action of beam forming was verified via actual HW/SW implementation.

본 논문에서는 사용자가 존재할 때만 LED 조명을 활성화해서 통신 기능을 수행하고 사용자가 없을 때에는 자동으로 LED 조명을 비활성화 할 수 있는 종합 제어 시스템 기반 기술을 디자인하고 구현하였다. 이 시스템은 마이크로 콘트롤러를 이용한 사용자 감지 기능을 수행하는 HW 모듈과 웹을 통한 종합 제어 시스템 SW 모듈로 구성된다. 마이크로 콘트롤러 보드는 ATMega2560를 이용하 여 설계하고, 적외선과 초음파 센서를 이용하여 사용자 유무를 판단하였으며, 웹 기반의 모니터링 시스템은 스마트 디바이스를 통해 서 제어할 수 있도록 구성하였다. 두 모듈의 연동 테스트를 통해서 본 시스템의 유효성을 검증하였다.

In this paper, we implemented total monitoring system in which LED light turned on only when user detected and LED light turned out only when user disappeared. This system is composed of two modules, one is HW board based on Micro Controller and the other is SW control system based on Web server. Micro controller board is based on ATMega2560 chip which is connected with Infra Red and Ultra sonic sensors. Web based monitoring system was designed can be used in smart device. The validity of this monitoring system was proved by integration test of two modules.

This paper presents an optical technique for the three-dimensional (3D) shape inspection of micro solder balls used in ball-grid array (BGA) packaging. The proposed technique uses an optical source composed of spatially arranged light-emitting diodes (LEDs) and the results are derived based on the specular reflection characteristics of the micro solder balls for BGA A vision system comprising a camera and LEDs is designed to capture the reflected images of multiple solder balls arranged arbitrarily on a tray and the locations of the LED point-light-source reflections in each ball are determined via image processing, for shape inspection. The proposed methodology aims to determine the presence of defects in 3D BGA shape using the statistical information of the relative positions of multiple BGA balls, which are included in the image. The presence of the BGA balls with large deviations in relative position imply the inconsistencies in their shape. Experiments were conducted to verify that the proposed method could be applied to inspection without sophisticated mechanism and productivity problem

LED-to-LED VLC(: Visible Light Communications) 기술은 기존의 VLC 기술과 달리 송신부와 수신부를 모두 LED로 사용한 기술이다. 본 논문에서는 Micro LED를 송신부로 사용한 Micro LED-to-LED VLC 기술을 연구하였다. Red, Yellow, Green, Blue, White 색상의 송/수신부 LED에 따라 통신 시스템의 상승 시간과 신호 대 잡음비를 측정하여 채널 용량을 비교하였다. 본 실험 결과에 따르면, 송신부 Micro LED가 Blue이고 수신부 LED가 Green일 때에 통신 시스템 중에서 가장 좋은 결과인 125 kbps의 채널 용량을 갖는 것을 확인하였다. 또한, OFDM(: Orthogonal Frequency Division Multiplexing) 신호를 사용하여 비트 에러율을 확인한 결과, 250 kbps까지는 통신이 가능함을 실험적으로 확인하였다.

LED-to-LED VLC is a technology that uses LEDs as both a transmitter and a receiver unlike the typical VLCs. In this paper, we experimentally demonstrate a micro LED-to-LED VLC using Micro LED. We tested all the possible VLC cases using red, yellow, green, blue, and white color LED as both a transmitter and a receiver, and measured rise time and SNR. Then we calculated channel capacity depending on the LED color sets. Our experimental results show that the best channel capacity is 125 kbps when the transmitter micro LED was blue and the receiver LED was green. We also measured BERs of VLCs using OFDM signal, and we showed a successful micro LED-to-LED VLC upto 250 kbps.

Micro-light emitting diode (micro-LED) displays offer numerous advantages such as high brightness, fast response, and low power consumption. Hence, they are spotlighted as the next-generation display. However, defective LEDs may be created due to non-uniform contact loads or LED alignment errors. Therefore, a repair process involving the replacement of defective LEDs with favorable ones is necessitated. The general repair process involves the removal of defective micro-LEDs, interconnection material transfer, as well as new micro-LED transfer and bonding. However, micro-LEDs are difficult to repair since their size decreases to a few tens of micron in width and less than 10 μm in thickness. The conventional nozzle-type dispenser for fluxes and the conventional vacuum chuck for LEDs are not applicable to the micro-LED repair process. In this study, transfer conditions are determined using a micro stamp for repairing micro-LEDs. Results show that the aging time should be set to within 60 min, based on measuring the aging time of the flux. Additionally, the micro-LEDs are subjected to a compression test, and the result shows that they should be transferred under 18.4 MPa. Finally, the I-V curves of micro-LEDs processed by the laser and hot plate reflows are measured to compare the electrical properties of the micro-LEDs based on the reflow methods. It was confirmed that the micro-LEDs processed by the laser reflow show similar electrical performance with that processed by the hot plate reflow. The results can provide guidance for the repair of micro-LEDs using micro stamps.

In this paper, in order to apply the CF (color filter) type of the micro light emitting device (Micro LED) display method, a study on the manufacturing process of red and green phosphor inks for the inkjet process was conducted. The blue light-emitting KSF and LuAG phosphors were respectively used to control the phosphor particle size to about 1㎛, and a phosphor ink was prepared by synthesizing with a low-viscosity solution (IPA/Eg). A chemical dispersion method was applied to selectively control the dispersion characteristics in the manufacture of phosphor inks, and in particular, phosphor inks with a dispersant applied a dispersant secured stable dispersion characteristic compared to phosphor inks without a dispersion process. Therefore, it seems possible to manufacture CF for Micro LED through an inkjet process capable of controlling the dispersion characteristics of phosphor ink.

차세대 디스플레이로 부상하고 있는 Micro LED는 현재까지 여러 기술적인 문제로 상용화가 어려운 상황이다. 특히 Micro 단위로 작은 LED를 디스플레이 기판으로 옮기는 전사(Transfer) 기술이 상용화를 가지고 올 수 있는 핵심 기술이며, 중요한 변수다. 전사기술의 핵심은 LED를 원하는 위치에 빠르고 정확하게 이동시키는 것에 있다. 따라서 Micro LED에서 가장 핵심 과제인 전사 기술에 대해 고민할 필요가 있으며, 전사 기술 중 하나인 Elastomer Stamp를 이용한 전사 기술에 대해서 분석해보고 정리해보았다.

Micro LED, which is emerging as a next generation display, is difficult to commercialize due to various technical problems to date. In particular, transfer technology that transfers small LED to a display substrate in micro units is a key technology that can bring commercialization and is an important variable. The core of transfer technology is to move the LED quickly and accurately to the desired location. Therefore, it is necessary to pay attention on the transfer technology which is the most important task of Micro LED, and we analyzed and summarized the transfer technology using Elastomer Stamp which is one of the advanced transfer technologies.

This research aimed to evaluate the adhesion properties of transfer films and target substrate materials and design an appropriate transfer process for flexible and transparent micro-LED displays. The adhesion between transfer films and the micro-LEDs should be smaller than the adhesion between the target substrate and the micro-LEDs for successful transfer process. Colorless Polyimide(CPI) film coated with a UV-curable adhesive was used as the target substrate. Various transfer films with a silicone-based adhesive layer were used to transfer micro-LED arrays. Pull-off forces on the transfer films and the target substrate were measured using a micro-tribometer at different contact load and separation velocity to predict the transfer of micro-LED arrays from the transfer films to the target substrate. The results showed that the pull-off forces on the transfer films were increased with contact load and separation velocity. On the other hand, the pull-off forces on the target substrate increased and decreased with UV exposure time. Based on the adhesion test results and the transfer test, the transfer film (TF-C) was used and the target substrate was cured for 90 s for the highest transfer yield. As a result, the transfer yield was achieved to 99.82% for monochromatic micro-LED arrays and 96.78% for RGB micro-LED arrays. This work demonstrates the feasibility of fabricating flexible and transparent micro-LED displays by the transfer process.

The core-shell InGaN/GaN Multi Quantum Well-Nanowires (MQW-NWs) that were selectively grown on oxide templates with perfectly circular hole patterns were highly crystalline and were shaped as high-aspect-ratio pyramids with semi-polar facets, indicating hexagonal symmetry. The formation of the InGaN active layer was characterized at its various locations for two types of the substrates, one containing defect-free MQW-NWs with GQDs and the other containing MQW-NWs with defects by using HRTEM. The TEM of the defect-free NW showed a typical diode behavior, much larger than that of the NW with defects, resulting in stronger EL from the former device, which holds promise for the realization of high-performance nonpolar core-shell InGaN/GaN MQW-NW substrates. These results suggest that well-defined nonpolar InGaN/GaN MQW-NWs can be utilized for the realization of high-performance LEDs.

Due to the wavelength shift problem of micro LED caused by the change of current density, the active matrix driving pixel circuit that is used in OLED cannot be applied to micro LED displays. Therefore, we need a gray scale method based on modulation of duration time of light emission. In this study, we propose the PWM-controlled micro LED pixel circuit based on CMOS thin film transistors (TFTs). By adopting CMOS inverter structure, we can reduce the number of storage capacitors from the circuit and make the operating speed of the circuit faster. Most of all, our circuit is designed to make operating speed of PWM circuit faster by adopting feedback effect through double gate TFT structure. As a result, it takes about 4.7ns to turn on the LED and about 5.6ns to turn it off. This operating time is short enough to avoid the color distortion and help the precise control of the gray scale.

Background: Photodynamic therapy (PDT) using topical photosensitizers has been widely used worldwide as a therapeutic modality for acne. However, there are no published reports on pho- todithazine PDT for acne treatment. Objective: We investigated the effectiveness of PDT with photodithazine and micro-LED treat- ment for acne bacteria-induced inflammation. Methods: We established an acne bacteria-induced inflammation model by injecting of Cutibac- terium acnes into the backs of HR-1 mice. The mouse models were divided into seven groups for control and comparison. Topical photosensitizer (photodithazine) was administered to the mice, and then their acne lesions were exposed to a micro?light-emitting diode (micro-LED) source. The effectiveness of the treatment on acne lesions was evaluated through clinical findings and measurements of acne inflammation biomarkers using semi-quantitative reverse transcription polymerase chain reaction, and Western blot. Additionally, hematoxylin and eosin staining and immunohistochemical staining were employed to evaluate the changes in inflammatory cells and biomarkers in skin tissues. Results: Compared with the control groups treated with either LED or photosensitizer alone, the acne lesions were significantly reduced in severity and number after PDT. The mRNA and protein levels of biomarkers (interleukin [IL]-1α, IL-1β, tumor necrosis factor-α, Toll-like recep- tor 2, matrix metalloproteinase-2, and IL-8) exhibited variable decreases in the PDT group rel- ative to the others. Moreover, there was a decline in inflammatory cells and biomarkers in skin specimens after PDT. Conclusion: This in vivo study demonstrated that PDT using photodithazine and micro-LED technology is effective against inflammation induced by acne bacteria.