풀 컬러 투명 마이크로 LED 디스플레이를 위한 색변환 기술 연구

Abstract

Micro‐LED display is expanding rapidly in recent years because of its outstanding features such as low power consumption, nanosecond response time, long lifetime, high dynamic range, and wide color gamut. However, the high‐yield mass transfer process of micro‐LEDs from semiconductor wafer to glass substrate remains a challenge. To achieve full‐color micro‐LED displays, the most commonly used method is to grow red, green, and blue (RGB) micro‐LEDs on different wafers, and then assemble them into matrices on the same thin‐film transistor (TFT)‐based glass substrates through mass‐transfer, which requires precise alignment for each pixel. Moreover, the light emission efficiency and degradation rate of RGB micro‐LEDs are different, as a result, it may need complicated driving circuit to maintain the color rendering index during operation. One approach to avoid mass‐transfer process is monolithic integration of RGB micro‐LEDs via adhesive bonding. However, the fabrication process is complicated since different semiconductor substrates are required for growing blue/green micro‐LEDs and red micro‐LED. Another simpler method utilizes single‐color micro‐LEDs to excite the color converters, such as phosphors or quantum dots (QDs). For example, UV LED array with pixelated RGB QDs can achieve high efficiency and wide color gamut because no color filter is needed. However, for complete color down‐conversion, QDs with a high optical density and a relatively thick layer are required. Much research has been carried out recently to achieve high resolution microdisplays by arranging RBG LED this films accurately and effectively, resulting in a pixel size of under a few tens of micrometers. Currently, the most widely attempted methods for creating micro-LEDs are picking and placing a number of LED thin films without a substrate via laser lift-off (LLO). The major technical issue with this approach is acquiring all of the RGB thin film LED pixels and placing them on the display panel without any mixing pixels. Therefore, many research groups are attempting to solve these problem of transfer yield. Nevertheless, the thin film LED pixel transfer method still has a limited resolution because it depends on the pick-and-place machine accuracy rather than photolithograpy resolution. In short, this thin LED pixel transfer method may be suitable for 100-400 pixel per inch PPI) level display applications, such as smart watch, TVs and mobile phones. However, higher resolution displays above 1000-2000 PPI such as head mount displays, smart glasses and pico projector, remain a challenge. In this paper, we propose the new red and green color conversion material and lithography-based color conversion layer coating method with using monolithic blue LEDs. In addition, we propose the new growth technique of MOS2 thin film at the low temperature below 450℃ with metal buffer to use in fabrication of transparent and flexible TFT.