CONFORMAL PHOSPHOR COATING USING MASKLESS LITHOGRAPHY FOR PHOSPHOR-CONVERTED WHITE LIGHT EMITTING DIODES

Abstract

In this dissertation, I introduce a novel conformal phosphor coating system for white Light emitting diodes (LEDs). In this system, I use maskless lithography technique to improve coating quality. And, making up for its weakness, I design and develop a novel exposure system called dual exposure image processed maskless lithography system (DE-IPML). The key idea is to confine the coating area to light arrival range and resin applied range. By actively regulating the light path using maskless lithograpy, it is possible to control the coating thickness of LEDs side wall. Also by confining the height of phosphor-resin mixture, I can control the top coating thickness of LEDs. First, I describe the importance of white LED, especially conformal phosphor coated white LED. And, I introduce two conventional techniques our group had developed, which are basic technologies of this novel coating system. Lastly, the main concept of this research is described, which combine the phosphor coating technique and lithography technique. Then, a conformal, chip-level phosphor coating technique using IPML is presented. Owing to the ability of the IPML system to perform real-time recognition of LED placement and to generate a corresponding pattern, conformal phosphor coating is successfully performed on an LED array on a blue tape. The fabricated white LEDs provide a uniform color distribution that can be controlled by altering the coating thickness and the phosphor concentration. As the blue tape used in this experiment represents a conventional form of the LED chip delivery in packaging processes, the use of this method minimizes the revision needed for the packaging process. In addition, as color binning can be carried out prior to the packaging process, the waste and packaging cost needed to supply LEDs with a target white color is reduced in this method. However, the coating method has some limitations, so the limitations are introduced. The problems are shape distortion and difficulty of image processing, and they are usually revealed in the condition which contains high phosphor concentration. So, an improved lithography technique, dual exposure IPML (DE-IPML), is proposed to solve the problem. DE-IPML setup is designed and manufactured actually. pc-WLED chips is produced by the new lithography setup, and improved physical and optical properties are measured and compared with those of the chips produced by normal IPML setup. As the result, we can see that DE-IPML improves the optical properties of pc-WLEDs dramatically compared with the chips fabricated IPML chips. Coating LED array with large area using DE-IPML coating process is also tried. For this, I resolve two main problems, which are related with alignment of array and optics. Then, I perform the coating process of 20 × 20 chip array and measured their properties. The throughput of the process is about 50,000 ~ 100,000 chips per day. To show the extensibility of this technology, various types of materials are demonstrated. Various phosphors and resins are tested for investigating the possibility to be used in DE-IPML coating system. Shape formation and long term stability are examined primarily. The accessability of material for commercialization is also considered. And, some modified processes to apply other materials are introduced. To improve the usability of the coating systems, more advance research of materials is needed in future work. I envision that this approach greatly improves the manufacturing process of white LED and also offers opportunity in that domestic LED industry advance the market of high quality white LED manufacturing.