1700 pixels per inch (PPI) Passive-Matrix Micro-LED Display Powered by ASIC Wing Cheung Chong*, Wai Keung Cho, Zhao Jun Liu, Chu Hong Wang a and Kei May Lau Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong a 3C Limited, 481 Castle Peak Road, Kowloon, Hong Kong *Email: eeeddie@ust.hk, Phone: +852 2358 8843 Abstract — We report the first 1700 pixels per inch (PPI) passive-matrix blue light-emitting diodes on silicon (LEDoS) micro-displays. By flip-chip bonding a micro-LED array onto an ASIC display driver, we successfully fabricated a 0.19- inch display with a resolution of 256 x 192, the highest ever reported in LED-based micro-display. In addition, the LEDoS micro-display can deliver brightness as high as 1300 mcd/m 2 and render images in 6-bit grayscale. The remarkable performance suggests the tremendous potential of LEDoS micro-displays for portable display applications which require high performance, small size and low power consumption. Index Terms — Passive-matrix, light-emitting diodes on Silicon (LEDoS), micro-display, micro-LED array, ASIC, high resolution, flip-chip. I. INTRODUCTION LED micro-display has received attention recently because of its great potential to augment other existing micro-display technologies in the market [1]-[8]. Unlike liquid crystal display (LCD), liquid crystal on silicon (LCoS), and digital light processor (DLP), LED micro- display is a self-emissive device which can generate bright images efficiently without external light sources and lossy optical components. While organic LED (OLED) is an attractive alternative for micro-display applications, semiconductor-based LED is more advantageous in terms of brightness, lifetime, thermal stability and robustness in extreme conditions. Development of inorganic LED micro-displays is thus highly desirable. Despite all the attractive advantages, it is challenging to achieve high-resolution inorganic LED micro-displays with high pixel yield. An et al. demonstrated a passive- matrix micro-LED array structure by flip-chip bonding of a gallium nitride (GaN) micro-LED array on a silicon submount with common p-electrode stripes [7]. In their design, each pixel in the micro-LED array is connected to the common p-electrode stripes via an individual solder bump. Due to the large thermal mismatch between GaN and silicon, severe bonding failures occur in the closely spaced bumps. Many pixels are physically disconnected from the p-electrode line, leading to numerous dead pixels in the display. Similar issues also appear in active-matrix InGaN micro-display in which the LED array was bonded onto the silicon side by high-density indium bumps [8]. To improve the integrity of LED-based micro-displays, an alternative bonding scheme which can reduce the solder bump density is of paramount importance. In this paper, we report, to our best knowledge, the first 1700 pixels per inch (PPI) blue passive-matrix light- emitting diodes on silicon (LEDoS) micro-displays powered by ASIC with 6-bit grayscale, as shown in Fig. 1. This is realized by flip-chip bonding of a micro- LED array onto a CMOS-based ASIC display driver. The LEDoS micro-display consists of 256 x 192 pixels within a display area of 0.19 inch in diagonal. In our design, all passive-matrix interconnects are implemented on the LED side, in sharp contrast to previous reports in which the interconnects were done on the driver side [7]. With this special architecture, all the solder bumps can be relocated to the peripheral areas of the micro-LED array where the bumps can be bigger and more spread out. At the same time, only 448 bumps are needed for our display with almost 50000 pixels. The huge reduction in bump density significantly improves the bonding reliability. This novel passive-matrix display design and bump arrangement make high resolution and high-yield LEDoS micro-display achievable for a variety of applications. Fig. 1. Passive-matrix LEDoS micro-display wire-bonded on a flex cable. 978-1-4799-3622-9/14/$31.00 ©2014 IEEE