# Micro-LEDs, a Manufacturing Perspective
## 1 Introduction  
Owing to the fact that III-nitride-based light-emitting diodes (LEDs) are more efficient, reliable, and have longer life span in comparison to traditional lighting sources, such as incandescent and fluorescent bulbs, they are increasingly being employed as an energy-saving solution for various applications, among which are general lighting, automobile, signal, and backlighting in displays. Conventionally, to provide sufficient light power for general lighting, lateral dimensions of LED chips are designed to be larger than 200 \(\upmu\)m \(\times\) 200 \(\upmu\)m. However, various emerging display applications, including wearable devices, head-mounted, and large-area displays require miniaturized chips composed of arrays of micro-scale LED elements (micro-LED) with high density with the lateral dimension down to less than 100 \(\upmu\)m \(\times\) 100 \(\upmu\)m [1; 2; 3; 4; 5]. Due to the benefits provided by miniaturization, micro-LEDs are considered to possess great potential in multi-site neuron stimulation [6; 7], miniaturized optoelectronic tweezers [8], optical cochlear implants [9], and mask-free lithography [10]. Additionally, a short differential carrier lifetime resulting from reduced capacitance and increased current density in micro-LEDs makes it also an appealing candidate for high-speed visible light communications (VLC) [11; 12; 13; 14]. Today, the mainstream flat panel displays (FPDs) are dominated by liquid crystal display (LCDs) and organic light emitting diodes (OLEDs). Traditional LCD display requires LED backlight to emit light through a matrix of liquid crystal to generate images, and it suffers from the issues of high power consumption, slow response time, low viewing angle, and low uniformity. Unlike LCDs, OLED displays are self-emissive, and each pixel can be switched on and off individually. However, due to its inorganic nature, it has limitations in brightness and stability. Compared with them, micro-LED displays possess potential advantages such as high contrast, fast response, and relatively wide color gamut, low power consumption, and long lifetime. The prospects of a wide range of applications of micro-LEDs have attracted a large number of manufactures, startups, and academicresearchers to establish programs in the field. Setting the display applications aside, since the early 2000s, micro-LEDs have been investigated to improve the luminescence efficiency for high-intensity lighting motivated by their relatively high light extraction efficiency, better current spreading, and lower self-heating effect [15; 16; 17; 18]. In recent years, tens of companies around the world have been involved in the micro-LED technology. For instance, Sony demonstrated its first 55\" full-HD micro-LED TV panel in 2012, which featured six million micro-LEDs. Compared with the existing Sony LCD and plasma displays, the micro-LED prototype features 3.5 times higher contrast ratio, 1.4 times wider color gamut, and 10 times faster response time. In 2018, Sony released its micro-LED crystal-LED \"CLEDIS\" integrated display with micro-LED chip size of 0.003 mm\({}^{2}\), which has a contrast ratio of over 1,000,000:1 and a black surface area of over 99%. CEA-Leti, a technology research institute in France, demonstrated its prototype wide video graphic array (WVGA) micro-display with 10 um pixel pitch in 2017, which is based on the patterning of monochrome (blue or green) GaN micro-LED arrays and hybridizing with complementary metal-oxide-semiconductor (CMOS) circuit. In 2018, PlayNitride demonstrated two full-color micro-LED prototypes, one is a 0.89 inch 64 \(\times\) 64 panel with 105 pixel per inch (PPI) resolution, and another is a 3.12 inch 256 \(\times\) 256 panel with 116 PPI resolution. AU Optronics Corp. has demonstrated 12.1 inch full-color micro-LED display with a 169 PPI and 1920 \(\times\) 720 resolution achieved by micro-LEDs with pixel size less than 30 microns. X-Celeprint has demonstrated both passive and active matrix full-color micro-LED displays.