# Micro-LEDs, a Manufacturing Perspective
## 2 Epitaxy and Chip Processing  
The main requirements on the epitaxial growth aspects of GaN-based micro-LEDs are the wavelength and drive current homogeneity across the wafer and low defect density. Both Aixtron and Veeco, two major manufacturers of deposition systems, have articulated their approaches to address the particular needs for epitaxy of micro-LEDs [20; 21]. Aixtron's Planetary Reactor@technology uses Gas Foil Rotation@for each wafer disks to enable uniform growth across an individual wafer and a second rotation to improve wafer-to-wafer uniformity [20]. By incorporating a Cl\({}_{2}\) in situ cleaning process between each run and cassette-to-cassette wafer automation, a density of defects (with a size larger than 1 um) as low as \(\sim\)0.1 defects/cm\({}^{2}\) has been achieved. Veeco's vertical-rotating disc TurboDisctm technology provides high 14 \(\times\) 6\" capacity with low defectivity and high uniformity [21]. A defectivity yield (percentage of killer defect number \(\leq\) 1 per Epi donor field) above 90% has been demonstrated for applications in smartphones and TVs, respectively. Desire for large diameter sapphire substrates to meet the needs of large-sized monolithic micro-displays brings about some difficulties due to increased bowing induced by the large thermal and lattice mismatches with the LED epilayer structure [22; 23]. The primary problem associated with wafer bowing is non-uniformity of the wafer surface temperature resulting in the spatially non-uniform distribution of the indium molar fractions in the InGaN/GaN MQWs over the wafer. A temperature non-uniformity of 1 \({}^{\circ}\)C could induce a wavelength variation of 1.8 and 2.5 nm in blue and green LEDs, respectively [24]. Wafer bowing also causes issues during the device fabrication process, such as wafer cracking and degradation of device homogeneity in dry etching and other like processes. To deal with these challenges, thicker substrates, optimized wafer holder designs, control of the coalescence stage,insertion of strain-compensation layers, and internally focused laser treatment, have been employed and or investigated [23; 24; 25; 26; 27]. For example, [PERSON] et al. have shown that photoluminescence (PL) peak wavelength uniformity of about 2 nm in standard deviation can be obtained in 2, 4, and 6 inch wafers by using proper wafer pocket design, except for the eight inch wafer, for which the pocket design needs to be improved, as shown in Figure 1. In another study, [PERSON] et al. have achieved reduction of the bowing in both GaN-on-sapphire and GaN-on-silicon substrates by stress implantation using an internally focused laser processing [27]. Due to the opposite bowing direction, laser-induced stress was implanted near the backside surface of the sapphire substrate to compensate for the strain generated by the GaN/sapphire system, while it was implanted near the interface between the GaN and silicon to reduce the bow of the GaN-on-silicon substrate. Uniformity in LED pixel electrical parameters such as forward current-voltage characteristics is also a very important factor for high quality displays. Optimized driving circuits, n- and p-electrode structures, and optical designs have been developed to minimize, if not totally eliminate, any electrical non-uniformity [28]. For example, in micro-LED arrays with a shared common n-electrode and individually addressable p-electrodes, non-uniform operating currents at the same applied voltage for each micro-LED occurs due to variation in series resistances arising from different distances between the common n-electrode and the respective micro-LEDs [29]. To minimize series-resistance difference arising from varying conductive paths, a configuration using a common p-electrode with individually addressable n-electrodes has been developed, which mitigates the problem somewhat as the conduction through n-GaN is less resistive.