Wafer-Scale Characterization of High-Density Blue Micro-LED Arrays with an Optimized ITO Layer

Micro-light-emitting diode (Micro-LED) technology continues to attract strong interest due to the high resolutions, outstanding luminous efficiency, remarkable brightness, and impressive durability. These features make these types of diodes a most promising platform in high-end display applications such as mobile phones, wearable watches, and augmented reality (AR)/virtual reality (VR) displays, which require high luminance, high refresh rates, and high pixel-per-inch (PPI) values. Despite the advancements in micro-LED technologies, several issues hinder their widespread application. For example, the application of near-eye displays to industrial mass production requires comprehensive wafer-scale characterization of blue micro-LED arrays. To the best of our knowledge, wafer-scale characterization of the blue micro-LED arrays has not yet been reported in detail. In addition, we still need to improve the optimization of indium tin oxide (ITO) films. This is associated with high power consumption resulting from the high forward voltage (V_F), attributed to the weakened optical and electrical properties of the ITO films. These motivations drove us to undertake the four-inch wafer-scale characterization of high-performance blue micro-LED arrays with a resolution of 1692 PPI to realize micro-LED displays with a high-density resolution.
This study presents the four-inch wafer-scale fabrication of blue micro-LED arrays on a sapphire substrate with a resolution of 1692 PPI, accomplished by optimizing the properties of e-beam-deposited ITO (E-ITO) and sputter-deposited ITO (S-ITO) as a current-spreading layer ultimately to realize high-performance micro-LED displays. The surface morphology of the S-ITO film was relatively smooth and dense, while that of the E-ITO films was rather rough. The roughness of the E-ITO films is approximately 7.72 times greater than that of the S-ITO films. Also, the measured resistivity of the S-ITO films is 4.86 x 10^-4 ohm cm, much lower than that of the E-ITO films, at 5.96 x 10^-3 ohm cm. Interestingly, although the S-ITO films exhibited a densely packed morphology and lower resistivity compared to the E-ITO films, the V_F values of a micro-LED created with the S-ITO films were higher than those of a micro-LED created with the E-ITO films. The V_F values of a single pixel with the optimized E-ITO layer from region 1 to region 5 on a four-inch wafer are 2.88, 2.81, 2.81, 2.82, and 2.79 V at 30 A/cm², respectively. Also, with four pixels, the corresponding V_F values of the five different regions are 2.89, 2.83, 2.83, 2.83, and 2.79 V at 30 A/cm². Surprisingly, the V_F variations of a single pixel and of four pixels with five different regions are only 3.13 % and 3.46 %, respectively. As the current density was increased from 30 to 1,500 A/cm², the blue shifts in the electroluminescence (EL) peaks wavelength were approximately 8.2, 7.5, 6.4, 8.2, and 6.3 nm from region 1 to region 5, respectively. In addition, the corresponding variations of the full width at half maximum (FWHM) values on a four-inch wafer were approximately 10.5, 9.9, 9.8, 8.9, and 11.2 nm, respectively. The values of V_F, the emission wavelength, and the FWHM are very low and show a narrow distribution on the four-inch wafer. Also, various emission images of passive matrix-type blue micro-LEDs utilizing the optimized E-ITO spreading layer at 583 pixels and 847 pixels simultaneously demonstrate good display uniformity and brightness. These observations highlight the immense potential of blue micro-LEDs for demanding display applications, showcasing their ability to meet rigorous criteria for superior image quality in practical applications.