South China University of Technology team: Challenges in high-yield MicroLED display manufacturing

Micro LED is a new display technology that has huge advantages over existing mainstream display technologies in terms of brightness, resolution, contrast, energy consumption and response speed, and is considered the next generation of display technology. However, in the manufacturing process of Micro LED displays, how to produce high-quality, defect-free Micro LED chips and achieve non-destructive processing throughout the entire long manufacturing chain to ensure the yield of Micro LED chips is a big problem, which greatly limits the commercialization of Micro LED displays.

Recently, a research team led by Li Zongtao and Li Jiasheng of South China University of Technology published a review article titled "Challenges of high-yield manufacture in micro-light-emitting diodes displays: chip fabrication, mass transfer, and detection" in the 2024 issue 4 of the international journal "Journal of Physics D: Applied Physics", which systematically summarized the cutting-edge technologies and challenges in manufacturing high-yield Micro LED displays in recent years.

Introduction

MMicro LED chips are often manufactured in the form of arrays. First, a suitable material is selected as a substrate, and each layer structure of the chip is epitaxially grown on the substrate. The etching process is then used to cut the LED structure on the epitaxial wafer into an array of Micro LED chips. In order to meet the high yield requirements of Micro LED displays, the manufactured Micro LED chips often need to be of high quality and even need to achieve near-zero defect levels.

In the chip manufacturing process, the technical challenges to achieve perfect yield include improving the quality of epitaxial wafers, reducing epitaxial wafer defects, and avoiding sidewall defects caused by etching. These factors that affect chip yield and cause chip defects all come from processes such as epitaxial growth and etching. The quality of chips can often be improved by improving these processes.

Mass transfer is a key step for the application of Micro LED in displays, and it is also a technical problem encountered in the current industrialization process of Micro LED displays. Mass transfer refers to separating Micro LED chips from the source substrate and picking them up in batches, and then transferring them individually or in groups to the corresponding pixel electrodes of the display substrate. Since industrial production requires a mass transfer yield rate of not less than 99.9999%, a chip transfer error of no more than ±1.5μm, and a transfer efficiency greater than 50~100M/h, traditional chip transfer, packaging and other technical means cannot meet industrial needs. Mass transfer technology has become a constraint for MicroThe technical bottleneck of LED display mass production.

In order to cope with the mass transfer challenges in Micro LED display manufacturing, a variety of mass transfer technologies such as precision pick-up transfer technology, self-aligned roller transfer technology, self-assembly transfer technology, and laser-assisted transfer technology have been developed. There are two core processes for mass transfer, one is to peel the chip from the substrate, and the other is to connect the chip to the display substrate. During the process of peeling off the Micro LED chip from the substrate, due to the peeling force, cracks, scratches and other damage may occur on the surface of the chip, thus affecting the quality and life of the chip.

During the process of transferring Micro LED chips to the target substrate, due to the limitations of self-assembly technology, the chips may suffer from defects such as misalignment and damage, thus affecting the final display yield. No matter which mass transfer technology is used, the main purpose is to improve the efficiency and yield of these two processes.

Using MOCVD technology to perform material deposition on the substrate for Micro LED structure epitaxy and etching, and then transferring a large amount of Micro LED chips to the display backplane and completing the connection between Micro LED and integrated devices, this is the core step in making Micro LED displays. Even if high-yield processes are used in each step of the production process, because Micro LEDs are used for display, the number of chips is millions or even tens of millions.The appearance of defective pixels is inevitable.

For industrial applications, any defects on the display are intolerable, so for the final Micro LED display yield, in addition to selecting low-defect processes at each step in the long manufacturing chain, bad pixels in the display chip also need to be detected and repaired in a timely manner during the production process to ensure it. The ideal detection method should reduce costs and increase efficiency while avoiding any interference and damage to LED functionality.

For traditional LEDs, the inspection process is easy because the large size of LEDs allows connections with probes and even bad pixels can be spotted with the naked eye and replaced with mechanical removal methods or chip bonders. As the size of LEDs decreases to the micron level, the probe size may greatly exceed the LED being inspected and cause inspection difficulties. At the same time, the reduction in LED size requires that connections such as circuits and sensors should also be miniaturized accordingly. For this reason, it is difficult to detect and repair a large number of Micro LED chips using traditional methods, and new detection and repair technologies are urgently needed.

Finally, the technical direction for manufacturing high-yield Micro LED displays was pointed out. Improving the manufacturing yield of Micro LED chip manufacturing requires precise control of parameters such as rotation speed, temperature and gas flow rate during the epitaxial growth process of epitaxial wafers. In the chip manufacturing process, this control is still a requirement for manufacturing to satisfy production requirements.The key research focus is to produce high-quality Micro LED chips with yield requirements. In terms of avoiding sidewall defects, the direct growth of micro-LED chip structures using nanoparticles and patterned templates shows promise and deserves further study.

In addition, the combination of QMAT substrate technology and LLO technology has the potential to improve transfer yield in subsequent stages. In the chip transfer process, LLO technology is a very promising high-yield chip stripping method. In the LLO lift-off process, the adjustment mechanism between laser parameters and interface adhesion needs to be further explored. Micropipe technology is well suited to the TFT process and has proven suitable for connecting stripped Micro LED chips on TFT backplanes.

For small Micro LED display products, monolithic integration technology can be considered to replace the mass transfer process. During the chip inspection process, an effective solution is to use AOI inspection technology to identify Micro LED chips with appearance defects and abnormal EL illumination brightness. This solution can detect a large number of Micro LED chips efficiently and accurately. Chip repair work relies heavily on chip transfer technology, which provides a convenient solution for replacing defective chips.

The three process steps of chip manufacturing, mass transfer and detection are interrelated and interdependent, and cooperation between them is required to ensure that Micro LED display yield.